Phase 2 PR 13 — port GalaxyRuntimeProbeManager state machine + wire per-platform ScanState probing. PR 8 gave operators the gateway-level transport signal (MxAccessClient.ConnectionStateChanged → OnHostStatusChanged tagged with the Wonderware client identity) — enough to detect when the entire MXAccess COM proxy dies, but silent when a specific or goes down while the gateway stays alive. This PR closes that gap: a pure-logic GalaxyRuntimeProbeManager (ported from v1's 472-LOC GalaxyRuntimeProbeManager.cs, distilled to ~240 LOC of state machine without the OPC UA node-manager entanglement) lives in Backend/Stability/, advises <TagName>.ScanState per WinPlatform + AppEngine gobject discovered during DiscoverAsync, runs the Unknown → Running → Stopped state machine with v1's documented semantics preserved verbatim, and raises a StateChanged event on transitions operators should react to. MxAccessGalaxyBackend instantiates the probe manager in the constructor with SubscribeAsync/UnsubscribeAsync delegate pointers into MxAccessClient, hooks StateChanged to forward each transition through the same OnHostStatusChanged IPC event the gateway signal uses (HostName = platform/engine TagName, RuntimeStatus = 'Running'|'Stopped'|'Unknown', LastObservedUtcUnixMs from the state-change timestamp), so Admin UI gets per-host signals flowing through the existing PR 8 wire with no additional IPC plumbing. State machine rules ported from v1 runtimestatus.md: (a) ScanState is on-change-only — a stably-Running host may go hours without a callback, so Running → Stopped is driven only by explicit ScanState=false, never by starvation; (b) Unknown → Running is a startup transition and does NOT fire StateChanged (would paint every host as 'just recovered' at startup, which is noise and can clear Bad quality set by a concurrently-stopping sibling); (c) Stopped → Running fires StateChanged for the real recovery case; (d) Running → Stopped fires StateChanged; (e) Unknown → Stopped fires StateChanged because that's the first-known-bad signal operators need when a host is down at our startup time. MxAccessGalaxyBackend.DiscoverAsync calls _probeManager.SyncAsync with the runtime-host subset of the hierarchy (CategoryId == 1 WinPlatform or 3 AppEngine) as a best-effort step after building the Discover response — probe failures are swallowed so Discover still returns the hierarchy even if a per-host advise fails; the gateway signal covers the critical rung. SyncAsync is idempotent (second call with the same set is a no-op) and handles the diff on re-Discover for tag rename / host add / host remove. Subscribe failure rolls back the host's state entry under the lock so a later probe callback for a never-advised tag can't transition a phantom state from Unknown to Stopped and fan out a false host-down signal (the same protection v1's GalaxyRuntimeProbeManager had at line 237-243 of v1 with a captured-probe-string comparison under the lock). MxAccessGalaxyBackend.Dispose unsubscribes the StateChanged handler before disposing the probe manager to prevent dangling invocation-list references across reconnects, same discipline as PR 8's ConnectionStateChanged and PR 6's SubscriptionReplayFailed. Tests (12 new GalaxyRuntimeProbeManagerTests): Sync_subscribes_to_ScanState_per_host verifies tag.ScanState subscriptions are advised per Platform/Engine; Sync_is_idempotent_on_repeat_call_with_same_set verifies no duplicate subscribes; Sync_unadvises_removed_hosts verifies the diff unadvises gone hosts; Subscribe_failure_rolls_back_host_entry_so_later_transitions_do_not_fire_stale_events covers the rollback-on-subscribe-fail guard; Unknown_to_Running_does_not_fire_StateChanged preserves the startup-noise rule; Running_to_Stopped_fires_StateChanged_with_both_states asserts OldState and NewState are both captured in the transition record; Stopped_to_Running_fires_StateChanged_for_recovery verifies the recovery case; Unknown_to_Stopped_fires_StateChanged_for_first_known_bad_signal preserves the first-bad rule; Repeated_Good_Running_callbacks_do_not_fire_duplicate_events verifies the state-tracking de-dup; Unknown_callback_for_non_tracked_probe_is_dropped asserts a foreign callback is silently ignored; Snapshot_reports_current_state_for_every_tracked_host covers the dashboard query hook; IsRuntimeHost_recognizes_WinPlatform_and_AppEngine_category_ids asserts the CategoryId filter. Galaxy.Host.Tests Unit suite 75 pass / 0 fail (12 new probe + 63 pre-existing). Galaxy.Host builds clean (0 errors / 0 warnings). Branches off v2.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/ZB.MOM.WW.OtOpcUa.Core.Abstractions/DriverAttributeInfo.cs

@@ -19,10 +19,17 @@ namespace ZB.MOM.WW.OtOpcUa.Core.Abstractions;
 /// <param name="ArrayDim">Declared array length when <see cref="IsArray"/> is true; null otherwise.</param>
 /// <param name="SecurityClass">Write-authorization tier for this attribute.</param>
 /// <param name="IsHistorized">True when this attribute is expected to feed historian / HistoryRead.</param>
+/// <param name="IsAlarm">
+///     True when this attribute represents an alarm condition (Galaxy: has an
+///     <c>AlarmExtension</c> primitive). The generic node-manager enriches the variable with an
+///     OPC UA <c>AlarmConditionState</c> when true. Defaults to false so existing non-Galaxy
+///     drivers aren't forced to flow a flag they don't produce.
+/// </param>
 public sealed record DriverAttributeInfo(
     string FullName,
     DriverDataType DriverDataType,
     bool IsArray,
     uint? ArrayDim,
     SecurityClassification SecurityClass,
-    bool IsHistorized);
+    bool IsHistorized,
+    bool IsAlarm = false);

src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host/Backend/DbBackedGalaxyBackend.cs

@@ -127,6 +127,33 @@ public sealed class DbBackedGalaxyBackend(GalaxyRepository repository) : IGalaxy
             Tags = System.Array.Empty<HistoryTagValues>(),
         });
 
+    public Task<HistoryReadProcessedResponse> HistoryReadProcessedAsync(
+        HistoryReadProcessedRequest req, CancellationToken ct)
+        => Task.FromResult(new HistoryReadProcessedResponse
+        {
+            Success = false,
+            Error = "MXAccess + Historian code lift pending (Phase 2 Task B.1)",
+            Values = System.Array.Empty<GalaxyDataValue>(),
+        });
+
+    public Task<HistoryReadAtTimeResponse> HistoryReadAtTimeAsync(
+        HistoryReadAtTimeRequest req, CancellationToken ct)
+        => Task.FromResult(new HistoryReadAtTimeResponse
+        {
+            Success = false,
+            Error = "MXAccess + Historian code lift pending (Phase 2 Task B.1)",
+            Values = System.Array.Empty<GalaxyDataValue>(),
+        });
+
+    public Task<HistoryReadEventsResponse> HistoryReadEventsAsync(
+        HistoryReadEventsRequest req, CancellationToken ct)
+        => Task.FromResult(new HistoryReadEventsResponse
+        {
+            Success = false,
+            Error = "MXAccess + Historian code lift pending (Phase 2 Task B.1)",
+            Events = System.Array.Empty<GalaxyHistoricalEvent>(),
+        });
+
     public Task<RecycleStatusResponse> RecycleAsync(RecycleHostRequest req, CancellationToken ct)
         => Task.FromResult(new RecycleStatusResponse { Accepted = true, GraceSeconds = 15 });
 
@@ -138,6 +165,7 @@ public sealed class DbBackedGalaxyBackend(GalaxyRepository repository) : IGalaxy
         ArrayDim = row.ArrayDimension is int d and > 0 ? (uint)d : null,
         SecurityClassification = row.SecurityClassification,
         IsHistorized = row.IsHistorized,
+        IsAlarm = row.IsAlarm,
     };
 
     /// <summary>

src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host/Backend/Historian/HistorianClusterEndpointPicker.cs

@@ -0,0 +1,129 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Backend.Historian
+{
+    /// <summary>
+    ///     Thread-safe, pure-logic endpoint picker for the Wonderware Historian cluster. Tracks which
+    ///     configured nodes are healthy, places failed nodes in a time-bounded cooldown, and hands
+    ///     out an ordered list of eligible candidates for the data source to try in sequence.
+    /// </summary>
+    internal sealed class HistorianClusterEndpointPicker
+    {
+        private readonly Func<DateTime> _clock;
+        private readonly TimeSpan _cooldown;
+        private readonly object _lock = new object();
+        private readonly List<NodeEntry> _nodes;
+
+        public HistorianClusterEndpointPicker(HistorianConfiguration config)
+            : this(config, () => DateTime.UtcNow) { }
+
+        internal HistorianClusterEndpointPicker(HistorianConfiguration config, Func<DateTime> clock)
+        {
+            _clock = clock ?? throw new ArgumentNullException(nameof(clock));
+            _cooldown = TimeSpan.FromSeconds(Math.Max(0, config.FailureCooldownSeconds));
+
+            var names = (config.ServerNames != null && config.ServerNames.Count > 0)
+                ? config.ServerNames
+                : new List<string> { config.ServerName };
+
+            _nodes = names
+                .Where(n => !string.IsNullOrWhiteSpace(n))
+                .Select(n => n.Trim())
+                .Distinct(StringComparer.OrdinalIgnoreCase)
+                .Select(n => new NodeEntry { Name = n })
+                .ToList();
+        }
+
+        public int NodeCount
+        {
+            get { lock (_lock) return _nodes.Count; }
+        }
+
+        public IReadOnlyList<string> GetHealthyNodes()
+        {
+            lock (_lock)
+            {
+                var now = _clock();
+                return _nodes.Where(n => IsHealthyAt(n, now)).Select(n => n.Name).ToList();
+            }
+        }
+
+        public int HealthyNodeCount
+        {
+            get
+            {
+                lock (_lock)
+                {
+                    var now = _clock();
+                    return _nodes.Count(n => IsHealthyAt(n, now));
+                }
+            }
+        }
+
+        public void MarkFailed(string node, string? error)
+        {
+            lock (_lock)
+            {
+                var entry = FindEntry(node);
+                if (entry == null) return;
+
+                var now = _clock();
+                entry.FailureCount++;
+                entry.LastError = error;
+                entry.LastFailureTime = now;
+                entry.CooldownUntil = _cooldown.TotalMilliseconds > 0 ? now + _cooldown : (DateTime?)null;
+            }
+        }
+
+        public void MarkHealthy(string node)
+        {
+            lock (_lock)
+            {
+                var entry = FindEntry(node);
+                if (entry == null) return;
+                entry.CooldownUntil = null;
+            }
+        }
+
+        public List<HistorianClusterNodeState> SnapshotNodeStates()
+        {
+            lock (_lock)
+            {
+                var now = _clock();
+                return _nodes.Select(n => new HistorianClusterNodeState
+                {
+                    Name = n.Name,
+                    IsHealthy = IsHealthyAt(n, now),
+                    CooldownUntil = IsHealthyAt(n, now) ? null : n.CooldownUntil,
+                    FailureCount = n.FailureCount,
+                    LastError = n.LastError,
+                    LastFailureTime = n.LastFailureTime
+                }).ToList();
+            }
+        }
+
+        private static bool IsHealthyAt(NodeEntry entry, DateTime now)
+        {
+            return entry.CooldownUntil == null || entry.CooldownUntil <= now;
+        }
+
+        private NodeEntry? FindEntry(string node)
+        {
+            for (var i = 0; i < _nodes.Count; i++)
+                if (string.Equals(_nodes[i].Name, node, StringComparison.OrdinalIgnoreCase))
+                    return _nodes[i];
+            return null;
+        }
+
+        private sealed class NodeEntry
+        {
+            public string Name { get; set; } = "";
+            public DateTime? CooldownUntil { get; set; }
+            public int FailureCount { get; set; }
+            public string? LastError { get; set; }
+            public DateTime? LastFailureTime { get; set; }
+        }
+    }
+}

src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host/Backend/Historian/HistorianClusterNodeState.cs

@@ -0,0 +1,18 @@
+using System;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Backend.Historian
+{
+    /// <summary>
+    ///     Point-in-time state of a single historian cluster node. One entry per configured node
+    ///     appears inside <see cref="HistorianHealthSnapshot"/>.
+    /// </summary>
+    public sealed class HistorianClusterNodeState
+    {
+        public string Name { get; set; } = "";
+        public bool IsHealthy { get; set; }
+        public DateTime? CooldownUntil { get; set; }
+        public int FailureCount { get; set; }
+        public string? LastError { get; set; }
+        public DateTime? LastFailureTime { get; set; }
+    }
+}

src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host/Backend/Historian/HistorianConfiguration.cs

@@ -0,0 +1,38 @@
+using System.Collections.Generic;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Backend.Historian
+{
+    /// <summary>
+    ///     Wonderware Historian SDK configuration. Populated from environment variables at Host
+    ///     startup (see <c>Program.cs</c>) or from the Proxy's <c>DriverInstance.DriverConfig</c>
+    ///     section passed during OpenSession. Kept OPC-UA-free — the Proxy side owns UA translation.
+    /// </summary>
+    public sealed class HistorianConfiguration
+    {
+        public bool Enabled { get; set; } = false;
+
+        /// <summary>Single-node fallback when <see cref="ServerNames"/> is empty.</summary>
+        public string ServerName { get; set; } = "localhost";
+
+        /// <summary>
+        ///     Ordered cluster nodes. When non-empty, the data source tries each in order on connect,
+        ///     falling through to the next on failure. A failed node is placed in cooldown for
+        ///     <see cref="FailureCooldownSeconds"/> before being re-eligible.
+        /// </summary>
+        public List<string> ServerNames { get; set; } = new();
+
+        public int FailureCooldownSeconds { get; set; } = 60;
+        public bool IntegratedSecurity { get; set; } = true;
+        public string? UserName { get; set; }
+        public string? Password { get; set; }
+        public int Port { get; set; } = 32568;
+        public int CommandTimeoutSeconds { get; set; } = 30;
+        public int MaxValuesPerRead { get; set; } = 10000;
+
+        /// <summary>
+        ///     Outer safety timeout applied to sync-over-async Historian operations. Must be
+        ///     comfortably larger than <see cref="CommandTimeoutSeconds"/>.
+        /// </summary>
+        public int RequestTimeoutSeconds { get; set; } = 60;
+    }
+}

src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host/Backend/Historian/HistorianDataSource.cs

@@ -0,0 +1,621 @@
+using System;
+using System.Collections.Generic;
+using StringCollection = System.Collections.Specialized.StringCollection;
+using System.Threading;
+using System.Threading.Tasks;
+using ArchestrA;
+using Serilog;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Backend.Historian
+{
+    /// <summary>
+    ///     Reads historical data from the Wonderware Historian via the aahClientManaged SDK.
+    ///     OPC-UA-free — emits <see cref="HistorianSample"/>/<see cref="HistorianAggregateSample"/>
+    ///     which the Proxy maps to OPC UA <c>DataValue</c> on its side of the IPC.
+    /// </summary>
+    public sealed class HistorianDataSource : IHistorianDataSource
+    {
+        private static readonly ILogger Log = Serilog.Log.ForContext<HistorianDataSource>();
+
+        private readonly HistorianConfiguration _config;
+        private readonly object _connectionLock = new object();
+        private readonly object _eventConnectionLock = new object();
+        private readonly IHistorianConnectionFactory _factory;
+        private HistorianAccess? _connection;
+        private HistorianAccess? _eventConnection;
+        private bool _disposed;
+
+        private readonly object _healthLock = new object();
+        private long _totalSuccesses;
+        private long _totalFailures;
+        private int _consecutiveFailures;
+        private DateTime? _lastSuccessTime;
+        private DateTime? _lastFailureTime;
+        private string? _lastError;
+        private string? _activeProcessNode;
+        private string? _activeEventNode;
+
+        private readonly HistorianClusterEndpointPicker _picker;
+
+        public HistorianDataSource(HistorianConfiguration config)
+            : this(config, new SdkHistorianConnectionFactory(), null) { }
+
+        internal HistorianDataSource(
+            HistorianConfiguration config,
+            IHistorianConnectionFactory factory,
+            HistorianClusterEndpointPicker? picker = null)
+        {
+            _config = config;
+            _factory = factory;
+            _picker = picker ?? new HistorianClusterEndpointPicker(config);
+        }
+
+        private (HistorianAccess Connection, string Node) ConnectToAnyHealthyNode(HistorianConnectionType type)
+        {
+            var candidates = _picker.GetHealthyNodes();
+            if (candidates.Count == 0)
+            {
+                var total = _picker.NodeCount;
+                throw new InvalidOperationException(
+                    total == 0
+                        ? "No historian nodes configured"
+                        : $"All {total} historian nodes are in cooldown — no healthy endpoints to connect to");
+            }
+
+            Exception? lastException = null;
+            foreach (var node in candidates)
+            {
+                var attemptConfig = CloneConfigWithServerName(node);
+                try
+                {
+                    var conn = _factory.CreateAndConnect(attemptConfig, type);
+                    _picker.MarkHealthy(node);
+                    return (conn, node);
+                }
+                catch (Exception ex)
+                {
+                    _picker.MarkFailed(node, ex.Message);
+                    lastException = ex;
+                    Log.Warning(ex, "Historian node {Node} failed during connect attempt; trying next candidate", node);
+                }
+            }
+
+            var inner = lastException?.Message ?? "(no detail)";
+            throw new InvalidOperationException(
+                $"All {candidates.Count} healthy historian candidate(s) failed during connect: {inner}",
+                lastException);
+        }
+
+        private HistorianConfiguration CloneConfigWithServerName(string serverName)
+        {
+            return new HistorianConfiguration
+            {
+                Enabled = _config.Enabled,
+                ServerName = serverName,
+                ServerNames = _config.ServerNames,
+                FailureCooldownSeconds = _config.FailureCooldownSeconds,
+                IntegratedSecurity = _config.IntegratedSecurity,
+                UserName = _config.UserName,
+                Password = _config.Password,
+                Port = _config.Port,
+                CommandTimeoutSeconds = _config.CommandTimeoutSeconds,
+                MaxValuesPerRead = _config.MaxValuesPerRead
+            };
+        }
+
+        public HistorianHealthSnapshot GetHealthSnapshot()
+        {
+            var nodeStates = _picker.SnapshotNodeStates();
+            var healthyCount = 0;
+            foreach (var n in nodeStates)
+                if (n.IsHealthy) healthyCount++;
+
+            lock (_healthLock)
+            {
+                return new HistorianHealthSnapshot
+                {
+                    TotalQueries = _totalSuccesses + _totalFailures,
+                    TotalSuccesses = _totalSuccesses,
+                    TotalFailures = _totalFailures,
+                    ConsecutiveFailures = _consecutiveFailures,
+                    LastSuccessTime = _lastSuccessTime,
+                    LastFailureTime = _lastFailureTime,
+                    LastError = _lastError,
+                    ProcessConnectionOpen = Volatile.Read(ref _connection) != null,
+                    EventConnectionOpen = Volatile.Read(ref _eventConnection) != null,
+                    ActiveProcessNode = _activeProcessNode,
+                    ActiveEventNode = _activeEventNode,
+                    NodeCount = nodeStates.Count,
+                    HealthyNodeCount = healthyCount,
+                    Nodes = nodeStates
+                };
+            }
+        }
+
+        private void RecordSuccess()
+        {
+            lock (_healthLock)
+            {
+                _totalSuccesses++;
+                _lastSuccessTime = DateTime.UtcNow;
+                _consecutiveFailures = 0;
+                _lastError = null;
+            }
+        }
+
+        private void RecordFailure(string error)
+        {
+            lock (_healthLock)
+            {
+                _totalFailures++;
+                _lastFailureTime = DateTime.UtcNow;
+                _consecutiveFailures++;
+                _lastError = error;
+            }
+        }
+
+        private void EnsureConnected()
+        {
+            if (_disposed)
+                throw new ObjectDisposedException(nameof(HistorianDataSource));
+
+            if (Volatile.Read(ref _connection) != null) return;
+
+            var (conn, winningNode) = ConnectToAnyHealthyNode(HistorianConnectionType.Process);
+
+            lock (_connectionLock)
+            {
+                if (_disposed)
+                {
+                    conn.CloseConnection(out _);
+                    conn.Dispose();
+                    throw new ObjectDisposedException(nameof(HistorianDataSource));
+                }
+
+                if (_connection != null)
+                {
+                    conn.CloseConnection(out _);
+                    conn.Dispose();
+                    return;
+                }
+
+                _connection = conn;
+                lock (_healthLock) _activeProcessNode = winningNode;
+                Log.Information("Historian SDK connection opened to {Server}:{Port}", winningNode, _config.Port);
+            }
+        }
+
+        private void HandleConnectionError(Exception? ex = null)
+        {
+            lock (_connectionLock)
+            {
+                if (_connection == null) return;
+
+                try
+                {
+                    _connection.CloseConnection(out _);
+                    _connection.Dispose();
+                }
+                catch (Exception disposeEx)
+                {
+                    Log.Debug(disposeEx, "Error disposing Historian SDK connection during error recovery");
+                }
+
+                _connection = null;
+                string? failedNode;
+                lock (_healthLock)
+                {
+                    failedNode = _activeProcessNode;
+                    _activeProcessNode = null;
+                }
+
+                if (failedNode != null) _picker.MarkFailed(failedNode, ex?.Message ?? "mid-query failure");
+                Log.Warning(ex, "Historian SDK connection reset (node={Node})", failedNode ?? "(unknown)");
+            }
+        }
+
+        private void EnsureEventConnected()
+        {
+            if (_disposed)
+                throw new ObjectDisposedException(nameof(HistorianDataSource));
+
+            if (Volatile.Read(ref _eventConnection) != null) return;
+
+            var (conn, winningNode) = ConnectToAnyHealthyNode(HistorianConnectionType.Event);
+
+            lock (_eventConnectionLock)
+            {
+                if (_disposed)
+                {
+                    conn.CloseConnection(out _);
+                    conn.Dispose();
+                    throw new ObjectDisposedException(nameof(HistorianDataSource));
+                }
+
+                if (_eventConnection != null)
+                {
+                    conn.CloseConnection(out _);
+                    conn.Dispose();
+                    return;
+                }
+
+                _eventConnection = conn;
+                lock (_healthLock) _activeEventNode = winningNode;
+                Log.Information("Historian SDK event connection opened to {Server}:{Port}", winningNode, _config.Port);
+            }
+        }
+
+        private void HandleEventConnectionError(Exception? ex = null)
+        {
+            lock (_eventConnectionLock)
+            {
+                if (_eventConnection == null) return;
+
+                try
+                {
+                    _eventConnection.CloseConnection(out _);
+                    _eventConnection.Dispose();
+                }
+                catch (Exception disposeEx)
+                {
+                    Log.Debug(disposeEx, "Error disposing Historian SDK event connection during error recovery");
+                }
+
+                _eventConnection = null;
+                string? failedNode;
+                lock (_healthLock)
+                {
+                    failedNode = _activeEventNode;
+                    _activeEventNode = null;
+                }
+
+                if (failedNode != null) _picker.MarkFailed(failedNode, ex?.Message ?? "mid-query failure");
+                Log.Warning(ex, "Historian SDK event connection reset (node={Node})", failedNode ?? "(unknown)");
+            }
+        }
+
+        public Task<List<HistorianSample>> ReadRawAsync(
+            string tagName, DateTime startTime, DateTime endTime, int maxValues,
+            CancellationToken ct = default)
+        {
+            var results = new List<HistorianSample>();
+
+            try
+            {
+                EnsureConnected();
+
+                using var query = _connection!.CreateHistoryQuery();
+                var args = new HistoryQueryArgs
+                {
+                    TagNames = new StringCollection { tagName },
+                    StartDateTime = startTime,
+                    EndDateTime = endTime,
+                    RetrievalMode = HistorianRetrievalMode.Full
+                };
+
+                if (maxValues > 0)
+                    args.BatchSize = (uint)maxValues;
+                else if (_config.MaxValuesPerRead > 0)
+                    args.BatchSize = (uint)_config.MaxValuesPerRead;
+
+                if (!query.StartQuery(args, out var error))
+                {
+                    Log.Warning("Historian SDK raw query start failed for {Tag}: {Error}", tagName, error.ErrorCode);
+                    RecordFailure($"raw StartQuery: {error.ErrorCode}");
+                    HandleConnectionError();
+                    return Task.FromResult(results);
+                }
+
+                var count = 0;
+                var limit = maxValues > 0 ? maxValues : _config.MaxValuesPerRead;
+
+                while (query.MoveNext(out error))
+                {
+                    ct.ThrowIfCancellationRequested();
+
+                    var result = query.QueryResult;
+                    var timestamp = DateTime.SpecifyKind(result.StartDateTime, DateTimeKind.Utc);
+
+                    object? value;
+                    if (!string.IsNullOrEmpty(result.StringValue) && result.Value == 0)
+                        value = result.StringValue;
+                    else
+                        value = result.Value;
+
+                    results.Add(new HistorianSample
+                    {
+                        Value = value,
+                        TimestampUtc = timestamp,
+                        Quality = (byte)(result.OpcQuality & 0xFF),
+                    });
+
+                    count++;
+                    if (limit > 0 && count >= limit) break;
+                }
+
+                query.EndQuery(out _);
+                RecordSuccess();
+            }
+            catch (OperationCanceledException) { throw; }
+            catch (ObjectDisposedException) { throw; }
+            catch (Exception ex)
+            {
+                Log.Warning(ex, "HistoryRead raw failed for {Tag}", tagName);
+                RecordFailure($"raw: {ex.Message}");
+                HandleConnectionError(ex);
+            }
+
+            Log.Debug("HistoryRead raw: {Tag} returned {Count} values ({Start} to {End})",
+                tagName, results.Count, startTime, endTime);
+
+            return Task.FromResult(results);
+        }
+
+        public Task<List<HistorianAggregateSample>> ReadAggregateAsync(
+            string tagName, DateTime startTime, DateTime endTime,
+            double intervalMs, string aggregateColumn,
+            CancellationToken ct = default)
+        {
+            var results = new List<HistorianAggregateSample>();
+
+            try
+            {
+                EnsureConnected();
+
+                using var query = _connection!.CreateAnalogSummaryQuery();
+                var args = new AnalogSummaryQueryArgs
+                {
+                    TagNames = new StringCollection { tagName },
+                    StartDateTime = startTime,
+                    EndDateTime = endTime,
+                    Resolution = (ulong)intervalMs
+                };
+
+                if (!query.StartQuery(args, out var error))
+                {
+                    Log.Warning("Historian SDK aggregate query start failed for {Tag}: {Error}", tagName, error.ErrorCode);
+                    RecordFailure($"aggregate StartQuery: {error.ErrorCode}");
+                    HandleConnectionError();
+                    return Task.FromResult(results);
+                }
+
+                while (query.MoveNext(out error))
+                {
+                    ct.ThrowIfCancellationRequested();
+
+                    var result = query.QueryResult;
+                    var timestamp = DateTime.SpecifyKind(result.StartDateTime, DateTimeKind.Utc);
+                    var value = ExtractAggregateValue(result, aggregateColumn);
+
+                    results.Add(new HistorianAggregateSample
+                    {
+                        Value = value,
+                        TimestampUtc = timestamp,
+                    });
+                }
+
+                query.EndQuery(out _);
+                RecordSuccess();
+            }
+            catch (OperationCanceledException) { throw; }
+            catch (ObjectDisposedException) { throw; }
+            catch (Exception ex)
+            {
+                Log.Warning(ex, "HistoryRead aggregate failed for {Tag}", tagName);
+                RecordFailure($"aggregate: {ex.Message}");
+                HandleConnectionError(ex);
+            }
+
+            Log.Debug("HistoryRead aggregate ({Aggregate}): {Tag} returned {Count} values",
+                aggregateColumn, tagName, results.Count);
+
+            return Task.FromResult(results);
+        }
+
+        public Task<List<HistorianSample>> ReadAtTimeAsync(
+            string tagName, DateTime[] timestamps,
+            CancellationToken ct = default)
+        {
+            var results = new List<HistorianSample>();
+
+            if (timestamps == null || timestamps.Length == 0)
+                return Task.FromResult(results);
+
+            try
+            {
+                EnsureConnected();
+
+                foreach (var timestamp in timestamps)
+                {
+                    ct.ThrowIfCancellationRequested();
+
+                    using var query = _connection!.CreateHistoryQuery();
+                    var args = new HistoryQueryArgs
+                    {
+                        TagNames = new StringCollection { tagName },
+                        StartDateTime = timestamp,
+                        EndDateTime = timestamp,
+                        RetrievalMode = HistorianRetrievalMode.Interpolated,
+                        BatchSize = 1
+                    };
+
+                    if (!query.StartQuery(args, out var error))
+                    {
+                        results.Add(new HistorianSample
+                        {
+                            Value = null,
+                            TimestampUtc = DateTime.SpecifyKind(timestamp, DateTimeKind.Utc),
+                            Quality = 0, // Bad
+                        });
+                        continue;
+                    }
+
+                    if (query.MoveNext(out error))
+                    {
+                        var result = query.QueryResult;
+                        object? value;
+                        if (!string.IsNullOrEmpty(result.StringValue) && result.Value == 0)
+                            value = result.StringValue;
+                        else
+                            value = result.Value;
+
+                        results.Add(new HistorianSample
+                        {
+                            Value = value,
+                            TimestampUtc = DateTime.SpecifyKind(timestamp, DateTimeKind.Utc),
+                            Quality = (byte)(result.OpcQuality & 0xFF),
+                        });
+                    }
+                    else
+                    {
+                        results.Add(new HistorianSample
+                        {
+                            Value = null,
+                            TimestampUtc = DateTime.SpecifyKind(timestamp, DateTimeKind.Utc),
+                            Quality = 0,
+                        });
+                    }
+
+                    query.EndQuery(out _);
+                }
+                RecordSuccess();
+            }
+            catch (OperationCanceledException) { throw; }
+            catch (ObjectDisposedException) { throw; }
+            catch (Exception ex)
+            {
+                Log.Warning(ex, "HistoryRead at-time failed for {Tag}", tagName);
+                RecordFailure($"at-time: {ex.Message}");
+                HandleConnectionError(ex);
+            }
+
+            Log.Debug("HistoryRead at-time: {Tag} returned {Count} values for {Timestamps} timestamps",
+                tagName, results.Count, timestamps.Length);
+
+            return Task.FromResult(results);
+        }
+
+        public Task<List<HistorianEventDto>> ReadEventsAsync(
+            string? sourceName, DateTime startTime, DateTime endTime, int maxEvents,
+            CancellationToken ct = default)
+        {
+            var results = new List<HistorianEventDto>();
+
+            try
+            {
+                EnsureEventConnected();
+
+                using var query = _eventConnection!.CreateEventQuery();
+                var args = new EventQueryArgs
+                {
+                    StartDateTime = startTime,
+                    EndDateTime = endTime,
+                    EventCount = maxEvents > 0 ? (uint)maxEvents : (uint)_config.MaxValuesPerRead,
+                    QueryType = HistorianEventQueryType.Events,
+                    EventOrder = HistorianEventOrder.Ascending
+                };
+
+                if (!string.IsNullOrEmpty(sourceName))
+                {
+                    query.AddEventFilter("Source", HistorianComparisionType.Equal, sourceName, out _);
+                }
+
+                if (!query.StartQuery(args, out var error))
+                {
+                    Log.Warning("Historian SDK event query start failed: {Error}", error.ErrorCode);
+                    RecordFailure($"events StartQuery: {error.ErrorCode}");
+                    HandleEventConnectionError();
+                    return Task.FromResult(results);
+                }
+
+                var count = 0;
+                while (query.MoveNext(out error))
+                {
+                    ct.ThrowIfCancellationRequested();
+                    results.Add(ToDto(query.QueryResult));
+                    count++;
+                    if (maxEvents > 0 && count >= maxEvents) break;
+                }
+
+                query.EndQuery(out _);
+                RecordSuccess();
+            }
+            catch (OperationCanceledException) { throw; }
+            catch (ObjectDisposedException) { throw; }
+            catch (Exception ex)
+            {
+                Log.Warning(ex, "HistoryRead events failed for source {Source}", sourceName ?? "(all)");
+                RecordFailure($"events: {ex.Message}");
+                HandleEventConnectionError(ex);
+            }
+
+            Log.Debug("HistoryRead events: source={Source} returned {Count} events ({Start} to {End})",
+                sourceName ?? "(all)", results.Count, startTime, endTime);
+
+            return Task.FromResult(results);
+        }
+
+        private static HistorianEventDto ToDto(HistorianEvent evt)
+        {
+            // The ArchestrA SDK marks these properties obsolete but still returns them; their
+            // successors aren't wired in the version we bind against. Using them is the documented
+            // v1 behavior — suppressed locally instead of project-wide so any non-event use of
+            // deprecated SDK surface still surfaces as an error.
+#pragma warning disable CS0618
+            return new HistorianEventDto
+            {
+                Id = evt.Id,
+                Source = evt.Source,
+                EventTime = evt.EventTime,
+                ReceivedTime = evt.ReceivedTime,
+                DisplayText = evt.DisplayText,
+                Severity = (ushort)evt.Severity
+            };
+#pragma warning restore CS0618
+        }
+
+        internal static double? ExtractAggregateValue(AnalogSummaryQueryResult result, string column)
+        {
+            switch (column)
+            {
+                case "Average": return result.Average;
+                case "Minimum": return result.Minimum;
+                case "Maximum": return result.Maximum;
+                case "ValueCount": return result.ValueCount;
+                case "First": return result.First;
+                case "Last": return result.Last;
+                case "StdDev": return result.StdDev;
+                default: return null;
+            }
+        }
+
+        public void Dispose()
+        {
+            if (_disposed) return;
+            _disposed = true;
+
+            try
+            {
+                _connection?.CloseConnection(out _);
+                _connection?.Dispose();
+            }
+            catch (Exception ex)
+            {
+                Log.Warning(ex, "Error closing Historian SDK connection");
+            }
+
+            try
+            {
+                _eventConnection?.CloseConnection(out _);
+                _eventConnection?.Dispose();
+            }
+            catch (Exception ex)
+            {
+                Log.Warning(ex, "Error closing Historian SDK event connection");
+            }
+
+            _connection = null;
+            _eventConnection = null;
+        }
+    }
+}

src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host/Backend/Historian/HistorianEventDto.cs

@@ -0,0 +1,18 @@
+using System;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Backend.Historian
+{
+    /// <summary>
+    ///     SDK-free representation of a Historian event record. Prevents ArchestrA types from
+    ///     leaking beyond <c>HistorianDataSource</c>.
+    /// </summary>
+    public sealed class HistorianEventDto
+    {
+        public Guid Id { get; set; }
+        public string? Source { get; set; }
+        public DateTime EventTime { get; set; }
+        public DateTime ReceivedTime { get; set; }
+        public string? DisplayText { get; set; }
+        public ushort Severity { get; set; }
+    }
+}

src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host/Backend/Historian/HistorianHealthSnapshot.cs

@@ -0,0 +1,27 @@
+using System;
+using System.Collections.Generic;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Backend.Historian
+{
+    /// <summary>
+    ///     Point-in-time runtime health of the historian subsystem — consumed by the status dashboard
+    ///     via an IPC health query (not wired in PR #5; deferred).
+    /// </summary>
+    public sealed class HistorianHealthSnapshot
+    {
+        public long TotalQueries { get; set; }
+        public long TotalSuccesses { get; set; }
+        public long TotalFailures { get; set; }
+        public int ConsecutiveFailures { get; set; }
+        public DateTime? LastSuccessTime { get; set; }
+        public DateTime? LastFailureTime { get; set; }
+        public string? LastError { get; set; }
+        public bool ProcessConnectionOpen { get; set; }
+        public bool EventConnectionOpen { get; set; }
+        public string? ActiveProcessNode { get; set; }
+        public string? ActiveEventNode { get; set; }
+        public int NodeCount { get; set; }
+        public int HealthyNodeCount { get; set; }
+        public List<HistorianClusterNodeState> Nodes { get; set; } = new();
+    }
+}

src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host/Backend/Historian/HistorianQualityMapper.cs

@@ -0,0 +1,46 @@
+namespace ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Backend.Historian;
+
+/// <summary>
+///     Maps a raw OPC DA quality byte (as returned by Wonderware Historian's <c>OpcQuality</c>)
+///     to an OPC UA <c>StatusCode</c> uint. Preserves specific codes (BadNotConnected,
+///     UncertainSubNormal, etc.) instead of collapsing to Good/Uncertain/Bad categories.
+///     Mirrors v1 <c>QualityMapper.MapToOpcUaStatusCode</c> without pulling in OPC UA types —
+///     the returned value is the 32-bit OPC UA <c>StatusCode</c> wire encoding that the Proxy
+///     surfaces directly as <c>DataValueSnapshot.StatusCode</c>.
+/// </summary>
+public static class HistorianQualityMapper
+{
+    /// <summary>
+    ///     Map an 8-bit OPC DA quality byte to the corresponding OPC UA StatusCode. The byte
+    ///     family bits decide the category (Good &gt;= 192, Uncertain 64-191, Bad 0-63); the
+    ///     low-nibble subcode selects the specific code.
+    /// </summary>
+    public static uint Map(byte q) => q switch
+    {
+        // Good family (192+)
+        192 => 0x00000000u,             // Good
+        216 => 0x00D80000u,             // Good_LocalOverride
+
+        // Uncertain family (64-191)
+        64 => 0x40000000u,              // Uncertain
+        68 => 0x40900000u,              // Uncertain_LastUsableValue
+        80 => 0x40930000u,              // Uncertain_SensorNotAccurate
+        84 => 0x40940000u,              // Uncertain_EngineeringUnitsExceeded
+        88 => 0x40950000u,              // Uncertain_SubNormal
+
+        // Bad family (0-63)
+        0  => 0x80000000u,              // Bad
+        4  => 0x80890000u,              // Bad_ConfigurationError
+        8  => 0x808A0000u,              // Bad_NotConnected
+        12 => 0x808B0000u,              // Bad_DeviceFailure
+        16 => 0x808C0000u,              // Bad_SensorFailure
+        20 => 0x80050000u,              // Bad_CommunicationError
+        24 => 0x808D0000u,              // Bad_OutOfService
+        32 => 0x80320000u,              // Bad_WaitingForInitialData
+
+        // Unknown code — fall back to the category so callers still get a sensible bucket.
+        _ when q >= 192 => 0x00000000u,
+        _ when q >= 64 => 0x40000000u,
+        _ => 0x80000000u,
+    };
+}

src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host/Backend/Historian/HistorianSample.cs

@@ -0,0 +1,30 @@
+using System;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Backend.Historian
+{
+    /// <summary>
+    ///     OPC-UA-free representation of a single historical data point. The Host returns these
+    ///     across the IPC boundary as <c>GalaxyDataValue</c>; the Proxy maps quality and value to
+    ///     OPC UA <c>DataValue</c>. Raw MX quality byte is preserved so the Proxy can use the same
+    ///     quality mapper it already uses for live reads.
+    /// </summary>
+    public sealed class HistorianSample
+    {
+        public object? Value { get; set; }
+
+        /// <summary>Raw OPC DA quality byte from the historian SDK (low 8 bits of OpcQuality).</summary>
+        public byte Quality { get; set; }
+
+        public DateTime TimestampUtc { get; set; }
+    }
+
+    /// <summary>
+    ///     Result of <see cref="IHistorianDataSource.ReadAggregateAsync"/>. When <see cref="Value"/> is
+    ///     null the aggregate is unavailable for that bucket (Proxy maps to <c>BadNoData</c>).
+    /// </summary>
+    public sealed class HistorianAggregateSample
+    {
+        public double? Value { get; set; }
+        public DateTime TimestampUtc { get; set; }
+    }
+}

src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host/Backend/Historian/IHistorianConnectionFactory.cs

@@ -0,0 +1,73 @@
+using System;
+using System.Threading;
+using ArchestrA;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Backend.Historian
+{
+    /// <summary>
+    ///     Creates and opens Historian SDK connections. Extracted so tests can inject fakes that
+    ///     control connection success, failure, and timeout behavior.
+    /// </summary>
+    internal interface IHistorianConnectionFactory
+    {
+        HistorianAccess CreateAndConnect(HistorianConfiguration config, HistorianConnectionType type);
+    }
+
+    /// <summary>Production implementation — opens real Historian SDK connections.</summary>
+    internal sealed class SdkHistorianConnectionFactory : IHistorianConnectionFactory
+    {
+        public HistorianAccess CreateAndConnect(HistorianConfiguration config, HistorianConnectionType type)
+        {
+            var conn = new HistorianAccess();
+
+            var args = new HistorianConnectionArgs
+            {
+                ServerName = config.ServerName,
+                TcpPort = (ushort)config.Port,
+                IntegratedSecurity = config.IntegratedSecurity,
+                UseArchestrAUser = config.IntegratedSecurity,
+                ConnectionType = type,
+                ReadOnly = true,
+                PacketTimeout = (uint)(config.CommandTimeoutSeconds * 1000)
+            };
+
+            if (!config.IntegratedSecurity)
+            {
+                args.UserName = config.UserName ?? string.Empty;
+                args.Password = config.Password ?? string.Empty;
+            }
+
+            if (!conn.OpenConnection(args, out var error))
+            {
+                conn.Dispose();
+                throw new InvalidOperationException(
+                    $"Failed to open Historian SDK connection to {config.ServerName}:{config.Port}: {error.ErrorCode}");
+            }
+
+            var timeoutMs = config.CommandTimeoutSeconds * 1000;
+            var elapsed = 0;
+            while (elapsed < timeoutMs)
+            {
+                var status = new HistorianConnectionStatus();
+                conn.GetConnectionStatus(ref status);
+
+                if (status.ConnectedToServer)
+                    return conn;
+
+                if (status.ErrorOccurred)
+                {
+                    conn.Dispose();
+                    throw new InvalidOperationException(
+                        $"Historian SDK connection failed: {status.Error}");
+                }
+
+                Thread.Sleep(250);
+                elapsed += 250;
+            }
+
+            conn.Dispose();
+            throw new TimeoutException(
+                $"Historian SDK connection to {config.ServerName}:{config.Port} timed out after {config.CommandTimeoutSeconds}s");
+        }
+    }
+}

src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host/Backend/Historian/IHistorianDataSource.cs

@@ -0,0 +1,34 @@
+using System;
+using System.Collections.Generic;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Backend.Historian
+{
+    /// <summary>
+    ///     OPC-UA-free surface for the Wonderware Historian subsystem inside Galaxy.Host.
+    ///     Implementations read via the aahClient* SDK; the Proxy side maps returned samples
+    ///     to OPC UA <c>DataValue</c>.
+    /// </summary>
+    public interface IHistorianDataSource : IDisposable
+    {
+        Task<List<HistorianSample>> ReadRawAsync(
+            string tagName, DateTime startTime, DateTime endTime, int maxValues,
+            CancellationToken ct = default);
+
+        Task<List<HistorianAggregateSample>> ReadAggregateAsync(
+            string tagName, DateTime startTime, DateTime endTime,
+            double intervalMs, string aggregateColumn,
+            CancellationToken ct = default);
+
+        Task<List<HistorianSample>> ReadAtTimeAsync(
+            string tagName, DateTime[] timestamps,
+            CancellationToken ct = default);
+
+        Task<List<HistorianEventDto>> ReadEventsAsync(
+            string? sourceName, DateTime startTime, DateTime endTime, int maxEvents,
+            CancellationToken ct = default);
+
+        HistorianHealthSnapshot GetHealthSnapshot();
+    }
+}

src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host/Backend/IGalaxyBackend.cs

@@ -38,6 +38,9 @@ public interface IGalaxyBackend
     Task AcknowledgeAlarmAsync(AlarmAckRequest req, CancellationToken ct);
 
     Task<HistoryReadResponse> HistoryReadAsync(HistoryReadRequest req, CancellationToken ct);
+    Task<HistoryReadProcessedResponse> HistoryReadProcessedAsync(HistoryReadProcessedRequest req, CancellationToken ct);
+    Task<HistoryReadAtTimeResponse> HistoryReadAtTimeAsync(HistoryReadAtTimeRequest req, CancellationToken ct);
+    Task<HistoryReadEventsResponse> HistoryReadEventsAsync(HistoryReadEventsRequest req, CancellationToken ct);
 
     Task<RecycleStatusResponse> RecycleAsync(RecycleHostRequest req, CancellationToken ct);
 }

src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host/Backend/MxAccess/MxAccessClient.cs

@@ -4,6 +4,7 @@ using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
 using ArchestrA.MxAccess;
+using Serilog;
 using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Sta;
 
 namespace ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Backend.MxAccess;
@@ -18,6 +19,8 @@ namespace ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Backend.MxAccess;
 /// </summary>
 public sealed class MxAccessClient : IDisposable
 {
+    private static readonly ILogger Log = Serilog.Log.ForContext<MxAccessClient>();
+
     private readonly StaPump _pump;
     private readonly IMxProxy _proxy;
     private readonly string _clientName;
@@ -40,6 +43,16 @@ public sealed class MxAccessClient : IDisposable
     /// <summary>Fires whenever the connection transitions Connected ↔ Disconnected.</summary>
     public event EventHandler<bool>? ConnectionStateChanged;
 
+    /// <summary>
+    ///     Fires once per failed subscription replay after a reconnect. Carries the tag reference
+    ///     and the exception so the backend can propagate the degradation signal (e.g. mark the
+    ///     subscription bad on the Proxy side rather than silently losing its callback). Added for
+    ///     PR 6 low finding #2 — the replay loop previously ate per-tag failures silently and an
+    ///     operator would only find out that a specific subscription stopped updating through a
+    ///     data-quality complaint from downstream.
+    /// </summary>
+    public event EventHandler<SubscriptionReplayFailedEventArgs>? SubscriptionReplayFailed;
+
     public MxAccessClient(StaPump pump, IMxProxy proxy, string clientName, MxAccessClientOptions? options = null)
     {
         _pump = pump;
@@ -54,6 +67,13 @@ public sealed class MxAccessClient : IDisposable
     public int SubscriptionCount => _subscriptions.Count;
     public int ReconnectCount => _reconnectCount;
 
+    /// <summary>
+    ///     Wonderware client identity used when registering with the LMXProxyServer. Surfaced so
+    ///     <see cref="Backend.MxAccessGalaxyBackend"/> can tag its <c>OnHostStatusChanged</c> IPC
+    ///     pushes with a stable gateway name per PR 8.
+    /// </summary>
+    public string ClientName => _clientName;
+
     /// <summary>Connects on the STA thread. Idempotent. Starts the reconnect monitor on first call.</summary>
     public async Task<int> ConnectAsync()
     {
@@ -117,16 +137,29 @@ public sealed class MxAccessClient : IDisposable
             if (idle <= _options.StaleThreshold) continue;
 
             // Probe: try a no-op COM call. If the proxy is dead, the call will throw — that's
-            // our reconnect signal.
+            // our reconnect signal. PR 6 low finding #1: AddItem allocates an MXAccess item
+            // handle; we must RemoveItem it on the same pump turn or the long-running monitor
+            // leaks one handle per probe cycle (one every MonitorInterval seconds, indefinitely).
             bool probeOk;
             try
             {
                 probeOk = await _pump.InvokeAsync(() =>
                 {
-                    // AddItem on the connection handle is cheap and round-trips through COM.
-                    // We use a sentinel "$Heartbeat" reference; if it fails the connection is gone.
-                    try { _proxy.AddItem(_connectionHandle, "$Heartbeat"); return true; }
+                    int probeHandle = 0;
+                    try
+                    {
+                        probeHandle = _proxy.AddItem(_connectionHandle, "$Heartbeat");
+                        return probeHandle > 0;
+                    }
                     catch { return false; }
+                    finally
+                    {
+                        if (probeHandle > 0)
+                        {
+                            try { _proxy.RemoveItem(_connectionHandle, probeHandle); }
+                            catch { /* proxy is dying; best-effort cleanup */ }
+                        }
+                    }
                 });
             }
             catch { probeOk = false; }
@@ -155,16 +188,33 @@ public sealed class MxAccessClient : IDisposable
                 _reconnectCount++;
                 ConnectionStateChanged?.Invoke(this, true);
 
-                // Replay every subscription that was active before the disconnect.
+                // Replay every subscription that was active before the disconnect. PR 6 low
+                // finding #2: surface per-tag failures — log them and raise
+                // SubscriptionReplayFailed so the backend can propagate the degraded state
+                // (previously swallowed silently; downstream quality dropped without a signal).
                 var snapshot = _addressToHandle.Keys.ToArray();
                 _addressToHandle.Clear();
                 _handleToAddress.Clear();
+                var failed = 0;
                 foreach (var fullRef in snapshot)
                 {
                     try { await SubscribeOnPumpAsync(fullRef); }
-                    catch { /* skip — operator can re-subscribe */ }
+                    catch (Exception subEx)
+                    {
+                        failed++;
+                        Log.Warning(subEx,
+                            "MXAccess subscription replay failed for {TagReference} after reconnect #{Reconnect}",
+                            fullRef, _reconnectCount);
+                        SubscriptionReplayFailed?.Invoke(this,
+                            new SubscriptionReplayFailedEventArgs(fullRef, subEx));
+                    }
                 }
 
+                if (failed > 0)
+                    Log.Warning("Subscription replay completed — {Failed} of {Total} failed", failed, snapshot.Length);
+                else
+                    Log.Information("Subscription replay completed — {Total} re-subscribed cleanly", snapshot.Length);
+
                 _lastObservedActivityUtc = DateTime.UtcNow;
             }
             catch

src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host/Backend/MxAccess/SubscriptionReplayFailedEventArgs.cs

@@ -0,0 +1,20 @@
+using System;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Backend.MxAccess;
+
+/// <summary>
+///     Fired by <see cref="MxAccessClient.SubscriptionReplayFailed"/> when a previously-active
+///     subscription fails to be restored after a reconnect. The backend should treat the tag as
+///     unhealthy until the next successful resubscribe.
+/// </summary>
+public sealed class SubscriptionReplayFailedEventArgs : EventArgs
+{
+    public SubscriptionReplayFailedEventArgs(string tagReference, Exception exception)
+    {
+        TagReference = tagReference;
+        Exception = exception;
+    }
+
+    public string TagReference { get; }
+    public Exception Exception { get; }
+}

src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host/Backend/MxAccessGalaxyBackend.cs

@@ -5,7 +5,9 @@ using System.Threading;
 using System.Threading.Tasks;
 using MessagePack;
 using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Backend.Galaxy;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Backend.Historian;
 using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Backend.MxAccess;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Backend.Stability;
 using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Shared.Contracts;
 
 namespace ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Backend;
@@ -18,10 +20,11 @@ namespace ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Backend;
 ///     MxAccess <c>AlarmExtension</c> primitives but the wire-up is also Phase 2 follow-up
 ///     (the v1 alarm subsystem is its own subtree).
 /// </summary>
-public sealed class MxAccessGalaxyBackend : IGalaxyBackend
+public sealed class MxAccessGalaxyBackend : IGalaxyBackend, IDisposable
 {
     private readonly GalaxyRepository _repository;
     private readonly MxAccessClient _mx;
+    private readonly IHistorianDataSource? _historian;
     private long _nextSessionId;
     private long _nextSubscriptionId;
 
@@ -32,17 +35,69 @@ public sealed class MxAccessGalaxyBackend : IGalaxyBackend
         _refToSubs = new(System.StringComparer.OrdinalIgnoreCase);
 
     public event System.EventHandler<OnDataChangeNotification>? OnDataChange;
-#pragma warning disable CS0067 // event not yet raised — alarm + host-status wire-up in PR #4 follow-up
+#pragma warning disable CS0067 // alarm wire-up deferred to PR 9
     public event System.EventHandler<GalaxyAlarmEvent>? OnAlarmEvent;
-    public event System.EventHandler<HostConnectivityStatus>? OnHostStatusChanged;
 #pragma warning restore CS0067
+    public event System.EventHandler<HostConnectivityStatus>? OnHostStatusChanged;
 
-    public MxAccessGalaxyBackend(GalaxyRepository repository, MxAccessClient mx)
+    private readonly System.EventHandler<bool> _onConnectionStateChanged;
+    private readonly GalaxyRuntimeProbeManager _probeManager;
+    private readonly System.EventHandler<HostStateTransition> _onProbeStateChanged;
+
+    public MxAccessGalaxyBackend(GalaxyRepository repository, MxAccessClient mx, IHistorianDataSource? historian = null)
     {
         _repository = repository;
         _mx = mx;
+        _historian = historian;
+
+        // PR 8: gateway-level host-status push. When the MXAccess COM proxy transitions
+        // connected↔disconnected, raise OnHostStatusChanged with a synthetic host entry named
+        // after the Wonderware client identity so the Admin UI surfaces top-level transport
+        // health even before per-platform/per-engine probing lands (deferred to a later PR that
+        // ports v1's GalaxyRuntimeProbeManager with ScanState subscriptions).
+        _onConnectionStateChanged = (_, connected) =>
+        {
+            OnHostStatusChanged?.Invoke(this, new HostConnectivityStatus
+            {
+                HostName = _mx.ClientName,
+                RuntimeStatus = connected ? "Running" : "Stopped",
+                LastObservedUtcUnixMs = DateTimeOffset.UtcNow.ToUnixTimeMilliseconds(),
+            });
+        };
+        _mx.ConnectionStateChanged += _onConnectionStateChanged;
+
+        // PR 13: per-platform runtime probes. ScanState subscriptions fire OnProbeCallback,
+        // which runs the state machine and raises StateChanged on transitions we care about.
+        // We forward each transition through the same OnHostStatusChanged IPC event that the
+        // gateway-level ConnectionStateChanged uses — tagged with the platform's TagName so the
+        // Admin UI can show per-host health independently from the top-level transport status.
+        _probeManager = new GalaxyRuntimeProbeManager(
+            subscribe: (probe, cb) => _mx.SubscribeAsync(probe, cb),
+            unsubscribe: probe => _mx.UnsubscribeAsync(probe));
+        _onProbeStateChanged = (_, t) =>
+        {
+            OnHostStatusChanged?.Invoke(this, new HostConnectivityStatus
+            {
+                HostName = t.TagName,
+                RuntimeStatus = t.NewState switch
+                {
+                    HostRuntimeState.Running => "Running",
+                    HostRuntimeState.Stopped => "Stopped",
+                    _ => "Unknown",
+                },
+                LastObservedUtcUnixMs = new DateTimeOffset(t.AtUtc, TimeSpan.Zero).ToUnixTimeMilliseconds(),
+            });
+        };
+        _probeManager.StateChanged += _onProbeStateChanged;
     }
 
+    /// <summary>
+    ///     Exposed for tests. Production flow: DiscoverAsync completes → backend calls
+    ///     <c>SyncProbesAsync</c> with the runtime hosts (WinPlatform + AppEngine gobjects) to
+    ///     advise ScanState per host.
+    /// </summary>
+    internal GalaxyRuntimeProbeManager ProbeManager => _probeManager;
+
     public async Task<OpenSessionResponse> OpenSessionAsync(OpenSessionRequest req, CancellationToken ct)
     {
         try
@@ -82,6 +137,21 @@ public sealed class MxAccessGalaxyBackend : IGalaxyBackend
                 Attributes = attrsByGobject.TryGetValue(o.GobjectId, out var a) ? a : Array.Empty<GalaxyAttributeInfo>(),
             }).ToArray();
 
+            // PR 13: Sync the per-platform probe manager against the just-discovered hierarchy
+            // so ScanState subscriptions track the current runtime set. Best-effort — probe
+            // failures don't block Discover from returning, since the gateway-level signal from
+            // MxAccessClient.ConnectionStateChanged still flows and the Admin UI degrades to
+            // that level if any per-host probe couldn't advise.
+            try
+            {
+                var targets = hierarchy
+                    .Where(o => o.CategoryId == GalaxyRuntimeProbeManager.CategoryWinPlatform
+                             || o.CategoryId == GalaxyRuntimeProbeManager.CategoryAppEngine)
+                    .Select(o => new HostProbeTarget(o.TagName, o.CategoryId));
+                await _probeManager.SyncAsync(targets).ConfigureAwait(false);
+            }
+            catch { /* swallow — Discover succeeded; probes are a diagnostic enrichment */ }
+
             return new DiscoverHierarchyResponse { Success = true, Objects = objects };
         }
         catch (Exception ex)
@@ -222,17 +292,174 @@ public sealed class MxAccessGalaxyBackend : IGalaxyBackend
     public Task SubscribeAlarmsAsync(AlarmSubscribeRequest req, CancellationToken ct) => Task.CompletedTask;
     public Task AcknowledgeAlarmAsync(AlarmAckRequest req, CancellationToken ct) => Task.CompletedTask;
 
-    public Task<HistoryReadResponse> HistoryReadAsync(HistoryReadRequest req, CancellationToken ct)
-        => Task.FromResult(new HistoryReadResponse
+    public async Task<HistoryReadResponse> HistoryReadAsync(HistoryReadRequest req, CancellationToken ct)
+    {
+        if (_historian is null)
+            return new HistoryReadResponse
+            {
+                Success = false,
+                Error = "Historian disabled — no OTOPCUA_HISTORIAN_ENABLED configuration",
+                Tags = Array.Empty<HistoryTagValues>(),
+            };
+
+        var start = DateTimeOffset.FromUnixTimeMilliseconds(req.StartUtcUnixMs).UtcDateTime;
+        var end = DateTimeOffset.FromUnixTimeMilliseconds(req.EndUtcUnixMs).UtcDateTime;
+        var tags = new List<HistoryTagValues>(req.TagReferences.Length);
+
+        try
         {
-            Success = false,
-            Error = "Wonderware Historian plugin loader not yet wired (Phase 2 Task B.1.h follow-up)",
-            Tags = Array.Empty<HistoryTagValues>(),
-        });
+            foreach (var reference in req.TagReferences)
+            {
+                var samples = await _historian.ReadRawAsync(reference, start, end, (int)req.MaxValuesPerTag, ct).ConfigureAwait(false);
+                tags.Add(new HistoryTagValues
+                {
+                    TagReference = reference,
+                    Values = samples.Select(s => ToWire(reference, s)).ToArray(),
+                });
+            }
+            return new HistoryReadResponse { Success = true, Tags = tags.ToArray() };
+        }
+        catch (OperationCanceledException) { throw; }
+        catch (Exception ex)
+        {
+            return new HistoryReadResponse
+            {
+                Success = false,
+                Error = $"Historian read failed: {ex.Message}",
+                Tags = tags.ToArray(),
+            };
+        }
+    }
+
+    public async Task<HistoryReadProcessedResponse> HistoryReadProcessedAsync(
+        HistoryReadProcessedRequest req, CancellationToken ct)
+    {
+        if (_historian is null)
+            return new HistoryReadProcessedResponse
+            {
+                Success = false,
+                Error = "Historian disabled — no OTOPCUA_HISTORIAN_ENABLED configuration",
+                Values = Array.Empty<GalaxyDataValue>(),
+            };
+
+        if (req.IntervalMs <= 0)
+            return new HistoryReadProcessedResponse
+            {
+                Success = false,
+                Error = "HistoryReadProcessed requires IntervalMs > 0",
+                Values = Array.Empty<GalaxyDataValue>(),
+            };
+
+        var start = DateTimeOffset.FromUnixTimeMilliseconds(req.StartUtcUnixMs).UtcDateTime;
+        var end = DateTimeOffset.FromUnixTimeMilliseconds(req.EndUtcUnixMs).UtcDateTime;
+
+        try
+        {
+            var samples = await _historian.ReadAggregateAsync(
+                req.TagReference, start, end, req.IntervalMs, req.AggregateColumn, ct).ConfigureAwait(false);
+
+            var wire = samples.Select(s => ToWire(req.TagReference, s)).ToArray();
+            return new HistoryReadProcessedResponse { Success = true, Values = wire };
+        }
+        catch (OperationCanceledException) { throw; }
+        catch (Exception ex)
+        {
+            return new HistoryReadProcessedResponse
+            {
+                Success = false,
+                Error = $"Historian aggregate read failed: {ex.Message}",
+                Values = Array.Empty<GalaxyDataValue>(),
+            };
+        }
+    }
+
+    public async Task<HistoryReadAtTimeResponse> HistoryReadAtTimeAsync(
+        HistoryReadAtTimeRequest req, CancellationToken ct)
+    {
+        if (_historian is null)
+            return new HistoryReadAtTimeResponse
+            {
+                Success = false,
+                Error = "Historian disabled — no OTOPCUA_HISTORIAN_ENABLED configuration",
+                Values = Array.Empty<GalaxyDataValue>(),
+            };
+
+        if (req.TimestampsUtcUnixMs.Length == 0)
+            return new HistoryReadAtTimeResponse { Success = true, Values = Array.Empty<GalaxyDataValue>() };
+
+        var timestamps = req.TimestampsUtcUnixMs
+            .Select(ms => DateTimeOffset.FromUnixTimeMilliseconds(ms).UtcDateTime)
+            .ToArray();
+
+        try
+        {
+            var samples = await _historian.ReadAtTimeAsync(req.TagReference, timestamps, ct).ConfigureAwait(false);
+            var wire = samples.Select(s => ToWire(req.TagReference, s)).ToArray();
+            return new HistoryReadAtTimeResponse { Success = true, Values = wire };
+        }
+        catch (OperationCanceledException) { throw; }
+        catch (Exception ex)
+        {
+            return new HistoryReadAtTimeResponse
+            {
+                Success = false,
+                Error = $"Historian at-time read failed: {ex.Message}",
+                Values = Array.Empty<GalaxyDataValue>(),
+            };
+        }
+    }
+
+    public async Task<HistoryReadEventsResponse> HistoryReadEventsAsync(
+        HistoryReadEventsRequest req, CancellationToken ct)
+    {
+        if (_historian is null)
+            return new HistoryReadEventsResponse
+            {
+                Success = false,
+                Error = "Historian disabled — no OTOPCUA_HISTORIAN_ENABLED configuration",
+                Events = Array.Empty<GalaxyHistoricalEvent>(),
+            };
+
+        var start = DateTimeOffset.FromUnixTimeMilliseconds(req.StartUtcUnixMs).UtcDateTime;
+        var end = DateTimeOffset.FromUnixTimeMilliseconds(req.EndUtcUnixMs).UtcDateTime;
+
+        try
+        {
+            var events = await _historian.ReadEventsAsync(req.SourceName, start, end, req.MaxEvents, ct).ConfigureAwait(false);
+            var wire = events.Select(e => new GalaxyHistoricalEvent
+            {
+                EventId = e.Id.ToString(),
+                SourceName = e.Source,
+                EventTimeUtcUnixMs = new DateTimeOffset(DateTime.SpecifyKind(e.EventTime, DateTimeKind.Utc), TimeSpan.Zero).ToUnixTimeMilliseconds(),
+                ReceivedTimeUtcUnixMs = new DateTimeOffset(DateTime.SpecifyKind(e.ReceivedTime, DateTimeKind.Utc), TimeSpan.Zero).ToUnixTimeMilliseconds(),
+                DisplayText = e.DisplayText,
+                Severity = e.Severity,
+            }).ToArray();
+            return new HistoryReadEventsResponse { Success = true, Events = wire };
+        }
+        catch (OperationCanceledException) { throw; }
+        catch (Exception ex)
+        {
+            return new HistoryReadEventsResponse
+            {
+                Success = false,
+                Error = $"Historian event read failed: {ex.Message}",
+                Events = Array.Empty<GalaxyHistoricalEvent>(),
+            };
+        }
+    }
 
     public Task<RecycleStatusResponse> RecycleAsync(RecycleHostRequest req, CancellationToken ct)
         => Task.FromResult(new RecycleStatusResponse { Accepted = true, GraceSeconds = 15 });
 
+    public void Dispose()
+    {
+        _probeManager.StateChanged -= _onProbeStateChanged;
+        _probeManager.Dispose();
+        _mx.ConnectionStateChanged -= _onConnectionStateChanged;
+        _historian?.Dispose();
+    }
+
     private static GalaxyDataValue ToWire(string reference, Vtq vtq) => new()
     {
         TagReference = reference,
@@ -243,6 +470,39 @@ public sealed class MxAccessGalaxyBackend : IGalaxyBackend
         ServerTimestampUtcUnixMs = DateTimeOffset.UtcNow.ToUnixTimeMilliseconds(),
     };
 
+    /// <summary>
+    ///     Maps a <see cref="HistorianSample"/> (raw historian row, OPC-UA-free) to the IPC wire
+    ///     shape. The Proxy decodes the MessagePack value and maps <see cref="HistorianSample.Quality"/>
+    ///     through <c>QualityMapper</c> on its side of the pipe — we keep the raw byte here so
+    ///     rich OPC DA status codes (e.g. <c>BadNotConnected</c>, <c>UncertainSubNormal</c>) survive
+    ///     the hop intact.
+    /// </summary>
+    private static GalaxyDataValue ToWire(string reference, HistorianSample sample) => new()
+    {
+        TagReference = reference,
+        ValueBytes = sample.Value is null ? null : MessagePackSerializer.Serialize(sample.Value),
+        ValueMessagePackType = 0,
+        StatusCode = HistorianQualityMapper.Map(sample.Quality),
+        SourceTimestampUtcUnixMs = new DateTimeOffset(sample.TimestampUtc, TimeSpan.Zero).ToUnixTimeMilliseconds(),
+        ServerTimestampUtcUnixMs = DateTimeOffset.UtcNow.ToUnixTimeMilliseconds(),
+    };
+
+
+    /// <summary>
+    ///     Maps a <see cref="HistorianAggregateSample"/> (one aggregate bucket) to the IPC wire
+    ///     shape. A null <see cref="HistorianAggregateSample.Value"/> means the aggregate was
+    ///     unavailable for the bucket — the Proxy translates that to OPC UA <c>BadNoData</c>.
+    /// </summary>
+    private static GalaxyDataValue ToWire(string reference, HistorianAggregateSample sample) => new()
+    {
+        TagReference = reference,
+        ValueBytes = sample.Value is null ? null : MessagePackSerializer.Serialize(sample.Value.Value),
+        ValueMessagePackType = 0,
+        StatusCode = sample.Value is null ? 0x800E0000u /* BadNoData */ : 0x00000000u,
+        SourceTimestampUtcUnixMs = new DateTimeOffset(sample.TimestampUtc, TimeSpan.Zero).ToUnixTimeMilliseconds(),
+        ServerTimestampUtcUnixMs = DateTimeOffset.UtcNow.ToUnixTimeMilliseconds(),
+    };
+
     private static GalaxyAttributeInfo MapAttribute(GalaxyAttributeRow row) => new()
     {
         AttributeName = row.AttributeName,
@@ -251,6 +511,7 @@ public sealed class MxAccessGalaxyBackend : IGalaxyBackend
         ArrayDim = row.ArrayDimension is int d and > 0 ? (uint)d : null,
         SecurityClassification = row.SecurityClassification,
         IsHistorized = row.IsHistorized,
+        IsAlarm = row.IsAlarm,
     };
 
     private static string MapCategory(int categoryId) => categoryId switch

src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host/Backend/Stability/GalaxyRuntimeProbeManager.cs

@@ -0,0 +1,273 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Backend.MxAccess;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Backend.Stability;
+
+/// <summary>
+///     Per-platform + per-AppEngine runtime probe. Subscribes to <c>&lt;TagName&gt;.ScanState</c>
+///     for each $WinPlatform and $AppEngine gobject, tracks Unknown → Running → Stopped
+///     transitions, and fires <see cref="StateChanged"/> so <see cref="Backend.MxAccessGalaxyBackend"/>
+///     can forward per-host events through the existing IPC <c>OnHostStatusChanged</c> event.
+///     Pure-logic state machine with an injected clock so it's deterministically testable —
+///     port of v1 <c>GalaxyRuntimeProbeManager</c> without the OPC UA node-manager coupling.
+/// </summary>
+/// <remarks>
+///     State machine rules (documented in v1's <c>runtimestatus.md</c> and preserved here):
+///     <list type="bullet">
+///         <item><c>ScanState</c> is on-change-only — a stably-Running host may go hours without a
+///         callback. Running → Stopped is driven by an explicit <c>ScanState=false</c> callback,
+///         never by starvation.</item>
+///         <item>Unknown → Running is a startup transition and does NOT fire StateChanged (would
+///         paint every host as "just recovered" at startup, which is noise).</item>
+///         <item>Stopped → Running and Running → Stopped fire StateChanged. Unknown → Stopped
+///         fires StateChanged because that's a first-known-bad signal operators need.</item>
+///         <item>All public methods are thread-safe. Callbacks fire outside the internal lock to
+///         avoid lock inversion with caller-owned state.</item>
+///     </list>
+/// </remarks>
+public sealed class GalaxyRuntimeProbeManager : IDisposable
+{
+    public const int CategoryWinPlatform = 1;
+    public const int CategoryAppEngine = 3;
+    public const string ProbeAttribute = ".ScanState";
+
+    private readonly Func<DateTime> _clock;
+    private readonly Func<string, Action<string, Vtq>, Task> _subscribe;
+    private readonly Func<string, Task> _unsubscribe;
+    private readonly object _lock = new();
+
+    // probe tag → per-host state
+    private readonly Dictionary<string, HostProbeState> _byProbe = new(StringComparer.OrdinalIgnoreCase);
+    // tag name → probe tag (for reverse lookup on the desired-set diff)
+    private readonly Dictionary<string, string> _probeByTagName = new(StringComparer.OrdinalIgnoreCase);
+    private bool _disposed;
+
+    /// <summary>
+    ///     Fires on every state transition that operators should react to. See class remarks
+    ///     for the rules on which transitions fire.
+    /// </summary>
+    public event EventHandler<HostStateTransition>? StateChanged;
+
+    public GalaxyRuntimeProbeManager(
+        Func<string, Action<string, Vtq>, Task> subscribe,
+        Func<string, Task> unsubscribe)
+        : this(subscribe, unsubscribe, () => DateTime.UtcNow) { }
+
+    internal GalaxyRuntimeProbeManager(
+        Func<string, Action<string, Vtq>, Task> subscribe,
+        Func<string, Task> unsubscribe,
+        Func<DateTime> clock)
+    {
+        _subscribe = subscribe ?? throw new ArgumentNullException(nameof(subscribe));
+        _unsubscribe = unsubscribe ?? throw new ArgumentNullException(nameof(unsubscribe));
+        _clock = clock ?? throw new ArgumentNullException(nameof(clock));
+    }
+
+    /// <summary>Number of probes currently advised. Test/dashboard hook.</summary>
+    public int ActiveProbeCount
+    {
+        get { lock (_lock) return _byProbe.Count; }
+    }
+
+    /// <summary>
+    ///     Snapshot every currently-tracked host's state. One entry per probe.
+    /// </summary>
+    public IReadOnlyList<HostProbeSnapshot> SnapshotStates()
+    {
+        lock (_lock)
+        {
+            return _byProbe.Select(kv => new HostProbeSnapshot(
+                TagName: kv.Value.TagName,
+                State: kv.Value.State,
+                LastChangedUtc: kv.Value.LastStateChangeUtc)).ToList();
+        }
+    }
+
+    /// <summary>
+    ///     Query the current runtime state for <paramref name="tagName"/>. Returns
+    ///     <see cref="HostRuntimeState.Unknown"/> when the host is not tracked.
+    /// </summary>
+    public HostRuntimeState GetState(string tagName)
+    {
+        lock (_lock)
+        {
+            if (_probeByTagName.TryGetValue(tagName, out var probe)
+                && _byProbe.TryGetValue(probe, out var state))
+                return state.State;
+            return HostRuntimeState.Unknown;
+        }
+    }
+
+    /// <summary>
+    ///     Diff the desired host set (filtered $WinPlatform / $AppEngine from the latest Discover)
+    ///     against the currently-tracked set and advise / unadvise as needed. Idempotent:
+    ///     calling twice with the same set does nothing.
+    /// </summary>
+    public async Task SyncAsync(IEnumerable<HostProbeTarget> desiredHosts)
+    {
+        if (_disposed) return;
+
+        var desired = desiredHosts
+            .Where(h => !string.IsNullOrWhiteSpace(h.TagName))
+            .ToDictionary(h => h.TagName, StringComparer.OrdinalIgnoreCase);
+
+        List<string> toAdvise;
+        List<string> toUnadvise;
+        lock (_lock)
+        {
+            toAdvise = desired.Keys
+                .Where(tag => !_probeByTagName.ContainsKey(tag))
+                .ToList();
+            toUnadvise = _probeByTagName.Keys
+                .Where(tag => !desired.ContainsKey(tag))
+                .Select(tag => _probeByTagName[tag])
+                .ToList();
+
+            foreach (var tag in toAdvise)
+            {
+                var probe = tag + ProbeAttribute;
+                _probeByTagName[tag] = probe;
+                _byProbe[probe] = new HostProbeState
+                {
+                    TagName = tag,
+                    State = HostRuntimeState.Unknown,
+                    LastStateChangeUtc = _clock(),
+                };
+            }
+
+            foreach (var probe in toUnadvise)
+            {
+                _byProbe.Remove(probe);
+            }
+
+            foreach (var removedTag in _probeByTagName.Keys.Where(t => !desired.ContainsKey(t)).ToList())
+            {
+                _probeByTagName.Remove(removedTag);
+            }
+        }
+
+        foreach (var tag in toAdvise)
+        {
+            var probe = tag + ProbeAttribute;
+            try
+            {
+                await _subscribe(probe, OnProbeCallback);
+            }
+            catch
+            {
+                // Rollback on subscribe failure so a later Tick can't transition a never-advised
+                // probe into a false Stopped state. Callers can re-Sync later to retry.
+                lock (_lock)
+                {
+                    _byProbe.Remove(probe);
+                    _probeByTagName.Remove(tag);
+                }
+            }
+        }
+
+        foreach (var probe in toUnadvise)
+        {
+            try { await _unsubscribe(probe); } catch { /* best-effort cleanup */ }
+        }
+    }
+
+    /// <summary>
+    ///     Public entry point for tests and internal callbacks. Production flow: MxAccessClient's
+    ///     SubscribeAsync delivers VTQ updates through the callback wired in <see cref="SyncAsync"/>,
+    ///     which calls this method under the lock to update state and fires
+    ///     <see cref="StateChanged"/> outside the lock for any transition that matters.
+    /// </summary>
+    public void OnProbeCallback(string probeTag, Vtq vtq)
+    {
+        if (_disposed) return;
+
+        HostStateTransition? transition = null;
+        lock (_lock)
+        {
+            if (!_byProbe.TryGetValue(probeTag, out var state)) return;
+
+            var isRunning = vtq.Quality >= 192 && vtq.Value is bool b && b;
+            var now = _clock();
+            var previous = state.State;
+            state.LastCallbackUtc = now;
+
+            if (isRunning)
+            {
+                state.GoodUpdateCount++;
+                if (previous != HostRuntimeState.Running)
+                {
+                    state.State = HostRuntimeState.Running;
+                    state.LastStateChangeUtc = now;
+                    if (previous == HostRuntimeState.Stopped)
+                    {
+                        transition = new HostStateTransition(state.TagName, previous, HostRuntimeState.Running, now);
+                    }
+                }
+            }
+            else
+            {
+                state.FailureCount++;
+                if (previous != HostRuntimeState.Stopped)
+                {
+                    state.State = HostRuntimeState.Stopped;
+                    state.LastStateChangeUtc = now;
+                    transition = new HostStateTransition(state.TagName, previous, HostRuntimeState.Stopped, now);
+                }
+            }
+        }
+
+        if (transition is { } t)
+        {
+            StateChanged?.Invoke(this, t);
+        }
+    }
+
+    public void Dispose()
+    {
+        if (_disposed) return;
+        _disposed = true;
+        lock (_lock)
+        {
+            _byProbe.Clear();
+            _probeByTagName.Clear();
+        }
+    }
+
+    private sealed class HostProbeState
+    {
+        public string TagName { get; set; } = "";
+        public HostRuntimeState State { get; set; }
+        public DateTime LastStateChangeUtc { get; set; }
+        public DateTime? LastCallbackUtc { get; set; }
+        public long GoodUpdateCount { get; set; }
+        public long FailureCount { get; set; }
+    }
+}
+
+public enum HostRuntimeState
+{
+    Unknown,
+    Running,
+    Stopped,
+}
+
+public sealed record HostStateTransition(
+    string TagName,
+    HostRuntimeState OldState,
+    HostRuntimeState NewState,
+    DateTime AtUtc);
+
+public sealed record HostProbeSnapshot(
+    string TagName,
+    HostRuntimeState State,
+    DateTime LastChangedUtc);
+
+public readonly record struct HostProbeTarget(string TagName, int CategoryId)
+{
+    public bool IsRuntimeHost =>
+        CategoryId == GalaxyRuntimeProbeManager.CategoryWinPlatform
+        || CategoryId == GalaxyRuntimeProbeManager.CategoryAppEngine;
+}

src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host/Backend/StubGalaxyBackend.cs

@@ -85,6 +85,33 @@ public sealed class StubGalaxyBackend : IGalaxyBackend
             Tags = System.Array.Empty<HistoryTagValues>(),
         });
 
+    public Task<HistoryReadProcessedResponse> HistoryReadProcessedAsync(
+        HistoryReadProcessedRequest req, CancellationToken ct)
+        => Task.FromResult(new HistoryReadProcessedResponse
+        {
+            Success = false,
+            Error = "stub: MXAccess code lift pending (Phase 2 Task B.1)",
+            Values = System.Array.Empty<GalaxyDataValue>(),
+        });
+
+    public Task<HistoryReadAtTimeResponse> HistoryReadAtTimeAsync(
+        HistoryReadAtTimeRequest req, CancellationToken ct)
+        => Task.FromResult(new HistoryReadAtTimeResponse
+        {
+            Success = false,
+            Error = "stub: MXAccess code lift pending (Phase 2 Task B.1)",
+            Values = System.Array.Empty<GalaxyDataValue>(),
+        });
+
+    public Task<HistoryReadEventsResponse> HistoryReadEventsAsync(
+        HistoryReadEventsRequest req, CancellationToken ct)
+        => Task.FromResult(new HistoryReadEventsResponse
+        {
+            Success = false,
+            Error = "stub: MXAccess code lift pending (Phase 2 Task B.1)",
+            Events = System.Array.Empty<GalaxyHistoricalEvent>(),
+        });
+
     public Task<RecycleStatusResponse> RecycleAsync(RecycleHostRequest req, CancellationToken ct)
         => Task.FromResult(new RecycleStatusResponse
         {

src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host/Ipc/GalaxyFrameHandler.cs

@@ -80,6 +80,27 @@ public sealed class GalaxyFrameHandler(IGalaxyBackend backend, ILogger logger) :
                     await writer.WriteAsync(MessageKind.HistoryReadResponse, resp, ct);
                     return;
                 }
+                case MessageKind.HistoryReadProcessedRequest:
+                {
+                    var resp = await backend.HistoryReadProcessedAsync(
+                        Deserialize<HistoryReadProcessedRequest>(body), ct);
+                    await writer.WriteAsync(MessageKind.HistoryReadProcessedResponse, resp, ct);
+                    return;
+                }
+                case MessageKind.HistoryReadAtTimeRequest:
+                {
+                    var resp = await backend.HistoryReadAtTimeAsync(
+                        Deserialize<HistoryReadAtTimeRequest>(body), ct);
+                    await writer.WriteAsync(MessageKind.HistoryReadAtTimeResponse, resp, ct);
+                    return;
+                }
+                case MessageKind.HistoryReadEventsRequest:
+                {
+                    var resp = await backend.HistoryReadEventsAsync(
+                        Deserialize<HistoryReadEventsRequest>(body), ct);
+                    await writer.WriteAsync(MessageKind.HistoryReadEventsResponse, resp, ct);
+                    return;
+                }
                 case MessageKind.RecycleHostRequest:
                 {
                     var resp = await backend.RecycleAsync(Deserialize<RecycleHostRequest>(body), ct);

src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host/Program.cs

@@ -4,6 +4,7 @@ using System.Threading;
 using Serilog;
 using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Backend;
 using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Backend.Galaxy;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Backend.Historian;
 using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Backend.MxAccess;
 using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Ipc;
 using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Sta;
@@ -66,9 +67,11 @@ public static class Program
                     pump = new StaPump("Galaxy.Sta");
                     pump.WaitForStartedAsync().GetAwaiter().GetResult();
                     mx = new MxAccessClient(pump, new MxProxyAdapter(), clientName);
+                    var historian = BuildHistorianIfEnabled();
                     backend = new MxAccessGalaxyBackend(
                         new GalaxyRepository(new GalaxyRepositoryOptions { ConnectionString = zbConn }),
-                        mx);
+                        mx,
+                        historian);
                     break;
             }
 
@@ -77,6 +80,7 @@ public static class Program
             try { server.RunAsync(handler, cts.Token).GetAwaiter().GetResult(); }
             finally
             {
+                (backend as IDisposable)?.Dispose();
                 mx?.Dispose();
                 pump?.Dispose();
             }
@@ -91,4 +95,45 @@ public static class Program
         }
         finally { Log.CloseAndFlush(); }
     }
+
+    /// <summary>
+    ///     Builds a <see cref="HistorianDataSource"/> from the OTOPCUA_HISTORIAN_* environment
+    ///     variables the supervisor passes at spawn time. Returns null when the historian is
+    ///     disabled (default) so <c>MxAccessGalaxyBackend.HistoryReadAsync</c> returns a clear
+    ///     "not configured" error instead of attempting an SDK connection to localhost.
+    /// </summary>
+    private static IHistorianDataSource? BuildHistorianIfEnabled()
+    {
+        var enabled = Environment.GetEnvironmentVariable("OTOPCUA_HISTORIAN_ENABLED");
+        if (!string.Equals(enabled, "true", StringComparison.OrdinalIgnoreCase) && enabled != "1")
+            return null;
+
+        var cfg = new HistorianConfiguration
+        {
+            Enabled = true,
+            ServerName = Environment.GetEnvironmentVariable("OTOPCUA_HISTORIAN_SERVER") ?? "localhost",
+            Port = TryParseInt("OTOPCUA_HISTORIAN_PORT", 32568),
+            IntegratedSecurity = !string.Equals(Environment.GetEnvironmentVariable("OTOPCUA_HISTORIAN_INTEGRATED"), "false", StringComparison.OrdinalIgnoreCase),
+            UserName = Environment.GetEnvironmentVariable("OTOPCUA_HISTORIAN_USER"),
+            Password = Environment.GetEnvironmentVariable("OTOPCUA_HISTORIAN_PASS"),
+            CommandTimeoutSeconds = TryParseInt("OTOPCUA_HISTORIAN_TIMEOUT_SEC", 30),
+            MaxValuesPerRead = TryParseInt("OTOPCUA_HISTORIAN_MAX_VALUES", 10000),
+            FailureCooldownSeconds = TryParseInt("OTOPCUA_HISTORIAN_COOLDOWN_SEC", 60),
+        };
+
+        var servers = Environment.GetEnvironmentVariable("OTOPCUA_HISTORIAN_SERVERS");
+        if (!string.IsNullOrWhiteSpace(servers))
+            cfg.ServerNames = new System.Collections.Generic.List<string>(
+                servers.Split(new[] { ',' }, StringSplitOptions.RemoveEmptyEntries));
+
+        Log.Information("Historian enabled — {NodeCount} configured node(s), port={Port}",
+            cfg.ServerNames.Count > 0 ? cfg.ServerNames.Count : 1, cfg.Port);
+        return new HistorianDataSource(cfg);
+    }
+
+    private static int TryParseInt(string envName, int defaultValue)
+    {
+        var raw = Environment.GetEnvironmentVariable(envName);
+        return int.TryParse(raw, out var parsed) ? parsed : defaultValue;
+    }
 }

src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.csproj

@@ -30,11 +30,43 @@
         <ProjectReference Include="..\ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Shared\ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Shared.csproj"/>
     </ItemGroup>
 
+    <ItemGroup>
+        <InternalsVisibleTo Include="ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Tests"/>
+    </ItemGroup>
+
     <ItemGroup>
         <Reference Include="ArchestrA.MxAccess">
             <HintPath>..\..\lib\ArchestrA.MxAccess.dll</HintPath>
             <Private>true</Private>
         </Reference>
+        <!-- Wonderware Historian SDK — consumed by Backend/Historian/ for HistoryReadAsync.
+             Previously lived in the v1 Historian.Aveva plugin; folded into Driver.Galaxy.Host
+             for PR #5 because this host is already Galaxy-specific. -->
+        <Reference Include="aahClientManaged">
+            <HintPath>..\..\lib\aahClientManaged.dll</HintPath>
+            <EmbedInteropTypes>false</EmbedInteropTypes>
+        </Reference>
+        <Reference Include="aahClientCommon">
+            <HintPath>..\..\lib\aahClientCommon.dll</HintPath>
+            <EmbedInteropTypes>false</EmbedInteropTypes>
+        </Reference>
+    </ItemGroup>
+
+    <ItemGroup>
+        <!-- Historian SDK native and satellite DLLs — staged beside the host exe so the
+             aahClientManaged wrapper can P/Invoke into them without an AssemblyResolve hook. -->
+        <None Include="..\..\lib\aahClient.dll">
+            <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+        </None>
+        <None Include="..\..\lib\Historian.CBE.dll">
+            <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+        </None>
+        <None Include="..\..\lib\Historian.DPAPI.dll">
+            <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+        </None>
+        <None Include="..\..\lib\ArchestrA.CloudHistorian.Contract.dll">
+            <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+        </None>
     </ItemGroup>
 
     <ItemGroup>

src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Proxy/GalaxyProxyDriver.cs

@@ -123,7 +123,8 @@ public sealed class GalaxyProxyDriver(GalaxyProxyOptions options)
                         IsArray: attr.IsArray,
                         ArrayDim: attr.ArrayDim,
                         SecurityClass: MapSecurity(attr.SecurityClassification),
-                        IsHistorized: attr.IsHistorized));
+                        IsHistorized: attr.IsHistorized,
+                        IsAlarm: attr.IsAlarm));
             }
         }
     }
@@ -296,10 +297,50 @@ public sealed class GalaxyProxyDriver(GalaxyProxyOptions options)
         return new HistoryReadResult(samples, ContinuationPoint: null);
     }
 
-    public Task<HistoryReadResult> ReadProcessedAsync(
+    public async Task<HistoryReadResult> ReadProcessedAsync(
         string fullReference, DateTime startUtc, DateTime endUtc, TimeSpan interval,
         HistoryAggregateType aggregate, CancellationToken cancellationToken)
-        => throw new NotSupportedException("Galaxy historian processed reads are not supported in v2; use ReadRawAsync.");
+    {
+        var client = RequireClient();
+        var column = MapAggregateToColumn(aggregate);
+
+        var resp = await client.CallAsync<HistoryReadProcessedRequest, HistoryReadProcessedResponse>(
+            MessageKind.HistoryReadProcessedRequest,
+            new HistoryReadProcessedRequest
+            {
+                SessionId = _sessionId,
+                TagReference = fullReference,
+                StartUtcUnixMs = new DateTimeOffset(startUtc, TimeSpan.Zero).ToUnixTimeMilliseconds(),
+                EndUtcUnixMs = new DateTimeOffset(endUtc, TimeSpan.Zero).ToUnixTimeMilliseconds(),
+                IntervalMs = (long)interval.TotalMilliseconds,
+                AggregateColumn = column,
+            },
+            MessageKind.HistoryReadProcessedResponse,
+            cancellationToken);
+
+        if (!resp.Success)
+            throw new InvalidOperationException($"Galaxy.Host HistoryReadProcessed failed: {resp.Error}");
+
+        IReadOnlyList<DataValueSnapshot> samples = [.. resp.Values.Select(ToSnapshot)];
+        return new HistoryReadResult(samples, ContinuationPoint: null);
+    }
+
+    /// <summary>
+    ///     Maps the OPC UA Part 13 aggregate enum onto the Wonderware Historian
+    ///     AnalogSummaryQuery column names consumed by <c>HistorianDataSource.ReadAggregateAsync</c>.
+    ///     Kept on the Proxy side so Galaxy.Host stays OPC-UA-free.
+    /// </summary>
+    internal static string MapAggregateToColumn(HistoryAggregateType aggregate) => aggregate switch
+    {
+        HistoryAggregateType.Average => "Average",
+        HistoryAggregateType.Minimum => "Minimum",
+        HistoryAggregateType.Maximum => "Maximum",
+        HistoryAggregateType.Count => "ValueCount",
+        HistoryAggregateType.Total => throw new NotSupportedException(
+            "HistoryAggregateType.Total is not supported by the Wonderware Historian AnalogSummary " +
+            "query — use Average × Count on the caller side, or switch to Average/Minimum/Maximum/Count."),
+        _ => throw new NotSupportedException($"Unknown HistoryAggregateType {aggregate}"),
+    };
 
     // ---- IRediscoverable ----
 

src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Proxy/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Proxy.csproj

@@ -16,6 +16,10 @@
         <ProjectReference Include="..\ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Shared\ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Shared.csproj"/>
     </ItemGroup>
 
+    <ItemGroup>
+        <InternalsVisibleTo Include="ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Proxy.Tests"/>
+    </ItemGroup>
+
     <ItemGroup>
         <NuGetAuditSuppress Include="https://github.com/advisories/GHSA-37gx-xxp4-5rgx"/>
         <NuGetAuditSuppress Include="https://github.com/advisories/GHSA-w3x6-4m5h-cxqf"/>

src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Shared/Contracts/Discovery.cs

@@ -30,6 +30,15 @@ public sealed class GalaxyAttributeInfo
     [Key(3)] public uint? ArrayDim { get; set; }
     [Key(4)] public int SecurityClassification { get; set; }
     [Key(5)] public bool IsHistorized { get; set; }
+
+    /// <summary>
+    ///     True when the attribute has an AlarmExtension primitive in the Galaxy repository
+    ///     (<c>primitive_definition.primitive_name = 'AlarmExtension'</c>). The generic
+    ///     node-manager uses this to enrich the variable's OPC UA node with an
+    ///     <c>AlarmConditionState</c> during address-space build. Added in PR 9 as the
+    ///     discovery-side foundation for the alarm event wire-up that follows in PR 10+.
+    /// </summary>
+    [Key(6)] public bool IsAlarm { get; set; }
 }
 
 [MessagePackObject]

src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Shared/Contracts/Framing.cs

@@ -48,8 +48,14 @@ public enum MessageKind : byte
     AlarmEvent                 = 0x51,
     AlarmAckRequest            = 0x52,
 
-    HistoryReadRequest         = 0x60,
-    HistoryReadResponse        = 0x61,
+    HistoryReadRequest           = 0x60,
+    HistoryReadResponse          = 0x61,
+    HistoryReadProcessedRequest  = 0x62,
+    HistoryReadProcessedResponse = 0x63,
+    HistoryReadAtTimeRequest     = 0x64,
+    HistoryReadAtTimeResponse    = 0x65,
+    HistoryReadEventsRequest     = 0x66,
+    HistoryReadEventsResponse    = 0x67,
 
     HostConnectivityStatus     = 0x70,
     RuntimeStatusChange        = 0x71,

src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Shared/Contracts/History.cs

@@ -26,3 +26,85 @@ public sealed class HistoryReadResponse
     [Key(1)] public string? Error { get; set; }
     [Key(2)] public HistoryTagValues[] Tags { get; set; } = System.Array.Empty<HistoryTagValues>();
 }
+
+/// <summary>
+///     Processed (aggregated) historian read — OPC UA HistoryReadProcessed service. The
+///     aggregate column is a string (e.g. "Average", "Minimum") mapped by the Proxy from the
+///     OPC UA HistoryAggregateType enum so Galaxy.Host stays OPC-UA-free.
+/// </summary>
+[MessagePackObject]
+public sealed class HistoryReadProcessedRequest
+{
+    [Key(0)] public long SessionId { get; set; }
+    [Key(1)] public string TagReference { get; set; } = string.Empty;
+    [Key(2)] public long StartUtcUnixMs { get; set; }
+    [Key(3)] public long EndUtcUnixMs { get; set; }
+    [Key(4)] public long IntervalMs { get; set; }
+    [Key(5)] public string AggregateColumn { get; set; } = "Average";
+}
+
+[MessagePackObject]
+public sealed class HistoryReadProcessedResponse
+{
+    [Key(0)] public bool Success { get; set; }
+    [Key(1)] public string? Error { get; set; }
+    [Key(2)] public GalaxyDataValue[] Values { get; set; } = System.Array.Empty<GalaxyDataValue>();
+}
+
+/// <summary>
+///     At-time historian read — OPC UA HistoryReadAtTime service. Returns one sample per
+///     requested timestamp (interpolated when no exact match exists). The per-timestamp array
+///     is flow-encoded as Unix milliseconds to avoid MessagePack DateTime quirks.
+/// </summary>
+[MessagePackObject]
+public sealed class HistoryReadAtTimeRequest
+{
+    [Key(0)] public long SessionId { get; set; }
+    [Key(1)] public string TagReference { get; set; } = string.Empty;
+    [Key(2)] public long[] TimestampsUtcUnixMs { get; set; } = System.Array.Empty<long>();
+}
+
+[MessagePackObject]
+public sealed class HistoryReadAtTimeResponse
+{
+    [Key(0)] public bool Success { get; set; }
+    [Key(1)] public string? Error { get; set; }
+    [Key(2)] public GalaxyDataValue[] Values { get; set; } = System.Array.Empty<GalaxyDataValue>();
+}
+
+/// <summary>
+///     Historical events read — OPC UA HistoryReadEvents service and Alarm &amp; Condition
+///     history. <c>SourceName</c> null means "all sources". Distinct from the live
+///     <see cref="GalaxyAlarmEvent"/> stream because historical rows carry both
+///     <c>EventTime</c> (when the event occurred in the process) and <c>ReceivedTime</c>
+///     (when the Historian persisted it) and have no StateTransition — the Historian logs
+///     the instantaneous event, not the OPC UA alarm lifecycle.
+/// </summary>
+[MessagePackObject]
+public sealed class HistoryReadEventsRequest
+{
+    [Key(0)] public long SessionId { get; set; }
+    [Key(1)] public string? SourceName { get; set; }
+    [Key(2)] public long StartUtcUnixMs { get; set; }
+    [Key(3)] public long EndUtcUnixMs { get; set; }
+    [Key(4)] public int MaxEvents { get; set; } = 1000;
+}
+
+[MessagePackObject]
+public sealed class GalaxyHistoricalEvent
+{
+    [Key(0)] public string EventId { get; set; } = string.Empty;
+    [Key(1)] public string? SourceName { get; set; }
+    [Key(2)] public long EventTimeUtcUnixMs { get; set; }
+    [Key(3)] public long ReceivedTimeUtcUnixMs { get; set; }
+    [Key(4)] public string? DisplayText { get; set; }
+    [Key(5)] public ushort Severity { get; set; }
+}
+
+[MessagePackObject]
+public sealed class HistoryReadEventsResponse
+{
+    [Key(0)] public bool Success { get; set; }
+    [Key(1)] public string? Error { get; set; }
+    [Key(2)] public GalaxyHistoricalEvent[] Events { get; set; } = System.Array.Empty<GalaxyHistoricalEvent>();
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Tests/AlarmDiscoveryTests.cs

@@ -0,0 +1,84 @@
+using System;
+using MessagePack;
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Shared.Contracts;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Tests;
+
+[Trait("Category", "Unit")]
+public sealed class AlarmDiscoveryTests
+{
+    /// <summary>
+    ///     PR 9 — IsAlarm must survive the MessagePack round-trip at Key=6 position.
+    ///     Regression guard: any reorder of keys in GalaxyAttributeInfo would silently corrupt
+    ///     the flag in the wire payload since MessagePack encodes by key number, not field name.
+    /// </summary>
+    [Fact]
+    public void GalaxyAttributeInfo_IsAlarm_round_trips_true_through_MessagePack()
+    {
+        var input = new GalaxyAttributeInfo
+        {
+            AttributeName = "TankLevel",
+            MxDataType = 2,
+            IsArray = false,
+            ArrayDim = null,
+            SecurityClassification = 1,
+            IsHistorized = true,
+            IsAlarm = true,
+        };
+
+        var bytes = MessagePackSerializer.Serialize(input);
+        var decoded = MessagePackSerializer.Deserialize<GalaxyAttributeInfo>(bytes);
+
+        decoded.IsAlarm.ShouldBeTrue();
+        decoded.IsHistorized.ShouldBeTrue();
+        decoded.AttributeName.ShouldBe("TankLevel");
+    }
+
+    [Fact]
+    public void GalaxyAttributeInfo_IsAlarm_round_trips_false_through_MessagePack()
+    {
+        var input = new GalaxyAttributeInfo { AttributeName = "ColorRgb", IsAlarm = false };
+        var bytes = MessagePackSerializer.Serialize(input);
+        var decoded = MessagePackSerializer.Deserialize<GalaxyAttributeInfo>(bytes);
+        decoded.IsAlarm.ShouldBeFalse();
+    }
+
+    /// <summary>
+    ///     Wire-compat guard: payloads serialized before PR 9 (which omit Key=6) must still
+    ///     deserialize cleanly — MessagePack treats missing keys as default. This lets a newer
+    ///     Proxy talk to an older Host during a rolling upgrade without a crash.
+    /// </summary>
+    [Fact]
+    public void Pre_PR9_payload_without_IsAlarm_key_deserializes_with_default_false()
+    {
+        // Build a 6-field payload (keys 0..5) matching the pre-PR9 shape by serializing a
+        // stand-in class with the same key layout but no Key=6.
+        var pre = new PrePR9Shape
+        {
+            AttributeName = "Legacy",
+            MxDataType = 1,
+            IsArray = false,
+            ArrayDim = null,
+            SecurityClassification = 0,
+            IsHistorized = false,
+        };
+        var bytes = MessagePackSerializer.Serialize(pre);
+
+        var decoded = MessagePackSerializer.Deserialize<GalaxyAttributeInfo>(bytes);
+        decoded.AttributeName.ShouldBe("Legacy");
+        decoded.IsAlarm.ShouldBeFalse();
+    }
+
+    [MessagePackObject]
+    public sealed class PrePR9Shape
+    {
+        [Key(0)] public string AttributeName { get; set; } = string.Empty;
+        [Key(1)] public int MxDataType { get; set; }
+        [Key(2)] public bool IsArray { get; set; }
+        [Key(3)] public uint? ArrayDim { get; set; }
+        [Key(4)] public int SecurityClassification { get; set; }
+        [Key(5)] public bool IsHistorized { get; set; }
+    }
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Tests/GalaxyRuntimeProbeManagerTests.cs

@@ -0,0 +1,231 @@
+using System;
+using System.Collections.Concurrent;
+using System.Collections.Generic;
+using System.Threading;
+using System.Threading.Tasks;
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Backend.MxAccess;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Backend.Stability;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Tests;
+
+[Trait("Category", "Unit")]
+public sealed class GalaxyRuntimeProbeManagerTests
+{
+    private sealed class FakeSubscriber
+    {
+        public readonly ConcurrentDictionary<string, Action<string, Vtq>> Subs = new();
+        public readonly ConcurrentQueue<string> UnsubCalls = new();
+        public bool FailSubscribeFor { get; set; }
+        public string? FailSubscribeTag { get; set; }
+
+        public Task Subscribe(string probe, Action<string, Vtq> cb)
+        {
+            if (FailSubscribeFor && string.Equals(probe, FailSubscribeTag, StringComparison.OrdinalIgnoreCase))
+                throw new InvalidOperationException("subscribe refused");
+            Subs[probe] = cb;
+            return Task.CompletedTask;
+        }
+
+        public Task Unsubscribe(string probe)
+        {
+            UnsubCalls.Enqueue(probe);
+            Subs.TryRemove(probe, out _);
+            return Task.CompletedTask;
+        }
+    }
+
+    private static Vtq Good(bool scanState) => new(scanState, DateTime.UtcNow, 192);
+    private static Vtq Bad() => new(null, DateTime.UtcNow, 0);
+
+    [Fact]
+    public async Task Sync_subscribes_to_ScanState_per_host()
+    {
+        var subs = new FakeSubscriber();
+        using var mgr = new GalaxyRuntimeProbeManager(subs.Subscribe, subs.Unsubscribe);
+
+        await mgr.SyncAsync(new[]
+        {
+            new HostProbeTarget("PlatformA", GalaxyRuntimeProbeManager.CategoryWinPlatform),
+            new HostProbeTarget("EngineB",   GalaxyRuntimeProbeManager.CategoryAppEngine),
+        });
+
+        mgr.ActiveProbeCount.ShouldBe(2);
+        subs.Subs.ShouldContainKey("PlatformA.ScanState");
+        subs.Subs.ShouldContainKey("EngineB.ScanState");
+    }
+
+    [Fact]
+    public async Task Sync_is_idempotent_on_repeat_call_with_same_set()
+    {
+        var subs = new FakeSubscriber();
+        using var mgr = new GalaxyRuntimeProbeManager(subs.Subscribe, subs.Unsubscribe);
+        var targets = new[] { new HostProbeTarget("PlatformA", 1) };
+
+        await mgr.SyncAsync(targets);
+        await mgr.SyncAsync(targets);
+
+        mgr.ActiveProbeCount.ShouldBe(1);
+        subs.Subs.Count.ShouldBe(1);
+        subs.UnsubCalls.Count.ShouldBe(0);
+    }
+
+    [Fact]
+    public async Task Sync_unadvises_removed_hosts()
+    {
+        var subs = new FakeSubscriber();
+        using var mgr = new GalaxyRuntimeProbeManager(subs.Subscribe, subs.Unsubscribe);
+
+        await mgr.SyncAsync(new[]
+        {
+            new HostProbeTarget("PlatformA", 1),
+            new HostProbeTarget("PlatformB", 1),
+        });
+        await mgr.SyncAsync(new[] { new HostProbeTarget("PlatformA", 1) });
+
+        mgr.ActiveProbeCount.ShouldBe(1);
+        subs.UnsubCalls.ShouldContain("PlatformB.ScanState");
+    }
+
+    [Fact]
+    public async Task Subscribe_failure_rolls_back_host_entry_so_later_transitions_do_not_fire_stale_events()
+    {
+        var subs = new FakeSubscriber { FailSubscribeFor = true, FailSubscribeTag = "PlatformA.ScanState" };
+        using var mgr = new GalaxyRuntimeProbeManager(subs.Subscribe, subs.Unsubscribe);
+
+        await mgr.SyncAsync(new[] { new HostProbeTarget("PlatformA", 1) });
+
+        mgr.ActiveProbeCount.ShouldBe(0); // rolled back
+        mgr.GetState("PlatformA").ShouldBe(HostRuntimeState.Unknown);
+    }
+
+    [Fact]
+    public async Task Unknown_to_Running_does_not_fire_StateChanged()
+    {
+        var now = new DateTime(2026, 4, 18, 10, 0, 0, DateTimeKind.Utc);
+        var subs = new FakeSubscriber();
+        using var mgr = new GalaxyRuntimeProbeManager(subs.Subscribe, subs.Unsubscribe, () => now);
+        var transitions = new ConcurrentQueue<HostStateTransition>();
+        mgr.StateChanged += (_, t) => transitions.Enqueue(t);
+
+        await mgr.SyncAsync(new[] { new HostProbeTarget("PlatformA", 1) });
+        subs.Subs["PlatformA.ScanState"]("PlatformA.ScanState", Good(true));
+
+        mgr.GetState("PlatformA").ShouldBe(HostRuntimeState.Running);
+        transitions.Count.ShouldBe(0); // startup transition, operators don't care
+    }
+
+    [Fact]
+    public async Task Running_to_Stopped_fires_StateChanged_with_both_states()
+    {
+        var now = new DateTime(2026, 4, 18, 10, 0, 0, DateTimeKind.Utc);
+        var subs = new FakeSubscriber();
+        using var mgr = new GalaxyRuntimeProbeManager(subs.Subscribe, subs.Unsubscribe, () => now);
+        var transitions = new ConcurrentQueue<HostStateTransition>();
+        mgr.StateChanged += (_, t) => transitions.Enqueue(t);
+
+        await mgr.SyncAsync(new[] { new HostProbeTarget("PlatformA", 1) });
+        subs.Subs["PlatformA.ScanState"]("PlatformA.ScanState", Good(true));  // Unknown→Running (silent)
+        subs.Subs["PlatformA.ScanState"]("PlatformA.ScanState", Good(false)); // Running→Stopped (fires)
+
+        transitions.Count.ShouldBe(1);
+        transitions.TryDequeue(out var t).ShouldBeTrue();
+        t!.TagName.ShouldBe("PlatformA");
+        t.OldState.ShouldBe(HostRuntimeState.Running);
+        t.NewState.ShouldBe(HostRuntimeState.Stopped);
+    }
+
+    [Fact]
+    public async Task Stopped_to_Running_fires_StateChanged_for_recovery()
+    {
+        var now = new DateTime(2026, 4, 18, 10, 0, 0, DateTimeKind.Utc);
+        var subs = new FakeSubscriber();
+        using var mgr = new GalaxyRuntimeProbeManager(subs.Subscribe, subs.Unsubscribe, () => now);
+        var transitions = new ConcurrentQueue<HostStateTransition>();
+        mgr.StateChanged += (_, t) => transitions.Enqueue(t);
+
+        await mgr.SyncAsync(new[] { new HostProbeTarget("PlatformA", 1) });
+        subs.Subs["PlatformA.ScanState"]("PlatformA.ScanState", Good(true));  // Unknown→Running (silent)
+        subs.Subs["PlatformA.ScanState"]("PlatformA.ScanState", Good(false)); // Running→Stopped (fires)
+        subs.Subs["PlatformA.ScanState"]("PlatformA.ScanState", Good(true));  // Stopped→Running (fires)
+
+        transitions.Count.ShouldBe(2);
+    }
+
+    [Fact]
+    public async Task Unknown_to_Stopped_fires_StateChanged_for_first_known_bad_signal()
+    {
+        var now = new DateTime(2026, 4, 18, 10, 0, 0, DateTimeKind.Utc);
+        var subs = new FakeSubscriber();
+        using var mgr = new GalaxyRuntimeProbeManager(subs.Subscribe, subs.Unsubscribe, () => now);
+        var transitions = new ConcurrentQueue<HostStateTransition>();
+        mgr.StateChanged += (_, t) => transitions.Enqueue(t);
+
+        await mgr.SyncAsync(new[] { new HostProbeTarget("PlatformA", 1) });
+        // First callback is bad-quality — we must flag the host Stopped so operators see it.
+        subs.Subs["PlatformA.ScanState"]("PlatformA.ScanState", Bad());
+
+        transitions.Count.ShouldBe(1);
+        transitions.TryDequeue(out var t).ShouldBeTrue();
+        t!.OldState.ShouldBe(HostRuntimeState.Unknown);
+        t.NewState.ShouldBe(HostRuntimeState.Stopped);
+    }
+
+    [Fact]
+    public async Task Repeated_Good_Running_callbacks_do_not_fire_duplicate_events()
+    {
+        var now = new DateTime(2026, 4, 18, 10, 0, 0, DateTimeKind.Utc);
+        var subs = new FakeSubscriber();
+        using var mgr = new GalaxyRuntimeProbeManager(subs.Subscribe, subs.Unsubscribe, () => now);
+        var count = 0;
+        mgr.StateChanged += (_, _) => Interlocked.Increment(ref count);
+
+        await mgr.SyncAsync(new[] { new HostProbeTarget("PlatformA", 1) });
+        for (var i = 0; i < 5; i++)
+            subs.Subs["PlatformA.ScanState"]("PlatformA.ScanState", Good(true));
+
+        count.ShouldBe(0); // only the silent Unknown→Running on the first, no events after
+    }
+
+    [Fact]
+    public async Task Unknown_callback_for_non_tracked_probe_is_dropped()
+    {
+        var subs = new FakeSubscriber();
+        using var mgr = new GalaxyRuntimeProbeManager(subs.Subscribe, subs.Unsubscribe);
+
+        mgr.OnProbeCallback("ProbeForSomeoneElse.ScanState", Good(true));
+
+        mgr.ActiveProbeCount.ShouldBe(0);
+    }
+
+    [Fact]
+    public async Task Snapshot_reports_current_state_for_every_tracked_host()
+    {
+        var now = new DateTime(2026, 4, 18, 10, 0, 0, DateTimeKind.Utc);
+        var subs = new FakeSubscriber();
+        using var mgr = new GalaxyRuntimeProbeManager(subs.Subscribe, subs.Unsubscribe, () => now);
+
+        await mgr.SyncAsync(new[]
+        {
+            new HostProbeTarget("PlatformA", 1),
+            new HostProbeTarget("EngineB",   3),
+        });
+        subs.Subs["PlatformA.ScanState"]("PlatformA.ScanState", Good(true));  // Running
+        subs.Subs["EngineB.ScanState"]("EngineB.ScanState", Bad());            // Stopped
+
+        var snap = mgr.SnapshotStates();
+        snap.Count.ShouldBe(2);
+        snap.ShouldContain(s => s.TagName == "PlatformA" && s.State == HostRuntimeState.Running);
+        snap.ShouldContain(s => s.TagName == "EngineB" && s.State == HostRuntimeState.Stopped);
+    }
+
+    [Fact]
+    public void IsRuntimeHost_recognizes_WinPlatform_and_AppEngine_category_ids()
+    {
+        new HostProbeTarget("X", GalaxyRuntimeProbeManager.CategoryWinPlatform).IsRuntimeHost.ShouldBeTrue();
+        new HostProbeTarget("X", GalaxyRuntimeProbeManager.CategoryAppEngine).IsRuntimeHost.ShouldBeTrue();
+        new HostProbeTarget("X", 4 /* $Area */).IsRuntimeHost.ShouldBeFalse();
+        new HostProbeTarget("X", 11 /* $ApplicationObject */).IsRuntimeHost.ShouldBeFalse();
+    }
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Tests/HistorianClusterEndpointPickerTests.cs

@@ -0,0 +1,94 @@
+using System;
+using System.Linq;
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Backend.Historian;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Tests
+{
+    [Trait("Category", "Unit")]
+    public sealed class HistorianClusterEndpointPickerTests
+    {
+        private static HistorianConfiguration Config(params string[] nodes) => new()
+        {
+            ServerName = "ignored",
+            ServerNames = nodes.ToList(),
+            FailureCooldownSeconds = 60,
+        };
+
+        [Fact]
+        public void Single_node_config_falls_back_to_ServerName_when_ServerNames_empty()
+        {
+            var cfg = new HistorianConfiguration { ServerName = "only-node", ServerNames = new() };
+            var p = new HistorianClusterEndpointPicker(cfg);
+            p.NodeCount.ShouldBe(1);
+            p.GetHealthyNodes().ShouldBe(new[] { "only-node" });
+        }
+
+        [Fact]
+        public void Failed_node_enters_cooldown_and_is_skipped()
+        {
+            var now = new DateTime(2026, 4, 18, 10, 0, 0, DateTimeKind.Utc);
+            var p = new HistorianClusterEndpointPicker(Config("a", "b"), () => now);
+
+            p.MarkFailed("a", "boom");
+            p.GetHealthyNodes().ShouldBe(new[] { "b" });
+        }
+
+        [Fact]
+        public void Cooldown_expires_after_configured_window()
+        {
+            var clock = new DateTime(2026, 4, 18, 10, 0, 0, DateTimeKind.Utc);
+            var p = new HistorianClusterEndpointPicker(Config("a", "b"), () => clock);
+            p.MarkFailed("a", "boom");
+            p.GetHealthyNodes().ShouldBe(new[] { "b" });
+
+            clock = clock.AddSeconds(61);
+            p.GetHealthyNodes().ShouldBe(new[] { "a", "b" });
+        }
+
+        [Fact]
+        public void MarkHealthy_immediately_clears_cooldown()
+        {
+            var now = new DateTime(2026, 4, 18, 10, 0, 0, DateTimeKind.Utc);
+            var p = new HistorianClusterEndpointPicker(Config("a"), () => now);
+            p.MarkFailed("a", "boom");
+            p.GetHealthyNodes().ShouldBeEmpty();
+            p.MarkHealthy("a");
+            p.GetHealthyNodes().ShouldBe(new[] { "a" });
+        }
+
+        [Fact]
+        public void All_nodes_in_cooldown_returns_empty_healthy_list()
+        {
+            var now = new DateTime(2026, 4, 18, 10, 0, 0, DateTimeKind.Utc);
+            var p = new HistorianClusterEndpointPicker(Config("a", "b"), () => now);
+            p.MarkFailed("a", "x");
+            p.MarkFailed("b", "y");
+            p.GetHealthyNodes().ShouldBeEmpty();
+            p.NodeCount.ShouldBe(2);
+        }
+
+        [Fact]
+        public void Snapshot_reports_failure_count_and_last_error()
+        {
+            var now = new DateTime(2026, 4, 18, 10, 0, 0, DateTimeKind.Utc);
+            var p = new HistorianClusterEndpointPicker(Config("a"), () => now);
+            p.MarkFailed("a", "first");
+            p.MarkFailed("a", "second");
+
+            var snap = p.SnapshotNodeStates().Single();
+            snap.FailureCount.ShouldBe(2);
+            snap.LastError.ShouldBe("second");
+            snap.IsHealthy.ShouldBeFalse();
+            snap.CooldownUntil.ShouldNotBeNull();
+        }
+
+        [Fact]
+        public void Duplicate_hostnames_are_deduplicated_case_insensitively()
+        {
+            var p = new HistorianClusterEndpointPicker(Config("NodeA", "nodea", "NodeB"));
+            p.NodeCount.ShouldBe(2);
+        }
+    }
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Tests/HistorianQualityMapperTests.cs

@@ -0,0 +1,61 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Backend.Historian;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Tests;
+
+[Trait("Category", "Unit")]
+public sealed class HistorianQualityMapperTests
+{
+    /// <summary>
+    ///     Rich mapping preserves specific OPC DA subcodes through the historian ToWire path.
+    ///     Before PR 12 the category-only fallback collapsed e.g. BadNotConnected(8) to
+    ///     Bad(0x80000000) so downstream OPC UA clients could not distinguish transport issues
+    ///     from sensor issues. After PR 12 every known subcode round-trips to its canonical
+    ///     uint32 StatusCode and Proxy translation stays byte-for-byte with v1 QualityMapper.
+    /// </summary>
+    [Theory]
+    [InlineData((byte)192, 0x00000000u)] // Good
+    [InlineData((byte)216, 0x00D80000u)] // Good_LocalOverride
+    [InlineData((byte)64, 0x40000000u)] // Uncertain
+    [InlineData((byte)68, 0x40900000u)] // Uncertain_LastUsableValue
+    [InlineData((byte)80, 0x40930000u)] // Uncertain_SensorNotAccurate
+    [InlineData((byte)84, 0x40940000u)] // Uncertain_EngineeringUnitsExceeded
+    [InlineData((byte)88, 0x40950000u)] // Uncertain_SubNormal
+    [InlineData((byte)0, 0x80000000u)] // Bad
+    [InlineData((byte)4, 0x80890000u)] // Bad_ConfigurationError
+    [InlineData((byte)8, 0x808A0000u)] // Bad_NotConnected
+    [InlineData((byte)12, 0x808B0000u)] // Bad_DeviceFailure
+    [InlineData((byte)16, 0x808C0000u)] // Bad_SensorFailure
+    [InlineData((byte)20, 0x80050000u)] // Bad_CommunicationError
+    [InlineData((byte)24, 0x808D0000u)] // Bad_OutOfService
+    [InlineData((byte)32, 0x80320000u)] // Bad_WaitingForInitialData
+    public void Maps_specific_OPC_DA_codes_to_canonical_StatusCode(byte quality, uint expected)
+    {
+        HistorianQualityMapper.Map(quality).ShouldBe(expected);
+    }
+
+    [Theory]
+    [InlineData((byte)200)] // Good — unknown subcode in Good family
+    [InlineData((byte)255)] // Good — unknown
+    public void Unknown_good_family_codes_fall_back_to_plain_Good(byte q)
+    {
+        HistorianQualityMapper.Map(q).ShouldBe(0x00000000u);
+    }
+
+    [Theory]
+    [InlineData((byte)100)] // Uncertain — unknown subcode
+    [InlineData((byte)150)] // Uncertain — unknown
+    public void Unknown_uncertain_family_codes_fall_back_to_plain_Uncertain(byte q)
+    {
+        HistorianQualityMapper.Map(q).ShouldBe(0x40000000u);
+    }
+
+    [Theory]
+    [InlineData((byte)1)] // Bad — unknown subcode
+    [InlineData((byte)50)] // Bad — unknown
+    public void Unknown_bad_family_codes_fall_back_to_plain_Bad(byte q)
+    {
+        HistorianQualityMapper.Map(q).ShouldBe(0x80000000u);
+    }
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Tests/HistorianWiringTests.cs

@@ -0,0 +1,109 @@
+using System;
+using System.Collections.Generic;
+using System.Threading;
+using System.Threading.Tasks;
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Backend;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Backend.Galaxy;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Backend.Historian;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Backend.MxAccess;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Sta;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Shared.Contracts;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Tests
+{
+    [Trait("Category", "Unit")]
+    public sealed class HistorianWiringTests
+    {
+        /// <summary>
+        ///     When the Proxy sends a HistoryRead but the supervisor never enabled the historian
+        ///     (OTOPCUA_HISTORIAN_ENABLED unset), we expect a clean Success=false with a
+        ///     self-explanatory error — not an exception or a hang against localhost.
+        /// </summary>
+        [Fact]
+        public async Task HistoryReadAsync_returns_disabled_error_when_no_historian_configured()
+        {
+            using var pump = new StaPump("Test.Sta");
+            await pump.WaitForStartedAsync();
+            var mx = new MxAccessClient(pump, new MxProxyAdapter(), "HistorianWiringTests");
+            using var backend = new MxAccessGalaxyBackend(
+                new GalaxyRepository(new GalaxyRepositoryOptions { ConnectionString = "Server=.;Database=ZB;Integrated Security=True;" }),
+                mx,
+                historian: null);
+
+            var resp = await backend.HistoryReadAsync(new HistoryReadRequest
+            {
+                TagReferences = new[] { "TestTag" },
+                StartUtcUnixMs = 0,
+                EndUtcUnixMs = DateTimeOffset.UtcNow.ToUnixTimeMilliseconds(),
+                MaxValuesPerTag = 100,
+            }, CancellationToken.None);
+
+            resp.Success.ShouldBeFalse();
+            resp.Error.ShouldContain("Historian disabled");
+            resp.Tags.ShouldBeEmpty();
+        }
+
+        /// <summary>
+        ///     When the historian is wired up, we expect the backend to call through and map
+        ///     samples onto the IPC wire shape. Uses a fake <see cref="IHistorianDataSource"/>
+        ///     that returns a single known-good sample so we can assert the mapping stays sane.
+        /// </summary>
+        [Fact]
+        public async Task HistoryReadAsync_maps_sample_to_GalaxyDataValue()
+        {
+            using var pump = new StaPump("Test.Sta");
+            await pump.WaitForStartedAsync();
+            var mx = new MxAccessClient(pump, new MxProxyAdapter(), "HistorianWiringTests");
+            var fake = new FakeHistorianDataSource(new HistorianSample
+            {
+                Value = 42.5,
+                Quality = 192, // Good
+                TimestampUtc = new DateTime(2026, 4, 18, 9, 0, 0, DateTimeKind.Utc),
+            });
+            using var backend = new MxAccessGalaxyBackend(
+                new GalaxyRepository(new GalaxyRepositoryOptions { ConnectionString = "Server=.;Database=ZB;Integrated Security=True;" }),
+                mx,
+                fake);
+
+            var resp = await backend.HistoryReadAsync(new HistoryReadRequest
+            {
+                TagReferences = new[] { "TankLevel" },
+                StartUtcUnixMs = 0,
+                EndUtcUnixMs = DateTimeOffset.UtcNow.ToUnixTimeMilliseconds(),
+                MaxValuesPerTag = 100,
+            }, CancellationToken.None);
+
+            resp.Success.ShouldBeTrue();
+            resp.Tags.Length.ShouldBe(1);
+            resp.Tags[0].TagReference.ShouldBe("TankLevel");
+            resp.Tags[0].Values.Length.ShouldBe(1);
+            resp.Tags[0].Values[0].StatusCode.ShouldBe(0u); // Good
+            resp.Tags[0].Values[0].ValueBytes.ShouldNotBeNull();
+            resp.Tags[0].Values[0].SourceTimestampUtcUnixMs.ShouldBe(
+                new DateTimeOffset(2026, 4, 18, 9, 0, 0, TimeSpan.Zero).ToUnixTimeMilliseconds());
+        }
+
+        private sealed class FakeHistorianDataSource : IHistorianDataSource
+        {
+            private readonly HistorianSample _sample;
+            public FakeHistorianDataSource(HistorianSample sample) => _sample = sample;
+
+            public Task<List<HistorianSample>> ReadRawAsync(string tagName, DateTime s, DateTime e, int max, CancellationToken ct)
+                => Task.FromResult(new List<HistorianSample> { _sample });
+
+            public Task<List<HistorianAggregateSample>> ReadAggregateAsync(string tagName, DateTime s, DateTime e, double ms, string col, CancellationToken ct)
+                => Task.FromResult(new List<HistorianAggregateSample>());
+
+            public Task<List<HistorianSample>> ReadAtTimeAsync(string tagName, DateTime[] ts, CancellationToken ct)
+                => Task.FromResult(new List<HistorianSample>());
+
+            public Task<List<HistorianEventDto>> ReadEventsAsync(string? src, DateTime s, DateTime e, int max, CancellationToken ct)
+                => Task.FromResult(new List<HistorianEventDto>());
+
+            public HistorianHealthSnapshot GetHealthSnapshot() => new();
+            public void Dispose() { }
+        }
+    }
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Tests/HistoryReadAtTimeTests.cs

@@ -0,0 +1,147 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading;
+using System.Threading.Tasks;
+using MessagePack;
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Backend;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Backend.Galaxy;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Backend.Historian;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Backend.MxAccess;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Sta;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Shared.Contracts;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Tests;
+
+[Trait("Category", "Unit")]
+public sealed class HistoryReadAtTimeTests
+{
+    private static MxAccessGalaxyBackend BuildBackend(IHistorianDataSource? historian, StaPump pump) =>
+        new(
+            new GalaxyRepository(new GalaxyRepositoryOptions { ConnectionString = "Server=.;Database=ZB;Integrated Security=True;" }),
+            new MxAccessClient(pump, new MxProxyAdapter(), "attime-test"),
+            historian);
+
+    [Fact]
+    public async Task Returns_disabled_error_when_no_historian_configured()
+    {
+        using var pump = new StaPump("Test.Sta");
+        await pump.WaitForStartedAsync();
+        using var backend = BuildBackend(null, pump);
+
+        var resp = await backend.HistoryReadAtTimeAsync(new HistoryReadAtTimeRequest
+        {
+            TagReference = "T",
+            TimestampsUtcUnixMs = new[] { 1L, 2L },
+        }, CancellationToken.None);
+
+        resp.Success.ShouldBeFalse();
+        resp.Error.ShouldContain("Historian disabled");
+    }
+
+    [Fact]
+    public async Task Empty_timestamp_list_short_circuits_to_success_with_no_values()
+    {
+        using var pump = new StaPump("Test.Sta");
+        await pump.WaitForStartedAsync();
+        var fake = new FakeHistorian();
+        using var backend = BuildBackend(fake, pump);
+
+        var resp = await backend.HistoryReadAtTimeAsync(new HistoryReadAtTimeRequest
+        {
+            TagReference = "T",
+            TimestampsUtcUnixMs = Array.Empty<long>(),
+        }, CancellationToken.None);
+
+        resp.Success.ShouldBeTrue();
+        resp.Values.ShouldBeEmpty();
+        fake.Calls.ShouldBe(0); // no round-trip to SDK for empty timestamp list
+    }
+
+    [Fact]
+    public async Task Timestamps_survive_Unix_ms_round_trip_to_DateTime()
+    {
+        using var pump = new StaPump("Test.Sta");
+        await pump.WaitForStartedAsync();
+        var t1 = new DateTime(2026, 4, 18, 10, 0, 0, DateTimeKind.Utc);
+        var t2 = new DateTime(2026, 4, 18, 10, 5, 0, DateTimeKind.Utc);
+        var fake = new FakeHistorian(
+            new HistorianSample { Value = 100.0, Quality = 192, TimestampUtc = t1 },
+            new HistorianSample { Value = 101.5, Quality = 192, TimestampUtc = t2 });
+        using var backend = BuildBackend(fake, pump);
+
+        var resp = await backend.HistoryReadAtTimeAsync(new HistoryReadAtTimeRequest
+        {
+            TagReference = "TankLevel",
+            TimestampsUtcUnixMs = new[]
+            {
+                new DateTimeOffset(t1, TimeSpan.Zero).ToUnixTimeMilliseconds(),
+                new DateTimeOffset(t2, TimeSpan.Zero).ToUnixTimeMilliseconds(),
+            },
+        }, CancellationToken.None);
+
+        resp.Success.ShouldBeTrue();
+        resp.Values.Length.ShouldBe(2);
+        resp.Values[0].SourceTimestampUtcUnixMs.ShouldBe(new DateTimeOffset(t1, TimeSpan.Zero).ToUnixTimeMilliseconds());
+        resp.Values[0].StatusCode.ShouldBe(0u); // Good (quality 192)
+        MessagePackSerializer.Deserialize<double>(resp.Values[0].ValueBytes!).ShouldBe(100.0);
+
+        fake.Calls.ShouldBe(1);
+        fake.LastTimestamps.Length.ShouldBe(2);
+        fake.LastTimestamps[0].ShouldBe(t1);
+        fake.LastTimestamps[1].ShouldBe(t2);
+    }
+
+    [Fact]
+    public async Task Missing_sample_maps_to_Bad_category()
+    {
+        using var pump = new StaPump("Test.Sta");
+        await pump.WaitForStartedAsync();
+        // Quality=0 means no sample at that timestamp per HistorianDataSource.ReadAtTimeAsync.
+        var fake = new FakeHistorian(new HistorianSample
+        {
+            Value = null,
+            Quality = 0,
+            TimestampUtc = DateTime.UtcNow,
+        });
+        using var backend = BuildBackend(fake, pump);
+
+        var resp = await backend.HistoryReadAtTimeAsync(new HistoryReadAtTimeRequest
+        {
+            TagReference = "T",
+            TimestampsUtcUnixMs = new[] { 1L },
+        }, CancellationToken.None);
+
+        resp.Success.ShouldBeTrue();
+        resp.Values.Length.ShouldBe(1);
+        resp.Values[0].StatusCode.ShouldBe(0x80000000u); // Bad category
+        resp.Values[0].ValueBytes.ShouldBeNull();
+    }
+
+    private sealed class FakeHistorian : IHistorianDataSource
+    {
+        private readonly HistorianSample[] _samples;
+        public int Calls { get; private set; }
+        public DateTime[] LastTimestamps { get; private set; } = Array.Empty<DateTime>();
+
+        public FakeHistorian(params HistorianSample[] samples) => _samples = samples;
+
+        public Task<List<HistorianSample>> ReadAtTimeAsync(string tag, DateTime[] ts, CancellationToken ct)
+        {
+            Calls++;
+            LastTimestamps = ts;
+            return Task.FromResult(new List<HistorianSample>(_samples));
+        }
+
+        public Task<List<HistorianSample>> ReadRawAsync(string tag, DateTime s, DateTime e, int max, CancellationToken ct)
+            => Task.FromResult(new List<HistorianSample>());
+        public Task<List<HistorianAggregateSample>> ReadAggregateAsync(string tag, DateTime s, DateTime e, double ms, string col, CancellationToken ct)
+            => Task.FromResult(new List<HistorianAggregateSample>());
+        public Task<List<HistorianEventDto>> ReadEventsAsync(string? src, DateTime s, DateTime e, int max, CancellationToken ct)
+            => Task.FromResult(new List<HistorianEventDto>());
+        public HistorianHealthSnapshot GetHealthSnapshot() => new();
+        public void Dispose() { }
+    }
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Tests/HistoryReadEventsTests.cs

@@ -0,0 +1,129 @@
+using System;
+using System.Collections.Generic;
+using System.Threading;
+using System.Threading.Tasks;
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Backend;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Backend.Galaxy;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Backend.Historian;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Backend.MxAccess;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Sta;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Shared.Contracts;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Tests;
+
+[Trait("Category", "Unit")]
+public sealed class HistoryReadEventsTests
+{
+    private static MxAccessGalaxyBackend BuildBackend(IHistorianDataSource? h, StaPump pump) =>
+        new(
+            new GalaxyRepository(new GalaxyRepositoryOptions { ConnectionString = "Server=.;Database=ZB;Integrated Security=True;" }),
+            new MxAccessClient(pump, new MxProxyAdapter(), "events-test"),
+            h);
+
+    [Fact]
+    public async Task Returns_disabled_error_when_no_historian_configured()
+    {
+        using var pump = new StaPump("Test.Sta");
+        await pump.WaitForStartedAsync();
+        using var backend = BuildBackend(null, pump);
+
+        var resp = await backend.HistoryReadEventsAsync(new HistoryReadEventsRequest
+        {
+            SourceName = "TankA",
+            StartUtcUnixMs = 0,
+            EndUtcUnixMs = DateTimeOffset.UtcNow.ToUnixTimeMilliseconds(),
+            MaxEvents = 100,
+        }, CancellationToken.None);
+
+        resp.Success.ShouldBeFalse();
+        resp.Error.ShouldContain("Historian disabled");
+    }
+
+    [Fact]
+    public async Task Maps_HistorianEventDto_to_GalaxyHistoricalEvent_wire_shape()
+    {
+        using var pump = new StaPump("Test.Sta");
+        await pump.WaitForStartedAsync();
+
+        var eventId = Guid.NewGuid();
+        var eventTime = new DateTime(2026, 4, 18, 10, 0, 0, DateTimeKind.Utc);
+        var receivedTime = eventTime.AddMilliseconds(150);
+        var fake = new FakeHistorian(new HistorianEventDto
+        {
+            Id = eventId,
+            Source = "TankA.Level.HiHi",
+            EventTime = eventTime,
+            ReceivedTime = receivedTime,
+            DisplayText = "HiHi alarm tripped",
+            Severity = 900,
+        });
+        using var backend = BuildBackend(fake, pump);
+
+        var resp = await backend.HistoryReadEventsAsync(new HistoryReadEventsRequest
+        {
+            SourceName = "TankA.Level.HiHi",
+            StartUtcUnixMs = 0,
+            EndUtcUnixMs = DateTimeOffset.UtcNow.ToUnixTimeMilliseconds(),
+            MaxEvents = 50,
+        }, CancellationToken.None);
+
+        resp.Success.ShouldBeTrue();
+        resp.Events.Length.ShouldBe(1);
+        var got = resp.Events[0];
+        got.EventId.ShouldBe(eventId.ToString());
+        got.SourceName.ShouldBe("TankA.Level.HiHi");
+        got.DisplayText.ShouldBe("HiHi alarm tripped");
+        got.Severity.ShouldBe<ushort>(900);
+        got.EventTimeUtcUnixMs.ShouldBe(new DateTimeOffset(eventTime, TimeSpan.Zero).ToUnixTimeMilliseconds());
+        got.ReceivedTimeUtcUnixMs.ShouldBe(new DateTimeOffset(receivedTime, TimeSpan.Zero).ToUnixTimeMilliseconds());
+
+        fake.LastSourceName.ShouldBe("TankA.Level.HiHi");
+        fake.LastMaxEvents.ShouldBe(50);
+    }
+
+    [Fact]
+    public async Task Null_source_name_is_passed_through_as_all_sources()
+    {
+        using var pump = new StaPump("Test.Sta");
+        await pump.WaitForStartedAsync();
+        var fake = new FakeHistorian();
+        using var backend = BuildBackend(fake, pump);
+
+        await backend.HistoryReadEventsAsync(new HistoryReadEventsRequest
+        {
+            SourceName = null,
+            StartUtcUnixMs = 0,
+            EndUtcUnixMs = 1,
+            MaxEvents = 10,
+        }, CancellationToken.None);
+
+        fake.LastSourceName.ShouldBeNull();
+    }
+
+    private sealed class FakeHistorian : IHistorianDataSource
+    {
+        private readonly HistorianEventDto[] _events;
+        public string? LastSourceName { get; private set; } = "<unset>";
+        public int LastMaxEvents { get; private set; }
+
+        public FakeHistorian(params HistorianEventDto[] events) => _events = events;
+
+        public Task<List<HistorianEventDto>> ReadEventsAsync(string? src, DateTime s, DateTime e, int max, CancellationToken ct)
+        {
+            LastSourceName = src;
+            LastMaxEvents = max;
+            return Task.FromResult(new List<HistorianEventDto>(_events));
+        }
+
+        public Task<List<HistorianSample>> ReadRawAsync(string tag, DateTime s, DateTime e, int max, CancellationToken ct)
+            => Task.FromResult(new List<HistorianSample>());
+        public Task<List<HistorianAggregateSample>> ReadAggregateAsync(string tag, DateTime s, DateTime e, double ms, string col, CancellationToken ct)
+            => Task.FromResult(new List<HistorianAggregateSample>());
+        public Task<List<HistorianSample>> ReadAtTimeAsync(string tag, DateTime[] ts, CancellationToken ct)
+            => Task.FromResult(new List<HistorianSample>());
+        public HistorianHealthSnapshot GetHealthSnapshot() => new();
+        public void Dispose() { }
+    }
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Tests/HistoryReadProcessedTests.cs

@@ -0,0 +1,158 @@
+using System;
+using System.Collections.Generic;
+using System.Threading;
+using System.Threading.Tasks;
+using MessagePack;
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Backend;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Backend.Galaxy;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Backend.Historian;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Backend.MxAccess;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Sta;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Shared.Contracts;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Tests;
+
+[Trait("Category", "Unit")]
+public sealed class HistoryReadProcessedTests
+{
+    [Fact]
+    public async Task ReturnsDisabledError_When_NoHistorianConfigured()
+    {
+        using var pump = new StaPump("Test.Sta");
+        await pump.WaitForStartedAsync();
+        var mx = new MxAccessClient(pump, new MxProxyAdapter(), "processed-test");
+        using var backend = new MxAccessGalaxyBackend(
+            new GalaxyRepository(new GalaxyRepositoryOptions { ConnectionString = "Server=.;Database=ZB;Integrated Security=True;" }),
+            mx,
+            historian: null);
+
+        var resp = await backend.HistoryReadProcessedAsync(new HistoryReadProcessedRequest
+        {
+            TagReference = "T",
+            StartUtcUnixMs = 0,
+            EndUtcUnixMs = DateTimeOffset.UtcNow.ToUnixTimeMilliseconds(),
+            IntervalMs = 1000,
+            AggregateColumn = "Average",
+        }, CancellationToken.None);
+
+        resp.Success.ShouldBeFalse();
+        resp.Error.ShouldContain("Historian disabled");
+    }
+
+    [Fact]
+    public async Task Rejects_NonPositiveInterval()
+    {
+        using var pump = new StaPump("Test.Sta");
+        await pump.WaitForStartedAsync();
+        var mx = new MxAccessClient(pump, new MxProxyAdapter(), "processed-test");
+        var fake = new FakeHistorianDataSource();
+        using var backend = new MxAccessGalaxyBackend(
+            new GalaxyRepository(new GalaxyRepositoryOptions { ConnectionString = "Server=.;Database=ZB;Integrated Security=True;" }),
+            mx,
+            fake);
+
+        var resp = await backend.HistoryReadProcessedAsync(new HistoryReadProcessedRequest
+        {
+            TagReference = "T",
+            IntervalMs = 0,
+            AggregateColumn = "Average",
+        }, CancellationToken.None);
+
+        resp.Success.ShouldBeFalse();
+        resp.Error.ShouldContain("IntervalMs");
+    }
+
+    [Fact]
+    public async Task Maps_AggregateSample_With_Value_To_Good()
+    {
+        using var pump = new StaPump("Test.Sta");
+        await pump.WaitForStartedAsync();
+        var mx = new MxAccessClient(pump, new MxProxyAdapter(), "processed-test");
+        var fake = new FakeHistorianDataSource(new HistorianAggregateSample
+        {
+            Value = 12.34,
+            TimestampUtc = new DateTime(2026, 4, 18, 10, 0, 0, DateTimeKind.Utc),
+        });
+        using var backend = new MxAccessGalaxyBackend(
+            new GalaxyRepository(new GalaxyRepositoryOptions { ConnectionString = "Server=.;Database=ZB;Integrated Security=True;" }),
+            mx,
+            fake);
+
+        var resp = await backend.HistoryReadProcessedAsync(new HistoryReadProcessedRequest
+        {
+            TagReference = "T",
+            StartUtcUnixMs = 0,
+            EndUtcUnixMs = DateTimeOffset.UtcNow.ToUnixTimeMilliseconds(),
+            IntervalMs = 60_000,
+            AggregateColumn = "Average",
+        }, CancellationToken.None);
+
+        resp.Success.ShouldBeTrue();
+        resp.Values.Length.ShouldBe(1);
+        resp.Values[0].StatusCode.ShouldBe(0u); // Good
+        resp.Values[0].ValueBytes.ShouldNotBeNull();
+        MessagePackSerializer.Deserialize<double>(resp.Values[0].ValueBytes!).ShouldBe(12.34);
+        fake.LastAggregateColumn.ShouldBe("Average");
+        fake.LastIntervalMs.ShouldBe(60_000d);
+    }
+
+    [Fact]
+    public async Task Maps_Null_Bucket_To_BadNoData()
+    {
+        using var pump = new StaPump("Test.Sta");
+        await pump.WaitForStartedAsync();
+        var mx = new MxAccessClient(pump, new MxProxyAdapter(), "processed-test");
+        var fake = new FakeHistorianDataSource(new HistorianAggregateSample
+        {
+            Value = null,
+            TimestampUtc = DateTime.UtcNow,
+        });
+        using var backend = new MxAccessGalaxyBackend(
+            new GalaxyRepository(new GalaxyRepositoryOptions { ConnectionString = "Server=.;Database=ZB;Integrated Security=True;" }),
+            mx,
+            fake);
+
+        var resp = await backend.HistoryReadProcessedAsync(new HistoryReadProcessedRequest
+        {
+            TagReference = "T",
+            IntervalMs = 1000,
+            AggregateColumn = "Minimum",
+        }, CancellationToken.None);
+
+        resp.Success.ShouldBeTrue();
+        resp.Values.Length.ShouldBe(1);
+        resp.Values[0].StatusCode.ShouldBe(0x800E0000u); // BadNoData
+        resp.Values[0].ValueBytes.ShouldBeNull();
+    }
+
+    private sealed class FakeHistorianDataSource : IHistorianDataSource
+    {
+        private readonly HistorianAggregateSample[] _samples;
+        public string? LastAggregateColumn { get; private set; }
+        public double LastIntervalMs { get; private set; }
+
+        public FakeHistorianDataSource(params HistorianAggregateSample[] samples) => _samples = samples;
+
+        public Task<List<HistorianSample>> ReadRawAsync(string tag, DateTime s, DateTime e, int max, CancellationToken ct)
+            => Task.FromResult(new List<HistorianSample>());
+
+        public Task<List<HistorianAggregateSample>> ReadAggregateAsync(
+            string tag, DateTime s, DateTime e, double intervalMs, string col, CancellationToken ct)
+        {
+            LastAggregateColumn = col;
+            LastIntervalMs = intervalMs;
+            return Task.FromResult(new List<HistorianAggregateSample>(_samples));
+        }
+
+        public Task<List<HistorianSample>> ReadAtTimeAsync(string tag, DateTime[] ts, CancellationToken ct)
+            => Task.FromResult(new List<HistorianSample>());
+
+        public Task<List<HistorianEventDto>> ReadEventsAsync(string? src, DateTime s, DateTime e, int max, CancellationToken ct)
+            => Task.FromResult(new List<HistorianEventDto>());
+
+        public HistorianHealthSnapshot GetHealthSnapshot() => new();
+        public void Dispose() { }
+    }
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Tests/HostStatusPushTests.cs

@@ -0,0 +1,91 @@
+using System;
+using System.Collections.Concurrent;
+using System.Threading;
+using System.Threading.Tasks;
+using ArchestrA.MxAccess;
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Backend;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Backend.Galaxy;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Backend.MxAccess;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Sta;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Shared.Contracts;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Tests;
+
+[Trait("Category", "Unit")]
+public sealed class HostStatusPushTests
+{
+    /// <summary>
+    ///     PR 8 — when MxAccessClient.ConnectionStateChanged fires false→true→false,
+    ///     MxAccessGalaxyBackend raises OnHostStatusChanged once per transition with
+    ///     HostName=ClientName, RuntimeStatus="Running"/"Stopped", and a timestamp.
+    ///     This is the gateway-level signal; per-platform ScanState probes are deferred.
+    /// </summary>
+    [Fact]
+    public async Task ConnectionStateChanged_raises_OnHostStatusChanged_with_gateway_name()
+    {
+        using var pump = new StaPump("Test.Sta");
+        await pump.WaitForStartedAsync();
+        var proxy = new FakeProxy();
+        var mx = new MxAccessClient(pump, proxy, "GatewayClient", new MxAccessClientOptions { AutoReconnect = false });
+        using var backend = new MxAccessGalaxyBackend(
+            new GalaxyRepository(new GalaxyRepositoryOptions { ConnectionString = "Server=.;Database=ZB;Integrated Security=True;" }),
+            mx,
+            historian: null);
+
+        var notifications = new ConcurrentQueue<HostConnectivityStatus>();
+        backend.OnHostStatusChanged += (_, s) => notifications.Enqueue(s);
+
+        await mx.ConnectAsync();
+        await mx.DisconnectAsync();
+
+        notifications.Count.ShouldBe(2);
+        notifications.TryDequeue(out var first).ShouldBeTrue();
+        first!.HostName.ShouldBe("GatewayClient");
+        first.RuntimeStatus.ShouldBe("Running");
+        first.LastObservedUtcUnixMs.ShouldBeGreaterThan(0);
+
+        notifications.TryDequeue(out var second).ShouldBeTrue();
+        second!.HostName.ShouldBe("GatewayClient");
+        second.RuntimeStatus.ShouldBe("Stopped");
+    }
+
+    [Fact]
+    public async Task Dispose_unsubscribes_so_post_dispose_state_changes_do_not_fire_events()
+    {
+        using var pump = new StaPump("Test.Sta");
+        await pump.WaitForStartedAsync();
+        var proxy = new FakeProxy();
+        var mx = new MxAccessClient(pump, proxy, "GatewayClient", new MxAccessClientOptions { AutoReconnect = false });
+        var backend = new MxAccessGalaxyBackend(
+            new GalaxyRepository(new GalaxyRepositoryOptions { ConnectionString = "Server=.;Database=ZB;Integrated Security=True;" }),
+            mx,
+            historian: null);
+
+        var count = 0;
+        backend.OnHostStatusChanged += (_, _) => Interlocked.Increment(ref count);
+
+        await mx.ConnectAsync();
+        count.ShouldBe(1);
+
+        backend.Dispose();
+        await mx.DisconnectAsync();
+
+        count.ShouldBe(1); // no second notification after Dispose
+    }
+
+    private sealed class FakeProxy : IMxProxy
+    {
+        private int _next = 1;
+        public int Register(string _) => 42;
+        public void Unregister(int _) { }
+        public int AddItem(int _, string __) => Interlocked.Increment(ref _next);
+        public void RemoveItem(int _, int __) { }
+        public void AdviseSupervisory(int _, int __) { }
+        public void UnAdviseSupervisory(int _, int __) { }
+        public void Write(int _, int __, object ___, int ____) { }
+        public event MxDataChangeHandler? OnDataChange { add { } remove { } }
+        public event MxWriteCompleteHandler? OnWriteComplete { add { } remove { } }
+    }
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Tests/MxAccessClientMonitorLoopTests.cs

@@ -0,0 +1,173 @@
+using System;
+using System.Collections.Concurrent;
+using System.Collections.Generic;
+using System.Threading;
+using System.Threading.Tasks;
+using ArchestrA.MxAccess;
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Backend.MxAccess;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Sta;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Tests;
+
+[Trait("Category", "Unit")]
+public sealed class MxAccessClientMonitorLoopTests
+{
+    /// <summary>
+    ///     PR 6 low finding #1 — every $Heartbeat probe must RemoveItem the item handle it
+    ///     allocated. Without that, the monitor leaks one handle per MonitorInterval seconds,
+    ///     which over a 24h uptime becomes thousands of leaked MXAccess handles and can
+    ///     eventually exhaust the runtime proxy's handle table.
+    /// </summary>
+    [Fact]
+    public async Task Heartbeat_probe_calls_RemoveItem_for_every_AddItem()
+    {
+        using var pump = new StaPump("Monitor.Sta");
+        await pump.WaitForStartedAsync();
+
+        var proxy = new CountingProxy();
+        var client = new MxAccessClient(pump, proxy, "probe-test", new MxAccessClientOptions
+        {
+            AutoReconnect = true,
+            MonitorInterval = TimeSpan.FromMilliseconds(150),
+            StaleThreshold = TimeSpan.FromMilliseconds(50),
+        });
+
+        await client.ConnectAsync();
+
+        // Wait past StaleThreshold, then let several monitor cycles fire.
+        await Task.Delay(700);
+
+        client.Dispose();
+
+        // One Heartbeat probe fires per monitor tick once the connection looks stale.
+        proxy.HeartbeatAddCount.ShouldBeGreaterThan(1);
+        // Every AddItem("$Heartbeat") must be matched by a RemoveItem on the same handle.
+        proxy.HeartbeatAddCount.ShouldBe(proxy.HeartbeatRemoveCount);
+        proxy.OutstandingHeartbeatHandles.ShouldBe(0);
+    }
+
+    /// <summary>
+    ///     PR 6 low finding #2 — after reconnect, per-subscription replay failures must raise
+    ///     SubscriptionReplayFailed so the backend can propagate the degradation, not get
+    ///     silently eaten.
+    /// </summary>
+    [Fact]
+    public async Task SubscriptionReplayFailed_fires_for_each_tag_that_fails_to_replay()
+    {
+        using var pump = new StaPump("Replay.Sta");
+        await pump.WaitForStartedAsync();
+
+        var proxy = new ReplayFailingProxy(failOnReplayForTags: new[] { "BadTag.A", "BadTag.B" });
+        var client = new MxAccessClient(pump, proxy, "replay-test", new MxAccessClientOptions
+        {
+            AutoReconnect = true,
+            MonitorInterval = TimeSpan.FromMilliseconds(120),
+            StaleThreshold = TimeSpan.FromMilliseconds(50),
+        });
+
+        var failures = new ConcurrentBag<SubscriptionReplayFailedEventArgs>();
+        client.SubscriptionReplayFailed += (_, e) => failures.Add(e);
+
+        await client.ConnectAsync();
+        await client.SubscribeAsync("GoodTag.X", (_, _) => { });
+        await client.SubscribeAsync("BadTag.A", (_, _) => { });
+        await client.SubscribeAsync("BadTag.B", (_, _) => { });
+
+        proxy.TriggerProbeFailureOnNextCall();
+
+        // Wait for the monitor loop to probe → fail → reconnect → replay.
+        await Task.Delay(800);
+
+        client.Dispose();
+
+        failures.Count.ShouldBe(2);
+        var names = new HashSet<string>();
+        foreach (var f in failures) names.Add(f.TagReference);
+        names.ShouldContain("BadTag.A");
+        names.ShouldContain("BadTag.B");
+    }
+
+    // ----- test doubles -----
+
+    private sealed class CountingProxy : IMxProxy
+    {
+        private int _next = 1;
+        private readonly ConcurrentDictionary<int, string> _live = new();
+
+        public int HeartbeatAddCount;
+        public int HeartbeatRemoveCount;
+        public int OutstandingHeartbeatHandles => _live.Count;
+
+        public event MxDataChangeHandler? OnDataChange { add { } remove { } }
+        public event MxWriteCompleteHandler? OnWriteComplete { add { } remove { } }
+
+        public int Register(string _) => 42;
+        public void Unregister(int _) { }
+
+        public int AddItem(int _, string address)
+        {
+            var h = Interlocked.Increment(ref _next);
+            _live[h] = address;
+            if (address == "$Heartbeat") Interlocked.Increment(ref HeartbeatAddCount);
+            return h;
+        }
+
+        public void RemoveItem(int _, int itemHandle)
+        {
+            if (_live.TryRemove(itemHandle, out var addr) && addr == "$Heartbeat")
+                Interlocked.Increment(ref HeartbeatRemoveCount);
+        }
+
+        public void AdviseSupervisory(int _, int __) { }
+        public void UnAdviseSupervisory(int _, int __) { }
+        public void Write(int _, int __, object ___, int ____) { }
+    }
+
+    /// <summary>
+    ///     Mock that lets us exercise the reconnect + replay path. TriggerProbeFailureOnNextCall
+    ///     flips a one-shot flag so the very next AddItem("$Heartbeat") throws — that drives the
+    ///     monitor loop into the reconnect-with-replay branch. During the replay, AddItem for the
+    ///     tags listed in failOnReplayForTags throws so SubscriptionReplayFailed should fire once
+    ///     per failing tag.
+    /// </summary>
+    private sealed class ReplayFailingProxy : IMxProxy
+    {
+        private int _next = 1;
+        private readonly HashSet<string> _failOnReplay;
+        private int _probeFailOnce;
+        private readonly ConcurrentDictionary<string, bool> _replayedOnce = new(StringComparer.OrdinalIgnoreCase);
+
+        public ReplayFailingProxy(IEnumerable<string> failOnReplayForTags)
+        {
+            _failOnReplay = new HashSet<string>(failOnReplayForTags, StringComparer.OrdinalIgnoreCase);
+        }
+
+        public void TriggerProbeFailureOnNextCall() => Interlocked.Exchange(ref _probeFailOnce, 1);
+
+        public event MxDataChangeHandler? OnDataChange { add { } remove { } }
+        public event MxWriteCompleteHandler? OnWriteComplete { add { } remove { } }
+
+        public int Register(string _) => 42;
+        public void Unregister(int _) { }
+
+        public int AddItem(int _, string address)
+        {
+            if (address == "$Heartbeat" && Interlocked.Exchange(ref _probeFailOnce, 0) == 1)
+                throw new InvalidOperationException("simulated probe failure");
+
+            // Fail only on the *replay* AddItem for listed tags — not the initial subscribe.
+            if (_failOnReplay.Contains(address) && _replayedOnce.ContainsKey(address))
+                throw new InvalidOperationException($"simulated replay failure for {address}");
+
+            if (_failOnReplay.Contains(address)) _replayedOnce[address] = true;
+            return Interlocked.Increment(ref _next);
+        }
+
+        public void RemoveItem(int _, int __) { }
+        public void AdviseSupervisory(int _, int __) { }
+        public void UnAdviseSupervisory(int _, int __) { }
+        public void Write(int _, int __, object ___, int ____) { }
+    }
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Tests/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.Tests.csproj

@@ -24,6 +24,11 @@
     <ItemGroup>
         <ProjectReference Include="..\..\src\ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host\ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Host.csproj"/>
         <Reference Include="System.ServiceProcess"/>
+        <!-- IMxProxy's delegate signatures mention ArchestrA.MxAccess.MXSTATUS_PROXY, so tests
+             implementing the interface must resolve that type at compile time. -->
+        <Reference Include="ArchestrA.MxAccess">
+            <HintPath>..\..\lib\ArchestrA.MxAccess.dll</HintPath>
+        </Reference>
     </ItemGroup>
 
     <ItemGroup>

tests/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Proxy.Tests/AggregateColumnMappingTests.cs

@@ -0,0 +1,27 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Proxy;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Proxy.Tests;
+
+[Trait("Category", "Unit")]
+public sealed class AggregateColumnMappingTests
+{
+    [Theory]
+    [InlineData(HistoryAggregateType.Average, "Average")]
+    [InlineData(HistoryAggregateType.Minimum, "Minimum")]
+    [InlineData(HistoryAggregateType.Maximum, "Maximum")]
+    [InlineData(HistoryAggregateType.Count, "ValueCount")]
+    public void Maps_OpcUa_enum_to_AnalogSummary_column(HistoryAggregateType aggregate, string expected)
+    {
+        GalaxyProxyDriver.MapAggregateToColumn(aggregate).ShouldBe(expected);
+    }
+
+    [Fact]
+    public void Total_is_not_supported()
+    {
+        Should.Throw<System.NotSupportedException>(
+            () => GalaxyProxyDriver.MapAggregateToColumn(HistoryAggregateType.Total));
+    }
+}