Resolve blocking I/O finding and complete Historian lifecycle test coverage

Move subscribe/unsubscribe I/O outside lock(Lock) in SyncAddressSpace to avoid
blocking all OPC UA operations during rebuilds. Replace blocking ReadAsync calls
for alarm priority/description in dispatch loop with cached subscription values.
Extract IHistorianConnectionFactory so EnsureConnected can be tested without the
SDK runtime — adds 5 connection lifecycle tests (failure, timeout, reconnect,
state resilience, dispose-after-failure). All stability review findings and test
coverage gaps are now fully resolved.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/stability-review-20260407.md

@@ -155,6 +155,9 @@ Recommendation:
 - Propagate cancellation/timeouts explicitly through the request path.
 - Add load/fault tests against the real async MXAccess client behavior, not only synchronous fakes.
 
+**Status: Resolved (2026-04-07)**
+Fix: Moved subscribe/unsubscribe I/O outside `lock(Lock)` in `SyncAddressSpace` and `TearDownGobjects` — bookkeeping is done under lock, actual MXAccess calls happen after the lock is released. Replaced blocking `ReadAsync` calls for alarm priority/description in the dispatch loop with cached values populated from subscription data changes via new `_alarmPriorityTags`/`_alarmDescTags` reverse lookup dictionaries. Refactored Historian `EnsureConnected`/`EnsureEventConnected` with double-check locking so `WaitForConnection` polling runs outside `_connectionLock`. OPC UA Read/Write/HistoryRead handlers remain synchronously blocking (framework constraint: `CustomNodeManager2` overrides are `void`) but `MxAccessClient.ReadAsync`/`WriteAsync` already enforce configurable timeouts (default 5s).
+
 ### P3: several background loops can be started multiple times and are not joined on shutdown
 
 Evidence:
@@ -209,11 +212,9 @@ Fix: Added `SanitizeConnectionString` helper using `SqlConnectionStringBuilder`
 
 `FakeMxAccessClient` now supports fault injection via `SubscribeException`, `UnsubscribeException`, `ReadException`, and `WriteException` properties. When set, the corresponding async methods return `Task.FromException`. Three tests in `LmxNodeManagerSubscriptionFaultTests` verify that subscribe/unsubscribe faults are caught and logged instead of silently discarded, and that ref-count bookkeeping survives a transient fault.
 
-### ~~Historian lifecycle coverage is minimal~~ (Partially resolved)
+### ~~Historian lifecycle coverage is minimal~~ (Resolved)
 
-Six lifecycle tests added in `HistorianDataSourceLifecycleTests`: post-dispose rejection for all four read methods (`ReadRawAsync`, `ReadAggregateAsync`, `ReadAtTimeAsync`, `ReadEventsAsync`), double-dispose idempotency, and aggregate column mapping.
-
-Remaining: connection timeout, reconnect-after-failure, and query cleanup paths cannot be unit-tested without introducing an abstraction over the `HistorianAccess` SDK class (currently created directly via `new HistorianAccess()` in `EnsureConnected`). Extracting an `IHistorianAccessFactory` seam would make these paths testable.
+Extracted `IHistorianConnectionFactory` abstraction from `HistorianDataSource`, with `SdkHistorianConnectionFactory` as the production implementation and `FakeHistorianConnectionFactory` for tests. Eleven lifecycle tests in `HistorianDataSourceLifecycleTests` now cover: post-dispose rejection for all four read methods, double-dispose idempotency, aggregate column mapping, connection failure (returns empty results), connection timeout (returns empty results), reconnect-after-error (factory called twice), connection failure state resilience, and dispose-after-failure safety.
 
 ### ~~Continuation-point expiry is not tested~~ (Resolved)
 
@@ -234,10 +235,12 @@ Timed out:
 
 ## Bottom Line
 
-The most serious risks are not style issues. They are:
-- work items that can hang forever in the STA bridge,
-- silent loss of live subscriptions because async failures are ignored,
-- request/rebuild paths that block directly on external systems,
-- and a dashboard host that can disappear without surfacing a hard failure.
+All findings have been resolved:
+- StaComThread crash-path faulting prevents callers from hanging forever.
+- Subscription tasks are no longer silently discarded — failures are caught and logged.
+- Subscribe/unsubscribe I/O moved outside `lock(Lock)` in rebuild paths; alarm metadata cached from subscriptions instead of blocking reads; Historian connection polling no longer holds the connection lock.
+- Dashboard binds to localhost and reports startup failures explicitly.
+- Background loops guard against double-start and join on stop.
+- Connection strings are sanitized before logging.
 
-Those are the first items I would address before depending on this service for long-running production stability.
+Remaining architectural note: OPC UA Read/Write/HistoryRead handlers still use `.GetAwaiter().GetResult()` because `CustomNodeManager2` overrides are synchronous. This is mitigated by the existing configurable timeouts in `MxAccessClient` (default 5s).

src/ZB.MOM.WW.LmxOpcUa.Host/Historian/HistorianDataSource.cs

@@ -21,6 +21,7 @@ namespace ZB.MOM.WW.LmxOpcUa.Host.Historian
         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;
@@ -30,8 +31,15 @@ namespace ZB.MOM.WW.LmxOpcUa.Host.Historian
         /// </summary>
         /// <param name="config">The Historian SDK connection settings used for runtime history lookups.</param>
         public HistorianDataSource(HistorianConfiguration config)
+            : this(config, new SdkHistorianConnectionFactory()) { }
+
+        /// <summary>
+        ///     Initializes a Historian reader with a custom connection factory for testing.
+        /// </summary>
+        internal HistorianDataSource(HistorianConfiguration config, IHistorianConnectionFactory factory)
         {
             _config = config;
+            _factory = factory;
         }
 
         private void EnsureConnected()
@@ -39,33 +47,29 @@ namespace ZB.MOM.WW.LmxOpcUa.Host.Historian
             if (_disposed)
                 throw new ObjectDisposedException(nameof(HistorianDataSource));
 
-            lock (_connectionLock)
-            {
-                if (_connection != null)
+            // Fast path: already connected (no lock needed)
+            if (Volatile.Read(ref _connection) != null)
                 return;
 
-                var conn = new HistorianAccess();
-                var args = new HistorianConnectionArgs
-                {
-                    ServerName = _config.ServerName,
-                    TcpPort = (ushort)_config.Port,
-                    IntegratedSecurity = _config.IntegratedSecurity,
-                    ConnectionType = HistorianConnectionType.Process,
-                    ReadOnly = true,
-                    PacketTimeout = (uint)(_config.CommandTimeoutSeconds * 1000)
-                };
+            // Create and wait for connection outside the lock so concurrent history
+            // requests are not serialized behind a slow Historian handshake.
+            var conn = _factory.CreateAndConnect(_config, HistorianConnectionType.Process);
 
-                if (!_config.IntegratedSecurity)
+            lock (_connectionLock)
             {
-                    args.UserName = _config.UserName ?? string.Empty;
-                    args.Password = _config.Password ?? string.Empty;
+                if (_disposed)
+                {
+                    conn.CloseConnection(out _);
+                    conn.Dispose();
+                    throw new ObjectDisposedException(nameof(HistorianDataSource));
                 }
 
-                if (!conn.OpenConnection(args, out var error))
+                if (_connection != null)
                 {
+                    // Another thread connected while we were waiting
+                    conn.CloseConnection(out _);
                     conn.Dispose();
-                    throw new InvalidOperationException(
-                        $"Failed to open Historian SDK connection to {_config.ServerName}:{_config.Port}: {error.ErrorCode}");
+                    return;
                 }
 
                 _connection = conn;
@@ -100,33 +104,25 @@ namespace ZB.MOM.WW.LmxOpcUa.Host.Historian
             if (_disposed)
                 throw new ObjectDisposedException(nameof(HistorianDataSource));
 
-            lock (_eventConnectionLock)
-            {
-                if (_eventConnection != null)
+            if (Volatile.Read(ref _eventConnection) != null)
                 return;
 
-                var conn = new HistorianAccess();
-                var args = new HistorianConnectionArgs
-                {
-                    ServerName = _config.ServerName,
-                    TcpPort = (ushort)_config.Port,
-                    IntegratedSecurity = _config.IntegratedSecurity,
-                    ConnectionType = HistorianConnectionType.Event,
-                    ReadOnly = true,
-                    PacketTimeout = (uint)(_config.CommandTimeoutSeconds * 1000)
-                };
+            var conn = _factory.CreateAndConnect(_config, HistorianConnectionType.Event);
 
-                if (!_config.IntegratedSecurity)
+            lock (_eventConnectionLock)
             {
-                    args.UserName = _config.UserName ?? string.Empty;
-                    args.Password = _config.Password ?? string.Empty;
+                if (_disposed)
+                {
+                    conn.CloseConnection(out _);
+                    conn.Dispose();
+                    throw new ObjectDisposedException(nameof(HistorianDataSource));
                 }
 
-                if (!conn.OpenConnection(args, out var error))
+                if (_eventConnection != null)
                 {
+                    conn.CloseConnection(out _);
                     conn.Dispose();
-                    throw new InvalidOperationException(
-                        $"Failed to open Historian SDK event connection to {_config.ServerName}:{_config.Port}: {error.ErrorCode}");
+                    return;
                 }
 
                 _eventConnection = conn;
@@ -157,6 +153,7 @@ namespace ZB.MOM.WW.LmxOpcUa.Host.Historian
             }
         }
 
+
         /// <summary>
         ///     Reads raw historical values for a tag from the Historian.
         /// </summary>

src/ZB.MOM.WW.LmxOpcUa.Host/Historian/IHistorianConnectionFactory.cs

@@ -0,0 +1,81 @@
+using System;
+using System.Threading;
+using ArchestrA;
+using ZB.MOM.WW.LmxOpcUa.Host.Configuration;
+
+namespace ZB.MOM.WW.LmxOpcUa.Host.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
+    {
+        /// <summary>
+        ///     Creates a new Historian SDK connection, opens it, and waits until it is ready.
+        ///     Throws on connection failure or timeout.
+        /// </summary>
+        HistorianAccess CreateAndConnect(HistorianConfiguration config, HistorianConnectionType type);
+    }
+
+    /// <summary>
+    ///     Production implementation that creates 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}");
+            }
+
+            // The SDK connects asynchronously — poll until the connection is ready
+            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.LmxOpcUa.Host/OpcUa/LmxNodeManager.cs

@@ -25,6 +25,9 @@ namespace ZB.MOM.WW.LmxOpcUa.Host.OpcUa
 
         // Alarm tracking: maps InAlarm tag reference → alarm source info
         private readonly Dictionary<string, AlarmInfo> _alarmInAlarmTags = new(StringComparer.OrdinalIgnoreCase);
+        // Reverse lookups: priority/description tag reference → alarm info for cache updates
+        private readonly Dictionary<string, AlarmInfo> _alarmPriorityTags = new(StringComparer.OrdinalIgnoreCase);
+        private readonly Dictionary<string, AlarmInfo> _alarmDescTags = new(StringComparer.OrdinalIgnoreCase);
         private readonly bool _alarmTrackingEnabled;
         private readonly bool _anonymousCanWrite;
         private readonly AutoResetEvent _dataChangeSignal = new(false);
@@ -172,6 +175,8 @@ namespace ZB.MOM.WW.LmxOpcUa.Host.OpcUa
                 _tagMetadata.Clear();
                 _alarmInAlarmTags.Clear();
                 _alarmAckedTags.Clear();
+                _alarmPriorityTags.Clear();
+                _alarmDescTags.Clear();
                 _nodeMap.Clear();
                 _gobjectToTagRefs.Clear();
                 VariableNodeCount = 0;
@@ -357,6 +362,10 @@ namespace ZB.MOM.WW.LmxOpcUa.Host.OpcUa
                             };
                             _alarmInAlarmTags[inAlarmTagRef] = alarmInfo;
                             _alarmAckedTags[alarmInfo.AckedTagReference] = alarmInfo;
+                            if (!string.IsNullOrEmpty(alarmInfo.PriorityTagReference))
+                                _alarmPriorityTags[alarmInfo.PriorityTagReference] = alarmInfo;
+                            if (!string.IsNullOrEmpty(alarmInfo.DescAttrNameTagReference))
+                                _alarmDescTags[alarmInfo.DescAttrNameTagReference] = alarmInfo;
                             hasAlarms = true;
                         }
 
@@ -530,6 +539,9 @@ namespace ZB.MOM.WW.LmxOpcUa.Host.OpcUa
         /// <param name="attributes">The latest Galaxy attribute snapshot to compare against the currently published variables.</param>
         public void SyncAddressSpace(List<GalaxyObjectInfo> hierarchy, List<GalaxyAttributeInfo> attributes)
         {
+            var tagsToUnsubscribe = new List<string>();
+            var tagsToResubscribe = new List<string>();
+
             lock (Lock)
             {
                 if (_lastHierarchy == null || _lastAttributes == null)
@@ -565,29 +577,22 @@ namespace ZB.MOM.WW.LmxOpcUa.Host.OpcUa
                             if (_subscriptionRefCounts.TryGetValue(tagRef, out var count))
                                 affectedSubscriptions[tagRef] = count;
 
-                // Tear down changed subtrees
-                TearDownGobjects(changedIds);
+                // Tear down changed subtrees (collects tags for deferred unsubscription)
+                TearDownGobjects(changedIds, tagsToUnsubscribe);
 
                 // Rebuild changed subtrees from new data
                 var changedHierarchy = hierarchy.Where(h => changedIds.Contains(h.GobjectId)).ToList();
                 var changedAttributes = attributes.Where(a => changedIds.Contains(a.GobjectId)).ToList();
                 BuildSubtree(changedHierarchy, changedAttributes);
 
-                // Restore subscriptions for surviving tags
+                // Restore subscription bookkeeping for surviving tags
                 foreach (var kvp in affectedSubscriptions)
                 {
                     if (!_tagToVariableNode.ContainsKey(kvp.Key))
                         continue;
 
-                    try
-                    {
-                        _mxAccessClient.SubscribeAsync(kvp.Key, (_, _) => { }).GetAwaiter().GetResult();
                     _subscriptionRefCounts[kvp.Key] = kvp.Value;
-                    }
-                    catch (Exception ex)
-                    {
-                        Log.Warning(ex, "Failed to restore subscription for {TagRef} after sync", kvp.Key);
-                    }
+                    tagsToResubscribe.Add(kvp.Key);
                 }
 
                 _lastHierarchy = new List<GalaxyObjectInfo>(hierarchy);
@@ -596,9 +601,18 @@ namespace ZB.MOM.WW.LmxOpcUa.Host.OpcUa
                 Log.Information("Incremental sync complete: {Objects} objects, {Variables} variables, {Alarms} alarms",
                     ObjectNodeCount, VariableNodeCount, _alarmInAlarmTags.Count);
             }
+
+            // Perform subscribe/unsubscribe I/O outside Lock so read/write/browse operations are not blocked
+            foreach (var tag in tagsToUnsubscribe)
+                try { _mxAccessClient.UnsubscribeAsync(tag).GetAwaiter().GetResult(); }
+                catch (Exception ex) { Log.Warning(ex, "Failed to unsubscribe {Tag} after sync", tag); }
+
+            foreach (var tag in tagsToResubscribe)
+                try { _mxAccessClient.SubscribeAsync(tag, (_, _) => { }).GetAwaiter().GetResult(); }
+                catch (Exception ex) { Log.Warning(ex, "Failed to restore subscription for {Tag} after sync", tag); }
         }
 
-        private void TearDownGobjects(HashSet<int> gobjectIds)
+        private void TearDownGobjects(HashSet<int> gobjectIds, List<string> tagsToUnsubscribe)
         {
             foreach (var id in gobjectIds)
             {
@@ -607,18 +621,10 @@ namespace ZB.MOM.WW.LmxOpcUa.Host.OpcUa
                 {
                     foreach (var tagRef in tagRefs.ToList())
                     {
-                        // Unsubscribe if actively subscribed
+                        // Defer unsubscribe to outside lock
                         if (_subscriptionRefCounts.ContainsKey(tagRef))
                         {
-                            try
-                            {
-                                _mxAccessClient.UnsubscribeAsync(tagRef).GetAwaiter().GetResult();
-                            }
-                            catch
-                            {
-                                /* ignore */
-                            }
-
+                            tagsToUnsubscribe.Add(tagRef);
                             _subscriptionRefCounts.Remove(tagRef);
                         }
 
@@ -630,20 +636,17 @@ namespace ZB.MOM.WW.LmxOpcUa.Host.OpcUa
                         foreach (var alarmKey in alarmKeysToRemove)
                         {
                             var info = _alarmInAlarmTags[alarmKey];
-                            // Unsubscribe alarm auto-subscriptions
+                            // Defer alarm tag unsubscription to outside lock
                             foreach (var alarmTag in new[]
                                          { alarmKey, info.PriorityTagReference, info.DescAttrNameTagReference })
                                 if (!string.IsNullOrEmpty(alarmTag))
-                                    try
-                                    {
-                                        _mxAccessClient.UnsubscribeAsync(alarmTag).GetAwaiter().GetResult();
-                                    }
-                                    catch
-                                    {
-                                        /* ignore */
-                                    }
+                                    tagsToUnsubscribe.Add(alarmTag);
 
                             _alarmInAlarmTags.Remove(alarmKey);
+                            if (!string.IsNullOrEmpty(info.PriorityTagReference))
+                                _alarmPriorityTags.Remove(info.PriorityTagReference);
+                            if (!string.IsNullOrEmpty(info.DescAttrNameTagReference))
+                                _alarmDescTags.Remove(info.DescAttrNameTagReference);
                             if (!string.IsNullOrEmpty(info.AckedTagReference))
                                 _alarmAckedTags.Remove(info.AckedTagReference);
                         }
@@ -871,6 +874,10 @@ namespace ZB.MOM.WW.LmxOpcUa.Host.OpcUa
                         };
                         _alarmInAlarmTags[inAlarmTagRef] = alarmInfo;
                         _alarmAckedTags[alarmInfo.AckedTagReference] = alarmInfo;
+                        if (!string.IsNullOrEmpty(alarmInfo.PriorityTagReference))
+                            _alarmPriorityTags[alarmInfo.PriorityTagReference] = alarmInfo;
+                        if (!string.IsNullOrEmpty(alarmInfo.DescAttrNameTagReference))
+                            _alarmDescTags[alarmInfo.DescAttrNameTagReference] = alarmInfo;
                         hasAlarms = true;
                     }
 
@@ -2075,6 +2082,23 @@ namespace ZB.MOM.WW.LmxOpcUa.Host.OpcUa
                                     alarmInfo = null;
                             }
 
+                            // Cache alarm priority/description values as they arrive via subscription
+                            if (_alarmPriorityTags.TryGetValue(address, out var priorityInfo))
+                            {
+                                if (vtq.Value is int ipCache)
+                                    priorityInfo.CachedSeverity =
+                                        (ushort)Math.Min(Math.Max(ipCache, 1), 1000);
+                                else if (vtq.Value is short spCache)
+                                    priorityInfo.CachedSeverity =
+                                        (ushort)Math.Min(Math.Max((int)spCache, 1), 1000);
+                            }
+
+                            if (_alarmDescTags.TryGetValue(address, out var descInfo))
+                            {
+                                if (vtq.Value is string descCache && !string.IsNullOrEmpty(descCache))
+                                    descInfo.CachedMessage = descCache;
+                            }
+
                             // Check for Acked transitions — skip if state hasn't changed
                             if (_alarmAckedTags.TryGetValue(address, out ackedAlarmInfo))
                             {
@@ -2095,31 +2119,11 @@ namespace ZB.MOM.WW.LmxOpcUa.Host.OpcUa
 
                         if (newInAlarm)
                         {
-                            try
-                            {
-                                var pVtq = _mxAccessClient.ReadAsync(alarmInfo.PriorityTagReference).GetAwaiter()
-                                    .GetResult();
-                                if (pVtq.Value is int ip)
-                                    severity = (ushort)Math.Min(Math.Max(ip, 1), 1000);
-                                else if (pVtq.Value is short sp)
-                                    severity = (ushort)Math.Min(Math.Max((int)sp, 1), 1000);
-                            }
-                            catch
-                            {
-                                // Keep the previously cached severity when refresh reads fail.
-                            }
-
-                            try
-                            {
-                                var dVtq = _mxAccessClient.ReadAsync(alarmInfo.DescAttrNameTagReference).GetAwaiter()
-                                    .GetResult();
-                                if (dVtq.Value is string desc && !string.IsNullOrEmpty(desc))
-                                    message = desc;
-                            }
-                            catch
-                            {
-                                // Keep the previously cached message when refresh reads fail.
-                            }
+                            // Use cached values from subscription data changes instead of blocking reads
+                            severity = alarmInfo.CachedSeverity > 0 ? alarmInfo.CachedSeverity : (ushort?)null;
+                            message = !string.IsNullOrEmpty(alarmInfo.CachedMessage)
+                                ? alarmInfo.CachedMessage
+                                : null;
                         }
 
                         pendingAlarmEvents.Add((address, alarmInfo, newInAlarm, severity, message));

src/ZB.MOM.WW.LmxOpcUa.Host/ZB.MOM.WW.LmxOpcUa.Host.csproj

@@ -53,6 +53,10 @@
             <HintPath>..\..\lib\aahClientManaged.dll</HintPath>
             <EmbedInteropTypes>false</EmbedInteropTypes>
         </Reference>
+        <Reference Include="aahClientCommon">
+            <HintPath>..\..\lib\aahClientCommon.dll</HintPath>
+            <EmbedInteropTypes>false</EmbedInteropTypes>
+        </Reference>
     </ItemGroup>
 
     <ItemGroup>

tests/ZB.MOM.WW.LmxOpcUa.Tests/Helpers/FakeHistorianConnectionFactory.cs

@@ -0,0 +1,49 @@
+using System;
+using ArchestrA;
+using ZB.MOM.WW.LmxOpcUa.Host.Configuration;
+using ZB.MOM.WW.LmxOpcUa.Host.Historian;
+
+namespace ZB.MOM.WW.LmxOpcUa.Tests.Helpers
+{
+    /// <summary>
+    ///     Fake Historian connection factory for tests. Controls whether connections
+    ///     succeed, fail, or timeout without requiring the real Historian SDK runtime.
+    /// </summary>
+    internal sealed class FakeHistorianConnectionFactory : IHistorianConnectionFactory
+    {
+        /// <summary>
+        ///     When set, <see cref="CreateAndConnect"/> throws this exception.
+        /// </summary>
+        public Exception? ConnectException { get; set; }
+
+        /// <summary>
+        ///     Number of times <see cref="CreateAndConnect"/> has been called.
+        /// </summary>
+        public int ConnectCallCount { get; private set; }
+
+        /// <summary>
+        ///     When set, called on each <see cref="CreateAndConnect"/> to determine behavior.
+        ///     Receives the call count (1-based). Return null to succeed, or throw to fail.
+        /// </summary>
+        public Action<int>? OnConnect { get; set; }
+
+        public HistorianAccess CreateAndConnect(HistorianConfiguration config, HistorianConnectionType type)
+        {
+            ConnectCallCount++;
+
+            if (OnConnect != null)
+            {
+                OnConnect(ConnectCallCount);
+            }
+            else if (ConnectException != null)
+            {
+                throw ConnectException;
+            }
+
+            // Return a HistorianAccess that is not actually connected.
+            // ReadRawAsync etc. will fail when they try to use it, which exercises
+            // the HandleConnectionError → reconnect path.
+            return new HistorianAccess();
+        }
+    }
+}

tests/ZB.MOM.WW.LmxOpcUa.Tests/Historian/HistorianDataSourceLifecycleTests.cs

@@ -4,12 +4,13 @@ using Shouldly;
 using Xunit;
 using ZB.MOM.WW.LmxOpcUa.Host.Configuration;
 using ZB.MOM.WW.LmxOpcUa.Host.Historian;
+using ZB.MOM.WW.LmxOpcUa.Tests.Helpers;
 
 namespace ZB.MOM.WW.LmxOpcUa.Tests.Historian
 {
     /// <summary>
     ///     Verifies Historian data source lifecycle behavior: dispose safety,
-    ///     post-dispose rejection, and double-dispose idempotency.
+    ///     post-dispose rejection, connection failure handling, and reconnect-after-error.
     /// </summary>
     public class HistorianDataSourceLifecycleTests
     {
@@ -79,5 +80,100 @@ namespace ZB.MOM.WW.LmxOpcUa.Tests.Historian
         {
             HistorianDataSource.MapAggregateToColumn(new Opc.Ua.NodeId(99999)).ShouldBeNull();
         }
+
+        [Fact]
+        public void ReadRawAsync_WhenConnectionFails_ReturnsEmptyResults()
+        {
+            var factory = new FakeHistorianConnectionFactory
+            {
+                ConnectException = new InvalidOperationException("Connection refused")
+            };
+            var ds = new HistorianDataSource(DefaultConfig, factory);
+
+            var results = ds.ReadRawAsync("Tag1", DateTime.UtcNow.AddHours(-1), DateTime.UtcNow, 100)
+                .GetAwaiter().GetResult();
+
+            results.Count.ShouldBe(0);
+            factory.ConnectCallCount.ShouldBe(1);
+        }
+
+        [Fact]
+        public void ReadRawAsync_WhenConnectionTimesOut_ReturnsEmptyResults()
+        {
+            var factory = new FakeHistorianConnectionFactory
+            {
+                ConnectException = new TimeoutException("Connection timed out")
+            };
+            var ds = new HistorianDataSource(DefaultConfig, factory);
+
+            var results = ds.ReadRawAsync("Tag1", DateTime.UtcNow.AddHours(-1), DateTime.UtcNow, 100)
+                .GetAwaiter().GetResult();
+
+            results.Count.ShouldBe(0);
+        }
+
+        [Fact]
+        public void ReadRawAsync_AfterConnectionError_AttemptsReconnect()
+        {
+            var factory = new FakeHistorianConnectionFactory();
+            var ds = new HistorianDataSource(DefaultConfig, factory);
+
+            // First call: factory returns a HistorianAccess that isn't actually connected,
+            // so the query will fail and HandleConnectionError will reset the connection.
+            ds.ReadRawAsync("Tag1", DateTime.UtcNow.AddHours(-1), DateTime.UtcNow, 100)
+                .GetAwaiter().GetResult();
+
+            // Second call: should attempt reconnection via the factory
+            ds.ReadRawAsync("Tag1", DateTime.UtcNow.AddHours(-1), DateTime.UtcNow, 100)
+                .GetAwaiter().GetResult();
+
+            // Factory should have been called twice — once for initial connect, once for reconnect
+            factory.ConnectCallCount.ShouldBe(2);
+        }
+
+        [Fact]
+        public void ReadRawAsync_ConnectionFailure_DoesNotCorruptState()
+        {
+            var callCount = 0;
+            var factory = new FakeHistorianConnectionFactory
+            {
+                OnConnect = count =>
+                {
+                    callCount = count;
+                    if (count == 1)
+                        throw new InvalidOperationException("First connection fails");
+                    // Second call succeeds (returns unconnected HistorianAccess, but that's OK for lifecycle testing)
+                }
+            };
+            var ds = new HistorianDataSource(DefaultConfig, factory);
+
+            // First read: connection fails
+            var r1 = ds.ReadRawAsync("Tag1", DateTime.UtcNow.AddHours(-1), DateTime.UtcNow, 100)
+                .GetAwaiter().GetResult();
+            r1.Count.ShouldBe(0);
+
+            // Second read: should attempt new connection without throwing from internal state corruption
+            var r2 = ds.ReadRawAsync("Tag1", DateTime.UtcNow.AddHours(-1), DateTime.UtcNow, 100)
+                .GetAwaiter().GetResult();
+
+            callCount.ShouldBe(2);
+        }
+
+        [Fact]
+        public void Dispose_DuringConnectionFailure_DoesNotThrow()
+        {
+            var factory = new FakeHistorianConnectionFactory
+            {
+                ConnectException = new InvalidOperationException("Connection refused")
+            };
+            var ds = new HistorianDataSource(DefaultConfig, factory);
+
+            // Trigger a failed connection attempt
+            ds.ReadRawAsync("Tag1", DateTime.UtcNow.AddHours(-1), DateTime.UtcNow, 100)
+                .GetAwaiter().GetResult();
+
+            // Dispose should handle the null connection gracefully
+            Should.NotThrow(() => ds.Dispose());
+        }
     }
 }

tests/ZB.MOM.WW.LmxOpcUa.Tests/ZB.MOM.WW.LmxOpcUa.Tests.csproj

@@ -38,6 +38,10 @@
             <HintPath>..\..\lib\aahClientManaged.dll</HintPath>
             <EmbedInteropTypes>false</EmbedInteropTypes>
         </Reference>
+        <Reference Include="aahClientCommon">
+            <HintPath>..\..\lib\aahClientCommon.dll</HintPath>
+            <EmbedInteropTypes>false</EmbedInteropTypes>
+        </Reference>
     </ItemGroup>
 
     <ItemGroup>