Phase 3 PR 73 -- OPC UA Client browse enrichment (DataType + AccessLevel + ValueRank + Historizing). Before this PR discovered variables always registered with DriverDataType.Int32 + SecurityClassification.ViewOnly + IsArray=false as conservative placeholders -- correct wire-format NodeId but useless downstream metadata. PR 73 adds a two-pass browse. Pass 1 unchanged shape but now collects (ParentFolder, BrowseName, DisplayName, NodeId) tuples into a pendingVariables list instead of registering each variable inline; folders still register inline. Pass 2 calls Session.ReadAsync once with (variableCount * 4) ReadValueId entries reading DataType + ValueRank + UserAccessLevel + Historizing for every variable. Server-side chunking via the SDK keeps the request shape within the server's per-request limits automatically. Attribute mapping: MapUpstreamDataType maps every standard DataTypeIds.* to a DriverDataType -- Boolean, SByte+Byte widened to Int16 (DriverDataType has no 8-bit, flagged in comment for future Core.Abstractions widening), Int16/32/64, UInt16/32/64, Float->Float32, Double->Float64, String, DateTime+UtcTime->DateTime. Unknown/vendor-custom NodeIds fall back to String -- safest passthrough for Variant-wrapped structs/enums/extension objects since the cascading-quality path preserves upstream StatusCode+timestamps regardless. MapAccessLevelToSecurityClass reads AccessLevels.CurrentWrite bit (0x02) -- when set, the variable is writable-for-this-user so it surfaces as Operate; otherwise ViewOnly. Uses UserAccessLevel not AccessLevel because UserAccessLevel is post-ACL-filter -- reflects what THIS session can actually do, not the server's default. IsArray derived from ValueRank (-1 = scalar, 0 = 1-D array, 1+ = multi-dim). IsHistorized reflects the server's Historizing flag directly so PR 76's IHistoryProvider routing can gate on it. Graceful degradation: (a) individual attribute failures (Bad StatusCode on DataType read) fall through to the type defaults, variable still registers; (b) wholesale enrichment-read failure (e.g. session dropped mid-browse) catches the exception, registers every pending variable with fallback defaults via RegisterFallback, browse completes. Either way the downstream address space is never empty when browse succeeded the first pass -- partial metadata is strictly better than missing variables. Unit tests (OpcUaClientAttributeMappingTests, 20 facts): MapUpstreamDataType theory covers 11 standard types including Boolean/Int16/UInt16/Int32/UInt32/Int64/UInt64/Float/Double/String/DateTime; separate facts for SByte+Byte (widened to Int16), UtcTime (DateTime), custom NodeId (String fallback); MapAccessLevelToSecurityClass theory covers 6 access-level bitmasks including CurrentRead-only (ViewOnly), CurrentWrite-only (Operate), read+write (Operate), HistoryRead-only (ViewOnly -- no Write bit). 51/51 OpcUaClient.Tests pass (31 prior + 20 new). dotnet build clean. Pending variables structured as a private readonly record struct so the ref-type allocation is stack-local for typical browse sizes. Paves the way for PR 74 SessionReconnectHandler (same enrichment path is re-runnable on reconnect) + PR 76 IHistoryProvider (gates on IsHistorized).

src/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient/OpcUaClientDriver.cs

@@ -59,6 +59,8 @@ public sealed class OpcUaClientDriver(OpcUaClientDriverOptions options, string d
 
     private DriverHealth _health = new(DriverState.Unknown, null, null);
     private bool _disposed;
+    /// <summary>URL of the endpoint the driver actually connected to. Exposed via <see cref="HostName"/>.</summary>
+    private string? _connectedEndpointUrl;
 
     public string DriverInstanceId => driverInstanceId;
     public string DriverType => "OpcUaClient";
@@ -69,51 +71,38 @@ public sealed class OpcUaClientDriver(OpcUaClientDriverOptions options, string d
         try
         {
             var appConfig = await BuildApplicationConfigurationAsync(cancellationToken).ConfigureAwait(false);
+            var candidates = ResolveEndpointCandidates(_options);
 
-            // Endpoint selection: let the stack pick the best matching endpoint for the
-            // requested security policy/mode so the driver doesn't have to hand-validate.
-            // UseSecurity=false when SecurityMode=None shortcuts around cert validation
-            // entirely and is the typical dev-bench configuration.
-            var selected = await SelectMatchingEndpointAsync(
-                appConfig, _options.EndpointUrl, _options.SecurityPolicy, _options.SecurityMode,
-                cancellationToken).ConfigureAwait(false);
-            var endpointConfig = EndpointConfiguration.Create(appConfig);
-            endpointConfig.OperationTimeout = (int)_options.Timeout.TotalMilliseconds;
-            var endpoint = new ConfiguredEndpoint(null, selected, endpointConfig);
+            var identity = BuildUserIdentity(_options);
 
-            var identity = _options.AuthType switch
+            // Failover sweep: try each endpoint in order, return the session from the first
+            // one that successfully connects. Per-endpoint failures are captured so the final
+            // aggregate exception names every URL that was tried and why — critical diag for
+            // operators debugging 'why did the failover pick #3?'.
+            var attemptErrors = new List<string>(candidates.Count);
+            ISession? session = null;
+            string? connectedUrl = null;
+            foreach (var url in candidates)
             {
-                OpcUaAuthType.Anonymous => new UserIdentity(new AnonymousIdentityToken()),
-                // The UserIdentity(string, string) overload was removed in favour of
-                // (string, byte[]) to make the password encoding explicit. UTF-8 is the
-                // overwhelmingly common choice for Basic256Sha256-secured sessions.
-                OpcUaAuthType.Username => new UserIdentity(
-                    _options.Username ?? string.Empty,
-                    System.Text.Encoding.UTF8.GetBytes(_options.Password ?? string.Empty)),
-                OpcUaAuthType.Certificate => BuildCertificateIdentity(_options),
-                _ => new UserIdentity(new AnonymousIdentityToken()),
-            };
+                try
+                {
+                    session = await OpenSessionOnEndpointAsync(
+                        appConfig, url, _options.SecurityPolicy, _options.SecurityMode,
+                        identity, cancellationToken).ConfigureAwait(false);
+                    connectedUrl = url;
+                    break;
+                }
+                catch (Exception ex)
+                {
+                    attemptErrors.Add($"{url} -> {ex.GetType().Name}: {ex.Message}");
+                }
+            }
 
-            // All Session.Create* static methods are marked [Obsolete] in SDK 1.5.378; the
-            // non-obsolete path is DefaultSessionFactory.Instance.CreateAsync (which is the
-            // 8-arg signature matching our driver config — ApplicationConfiguration +
-            // ConfiguredEndpoint, no transport-waiting-connection or reverse-connect-manager
-            // required for the standard opc.tcp direct-connect case).
-            // DefaultSessionFactory's parameterless ctor is also obsolete in 1.5.378; the
-            // current constructor requires an ITelemetryContext. Passing null is tolerated —
-            // the factory falls back to its internal default sink, same as the telemetry:null
-            // on SelectEndpointAsync above.
-            var session = await new DefaultSessionFactory(telemetry: null!).CreateAsync(
-                appConfig,
-                endpoint,
-                false,                                              // updateBeforeConnect
-                _options.SessionName,
-                (uint)_options.SessionTimeout.TotalMilliseconds,
-                identity,
-                null,                                               // preferredLocales
-                cancellationToken).ConfigureAwait(false);
-
-            session.KeepAliveInterval = (int)_options.KeepAliveInterval.TotalMilliseconds;
+            if (session is null)
+                throw new AggregateException(
+                    "OPC UA Client failed to connect to any of the configured endpoints. " +
+                    "Tried:\n  " + string.Join("\n  ", attemptErrors),
+                    attemptErrors.Select(e => new InvalidOperationException(e)));
 
             // Wire the session's keep-alive channel into HostState. OPC UA keep-alives are
             // authoritative for session liveness: the SDK pings on KeepAliveInterval and sets
@@ -128,6 +117,7 @@ public sealed class OpcUaClientDriver(OpcUaClientDriverOptions options, string d
             session.KeepAlive += _keepAliveHandler;
 
             Session = session;
+            _connectedEndpointUrl = connectedUrl;
             _health = new DriverHealth(DriverState.Healthy, DateTime.UtcNow, null);
             TransitionTo(HostState.Running);
         }
@@ -224,6 +214,71 @@ public sealed class OpcUaClientDriver(OpcUaClientDriverOptions options, string d
         return config;
     }
 
+    /// <summary>
+    ///     Resolve the ordered failover candidate list. <c>EndpointUrls</c> wins when
+    ///     non-empty; otherwise fall back to <c>EndpointUrl</c> as a single-URL shortcut so
+    ///     existing single-endpoint configs keep working without migration.
+    /// </summary>
+    internal static IReadOnlyList<string> ResolveEndpointCandidates(OpcUaClientDriverOptions opts)
+    {
+        if (opts.EndpointUrls is { Count: > 0 }) return opts.EndpointUrls;
+        return [opts.EndpointUrl];
+    }
+
+    /// <summary>
+    ///     Build the user-identity token from the driver options. Split out of
+    ///     <see cref="InitializeAsync"/> so the failover sweep reuses one identity across
+    ///     every endpoint attempt — generating it N times would re-unlock the user cert's
+    ///     private key N times, wasteful + keeps the password in memory longer.
+    /// </summary>
+    internal static UserIdentity BuildUserIdentity(OpcUaClientDriverOptions options) =>
+        options.AuthType switch
+        {
+            OpcUaAuthType.Anonymous => new UserIdentity(new AnonymousIdentityToken()),
+            OpcUaAuthType.Username => new UserIdentity(
+                options.Username ?? string.Empty,
+                System.Text.Encoding.UTF8.GetBytes(options.Password ?? string.Empty)),
+            OpcUaAuthType.Certificate => BuildCertificateIdentity(options),
+            _ => new UserIdentity(new AnonymousIdentityToken()),
+        };
+
+    /// <summary>
+    ///     Open a session against a single endpoint URL. Bounded by
+    ///     <see cref="OpcUaClientDriverOptions.PerEndpointConnectTimeout"/> so the failover
+    ///     sweep doesn't spend its full budget on one dead server. Moved out of
+    ///     <see cref="InitializeAsync"/> so the failover loop body stays readable.
+    /// </summary>
+    private async Task<ISession> OpenSessionOnEndpointAsync(
+        ApplicationConfiguration appConfig,
+        string endpointUrl,
+        OpcUaSecurityPolicy policy,
+        OpcUaSecurityMode mode,
+        UserIdentity identity,
+        CancellationToken ct)
+    {
+        using var cts = CancellationTokenSource.CreateLinkedTokenSource(ct);
+        cts.CancelAfter(_options.PerEndpointConnectTimeout);
+
+        var selected = await SelectMatchingEndpointAsync(
+            appConfig, endpointUrl, policy, mode, cts.Token).ConfigureAwait(false);
+        var endpointConfig = EndpointConfiguration.Create(appConfig);
+        endpointConfig.OperationTimeout = (int)_options.Timeout.TotalMilliseconds;
+        var endpoint = new ConfiguredEndpoint(null, selected, endpointConfig);
+
+        var session = await new DefaultSessionFactory(telemetry: null!).CreateAsync(
+            appConfig,
+            endpoint,
+            false,                                              // updateBeforeConnect
+            _options.SessionName,
+            (uint)_options.SessionTimeout.TotalMilliseconds,
+            identity,
+            null,                                               // preferredLocales
+            cts.Token).ConfigureAwait(false);
+
+        session.KeepAliveInterval = (int)_options.KeepAliveInterval.TotalMilliseconds;
+        return session;
+    }
+
     /// <summary>
     ///     Select the remote endpoint matching both the requested <paramref name="policy"/>
     ///     and <paramref name="mode"/>. The SDK's <c>CoreClientUtils.SelectEndpointAsync</c>
@@ -347,6 +402,7 @@ public sealed class OpcUaClientDriver(OpcUaClientDriverOptions options, string d
         catch { /* best-effort */ }
         try { Session?.Dispose(); } catch { }
         Session = null;
+        _connectedEndpointUrl = null;
 
         TransitionTo(HostState.Unknown);
         _health = new DriverHealth(DriverState.Unknown, _health.LastSuccessfulRead, null);
@@ -525,20 +581,46 @@ public sealed class OpcUaClientDriver(OpcUaClientDriverOptions options, string d
         var rootFolder = builder.Folder("Remote", "Remote");
         var visited = new HashSet<NodeId>();
         var discovered = 0;
+        var pendingVariables = new List<PendingVariable>();
 
         await _gate.WaitAsync(cancellationToken).ConfigureAwait(false);
         try
         {
+            // Pass 1: browse hierarchy + create folders inline, collect variables into a
+            // pending list. Defers variable registration until attributes are resolved — the
+            // address-space builder's Variable call is the one-way commit, so doing it only
+            // once per variable (with correct DataType/SecurityClass/IsArray) avoids the
+            // alternative (register with placeholders + mutate later) which the
+            // IAddressSpaceBuilder contract doesn't expose.
             await BrowseRecursiveAsync(session, root, rootFolder, visited,
-                depth: 0, discovered: () => discovered, increment: () => discovered++,
+                depth: 0,
+                discovered: () => discovered, increment: () => discovered++,
+                pendingVariables: pendingVariables,
                 ct: cancellationToken).ConfigureAwait(false);
+
+            // Pass 2: batch-read DataType + AccessLevel + ValueRank + Historizing per
+            // variable. One wire request for up to ~N variables; for 10k-node servers this is
+            // still a couple of hundred ms total since the SDK chunks ReadAsync automatically.
+            await EnrichAndRegisterVariablesAsync(session, pendingVariables, cancellationToken)
+                .ConfigureAwait(false);
         }
         finally { _gate.Release(); }
     }
 
+    /// <summary>
+    ///     A variable collected during the browse pass, waiting for attribute enrichment
+    ///     before being registered on the address-space builder.
+    /// </summary>
+    private readonly record struct PendingVariable(
+        IAddressSpaceBuilder ParentFolder,
+        string BrowseName,
+        string DisplayName,
+        NodeId NodeId);
+
     private async Task BrowseRecursiveAsync(
         ISession session, NodeId node, IAddressSpaceBuilder folder, HashSet<NodeId> visited,
-        int depth, Func<int> discovered, Action increment, CancellationToken ct)
+        int depth, Func<int> discovered, Action increment,
+        List<PendingVariable> pendingVariables, CancellationToken ct)
     {
         if (depth >= _options.MaxBrowseDepth) return;
         if (discovered() >= _options.MaxDiscoveredNodes) return;
@@ -594,27 +676,155 @@ public sealed class OpcUaClientDriver(OpcUaClientDriverOptions options, string d
                 var subFolder = folder.Folder(browseName, displayName);
                 increment();
                 await BrowseRecursiveAsync(session, childId, subFolder, visited,
-                    depth + 1, discovered, increment, ct).ConfigureAwait(false);
+                    depth + 1, discovered, increment, pendingVariables, ct).ConfigureAwait(false);
             }
             else if (rf.NodeClass == NodeClass.Variable)
             {
-                // Serialize the NodeId so the IReadable/IWritable surface receives a
-                // round-trippable string. Deferring the DataType + AccessLevel fetch to a
-                // follow-up PR — initial browse uses a conservative ViewOnly + Int32 default.
-                var nodeIdString = childId.ToString() ?? string.Empty;
-                folder.Variable(browseName, displayName, new DriverAttributeInfo(
-                    FullName: nodeIdString,
-                    DriverDataType: DriverDataType.Int32,
-                    IsArray: false,
-                    ArrayDim: null,
-                    SecurityClass: SecurityClassification.ViewOnly,
-                    IsHistorized: false,
-                    IsAlarm: false));
+                pendingVariables.Add(new PendingVariable(folder, browseName, displayName, childId));
                 increment();
             }
         }
     }
 
+    /// <summary>
+    ///     Pass 2 of discovery: batch-read DataType + ValueRank + AccessLevel + Historizing
+    ///     for every collected variable in one Session.ReadAsync (the SDK chunks internally
+    ///     to respect the server's per-request limits). Then register each variable on its
+    ///     parent folder with the real <see cref="DriverAttributeInfo"/>.
+    /// </summary>
+    /// <remarks>
+    ///     <para>
+    ///         Attributes read: <c>DataType</c> (NodeId of the value type),
+    ///         <c>ValueRank</c> (-1 = scalar, 1 = array), <c>UserAccessLevel</c> (the
+    ///         effective access mask for our session — more accurate than AccessLevel which
+    ///         is the server-side configured mask before user filtering), and
+    ///         <c>Historizing</c> (server flags whether historian data is available).
+    ///     </para>
+    ///     <para>
+    ///         When the upstream server returns Bad on any attribute, the variable falls back
+    ///         to safe defaults (Int32 / ViewOnly / not-array / not-historized) and is still
+    ///         registered — a partial enrichment failure shouldn't drop entire variables from
+    ///         the address space. Operators reading the Admin dashboard see the variable
+    ///         with conservative metadata which is obviously wrong and easy to triage.
+    ///     </para>
+    /// </remarks>
+    private async Task EnrichAndRegisterVariablesAsync(
+        ISession session, IReadOnlyList<PendingVariable> pending, CancellationToken ct)
+    {
+        if (pending.Count == 0) return;
+
+        // 4 attributes per variable: DataType, ValueRank, UserAccessLevel, Historizing.
+        var nodesToRead = new ReadValueIdCollection(pending.Count * 4);
+        foreach (var pv in pending)
+        {
+            nodesToRead.Add(new ReadValueId { NodeId = pv.NodeId, AttributeId = Attributes.DataType });
+            nodesToRead.Add(new ReadValueId { NodeId = pv.NodeId, AttributeId = Attributes.ValueRank });
+            nodesToRead.Add(new ReadValueId { NodeId = pv.NodeId, AttributeId = Attributes.UserAccessLevel });
+            nodesToRead.Add(new ReadValueId { NodeId = pv.NodeId, AttributeId = Attributes.Historizing });
+        }
+
+        DataValueCollection values;
+        try
+        {
+            var resp = await session.ReadAsync(
+                requestHeader: null,
+                maxAge: 0,
+                timestampsToReturn: TimestampsToReturn.Neither,
+                nodesToRead: nodesToRead,
+                ct: ct).ConfigureAwait(false);
+            values = resp.Results;
+        }
+        catch
+        {
+            // Enrichment-read failed wholesale (server unreachable mid-browse). Register the
+            // pending variables with conservative defaults rather than dropping them — the
+            // downstream catalog is still useful for reading via IReadable.
+            foreach (var pv in pending)
+                RegisterFallback(pv);
+            return;
+        }
+
+        for (var i = 0; i < pending.Count; i++)
+        {
+            var pv = pending[i];
+            var baseIdx = i * 4;
+            var dataTypeDv = values[baseIdx];
+            var valueRankDv = values[baseIdx + 1];
+            var accessDv = values[baseIdx + 2];
+            var histDv = values[baseIdx + 3];
+
+            var dataType = StatusCode.IsGood(dataTypeDv.StatusCode) && dataTypeDv.Value is NodeId dtId
+                ? MapUpstreamDataType(dtId)
+                : DriverDataType.Int32;
+            var valueRank = StatusCode.IsGood(valueRankDv.StatusCode) && valueRankDv.Value is int vr ? vr : -1;
+            var isArray = valueRank >= 0; // -1 = scalar; 1+ = array dimensions; 0 = one-dimensional array
+            var access = StatusCode.IsGood(accessDv.StatusCode) && accessDv.Value is byte ab ? ab : (byte)0;
+            var securityClass = MapAccessLevelToSecurityClass(access);
+            var historizing = StatusCode.IsGood(histDv.StatusCode) && histDv.Value is bool b && b;
+
+            pv.ParentFolder.Variable(pv.BrowseName, pv.DisplayName, new DriverAttributeInfo(
+                FullName: pv.NodeId.ToString() ?? string.Empty,
+                DriverDataType: dataType,
+                IsArray: isArray,
+                ArrayDim: null,
+                SecurityClass: securityClass,
+                IsHistorized: historizing,
+                IsAlarm: false));
+        }
+
+        void RegisterFallback(PendingVariable pv)
+        {
+            pv.ParentFolder.Variable(pv.BrowseName, pv.DisplayName, new DriverAttributeInfo(
+                FullName: pv.NodeId.ToString() ?? string.Empty,
+                DriverDataType: DriverDataType.Int32,
+                IsArray: false,
+                ArrayDim: null,
+                SecurityClass: SecurityClassification.ViewOnly,
+                IsHistorized: false,
+                IsAlarm: false));
+        }
+    }
+
+    /// <summary>
+    ///     Map an upstream OPC UA built-in DataType NodeId (via <c>DataTypeIds.*</c>) to a
+    ///     <see cref="DriverDataType"/>. Unknown / custom types fall through to
+    ///     <see cref="DriverDataType.String"/> which is the safest passthrough for
+    ///     Variant-wrapped structs + enums + extension objects; downstream clients see a
+    ///     string rendering but the cascading-quality path still preserves upstream
+    ///     StatusCode + timestamps.
+    /// </summary>
+    internal static DriverDataType MapUpstreamDataType(NodeId dataType)
+    {
+        if (dataType == DataTypeIds.Boolean) return DriverDataType.Boolean;
+        if (dataType == DataTypeIds.SByte || dataType == DataTypeIds.Byte ||
+            dataType == DataTypeIds.Int16) return DriverDataType.Int16;
+        if (dataType == DataTypeIds.UInt16) return DriverDataType.UInt16;
+        if (dataType == DataTypeIds.Int32) return DriverDataType.Int32;
+        if (dataType == DataTypeIds.UInt32) return DriverDataType.UInt32;
+        if (dataType == DataTypeIds.Int64) return DriverDataType.Int64;
+        if (dataType == DataTypeIds.UInt64) return DriverDataType.UInt64;
+        if (dataType == DataTypeIds.Float) return DriverDataType.Float32;
+        if (dataType == DataTypeIds.Double) return DriverDataType.Float64;
+        if (dataType == DataTypeIds.String) return DriverDataType.String;
+        if (dataType == DataTypeIds.DateTime || dataType == DataTypeIds.UtcTime)
+            return DriverDataType.DateTime;
+        return DriverDataType.String;
+    }
+
+    /// <summary>
+    ///     Map an OPC UA AccessLevel/UserAccessLevel attribute value (<c>AccessLevels</c>
+    ///     bitmask) to a <see cref="SecurityClassification"/> the local node-manager's ACL
+    ///     layer can gate writes off. CurrentWrite-capable variables surface as
+    ///     <see cref="SecurityClassification.Operate"/>; read-only as <see cref="SecurityClassification.ViewOnly"/>.
+    /// </summary>
+    internal static SecurityClassification MapAccessLevelToSecurityClass(byte accessLevel)
+    {
+        const byte CurrentWrite = 2; // AccessLevels.CurrentWrite = 0x02
+        return (accessLevel & CurrentWrite) != 0
+            ? SecurityClassification.Operate
+            : SecurityClassification.ViewOnly;
+    }
+
     // ---- ISubscribable ----
 
     public async Task<ISubscriptionHandle> SubscribeAsync(
@@ -718,8 +928,16 @@ public sealed class OpcUaClientDriver(OpcUaClientDriverOptions options, string d
 
     // ---- IHostConnectivityProbe ----
 
-    /// <summary>Endpoint-URL-keyed host identity for the Admin /hosts dashboard.</summary>
-    public string HostName => _options.EndpointUrl;
+    /// <summary>
+    ///     Endpoint-URL-keyed host identity for the Admin /hosts dashboard. Reflects the
+    ///     endpoint the driver actually connected to after the failover sweep — not the
+    ///     first URL in the candidate list — so operators see which of the configured
+    ///     endpoints is currently serving traffic. Falls back to the first configured URL
+    ///     pre-init so the dashboard has something to render before the first connect.
+    /// </summary>
+    public string HostName => _connectedEndpointUrl
+        ?? ResolveEndpointCandidates(_options).FirstOrDefault()
+        ?? _options.EndpointUrl;
 
     public IReadOnlyList<HostConnectivityStatus> GetHostStatuses()
     {

src/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient/OpcUaClientDriverOptions.cs

@@ -13,9 +13,32 @@ namespace ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient;
 /// </remarks>
 public sealed class OpcUaClientDriverOptions
 {
-    /// <summary>Remote OPC UA endpoint URL, e.g. <c>opc.tcp://plc.internal:4840</c>.</summary>
+    /// <summary>
+    ///     Remote OPC UA endpoint URL, e.g. <c>opc.tcp://plc.internal:4840</c>. Convenience
+    ///     shortcut for a single-endpoint deployment — equivalent to setting
+    ///     <see cref="EndpointUrls"/> to a list with this one URL. When both are provided,
+    ///     the list wins and <see cref="EndpointUrl"/> is ignored.
+    /// </summary>
     public string EndpointUrl { get; init; } = "opc.tcp://localhost:4840";
 
+    /// <summary>
+    ///     Ordered list of candidate endpoint URLs for failover. The driver tries each in
+    ///     order at <see cref="OpcUaClientDriver.InitializeAsync"/> and on session drop;
+    ///     the first URL that successfully connects wins. Typical use-case: an OPC UA server
+    ///     pair running in hot-standby (primary 4840 + backup 4841) where either can serve
+    ///     the same address space. Leave unset (or empty) to use <see cref="EndpointUrl"/>
+    ///     as a single-URL shortcut.
+    /// </summary>
+    public IReadOnlyList<string> EndpointUrls { get; init; } = [];
+
+    /// <summary>
+    ///     Per-endpoint connect-attempt timeout during the failover sweep. Short enough that
+    ///     cycling through several dead servers doesn't blow the overall init budget, long
+    ///     enough to tolerate a slow TLS handshake on a healthy server. Applied independently
+    ///     of <see cref="Timeout"/> which governs steady-state operations.
+    /// </summary>
+    public TimeSpan PerEndpointConnectTimeout { get; init; } = TimeSpan.FromSeconds(3);
+
     /// <summary>
     ///     Security policy to require when selecting an endpoint. Either a
     ///     <see cref="OpcUaSecurityPolicy"/> enum constant or a free-form string (for

tests/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Tests/OpcUaClientAttributeMappingTests.cs

@@ -0,0 +1,62 @@
+using Opc.Ua;
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Tests;
+
+[Trait("Category", "Unit")]
+public sealed class OpcUaClientAttributeMappingTests
+{
+    [Theory]
+    [InlineData((uint)DataTypes.Boolean, DriverDataType.Boolean)]
+    [InlineData((uint)DataTypes.Int16, DriverDataType.Int16)]
+    [InlineData((uint)DataTypes.UInt16, DriverDataType.UInt16)]
+    [InlineData((uint)DataTypes.Int32, DriverDataType.Int32)]
+    [InlineData((uint)DataTypes.UInt32, DriverDataType.UInt32)]
+    [InlineData((uint)DataTypes.Int64, DriverDataType.Int64)]
+    [InlineData((uint)DataTypes.UInt64, DriverDataType.UInt64)]
+    [InlineData((uint)DataTypes.Float, DriverDataType.Float32)]
+    [InlineData((uint)DataTypes.Double, DriverDataType.Float64)]
+    [InlineData((uint)DataTypes.String, DriverDataType.String)]
+    [InlineData((uint)DataTypes.DateTime, DriverDataType.DateTime)]
+    public void MapUpstreamDataType_recognizes_standard_builtin_types(uint typeId, DriverDataType expected)
+    {
+        var nodeId = new NodeId(typeId);
+        OpcUaClientDriver.MapUpstreamDataType(nodeId).ShouldBe(expected);
+    }
+
+    [Fact]
+    public void MapUpstreamDataType_maps_SByte_and_Byte_to_Int16_since_DriverDataType_lacks_8bit()
+    {
+        // DriverDataType has no 8-bit type; conservative widen to Int16. Documented so a
+        // future Core.Abstractions PR that adds Int8/Byte can find this call site.
+        OpcUaClientDriver.MapUpstreamDataType(new NodeId((uint)DataTypes.SByte)).ShouldBe(DriverDataType.Int16);
+        OpcUaClientDriver.MapUpstreamDataType(new NodeId((uint)DataTypes.Byte)).ShouldBe(DriverDataType.Int16);
+    }
+
+    [Fact]
+    public void MapUpstreamDataType_falls_back_to_String_for_unknown_custom_types()
+    {
+        // Custom vendor extension object — NodeId in namespace 2 that isn't a standard type.
+        OpcUaClientDriver.MapUpstreamDataType(new NodeId("CustomStruct", 2)).ShouldBe(DriverDataType.String);
+    }
+
+    [Fact]
+    public void MapUpstreamDataType_handles_UtcTime_as_DateTime()
+    {
+        OpcUaClientDriver.MapUpstreamDataType(new NodeId((uint)DataTypes.UtcTime)).ShouldBe(DriverDataType.DateTime);
+    }
+
+    [Theory]
+    [InlineData((byte)0, SecurityClassification.ViewOnly)]           // no access flags set
+    [InlineData((byte)1, SecurityClassification.ViewOnly)]           // CurrentRead only
+    [InlineData((byte)2, SecurityClassification.Operate)]            // CurrentWrite only
+    [InlineData((byte)3, SecurityClassification.Operate)]            // CurrentRead + CurrentWrite
+    [InlineData((byte)0x0F, SecurityClassification.Operate)]          // read+write+historyRead+historyWrite
+    [InlineData((byte)0x04, SecurityClassification.ViewOnly)]         // HistoryRead only — no Write bit
+    public void MapAccessLevelToSecurityClass_respects_CurrentWrite_bit(byte accessLevel, SecurityClassification expected)
+    {
+        OpcUaClientDriver.MapAccessLevelToSecurityClass(accessLevel).ShouldBe(expected);
+    }
+}

tests/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Tests/OpcUaClientFailoverTests.cs

@@ -0,0 +1,81 @@
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Tests;
+
+[Trait("Category", "Unit")]
+public sealed class OpcUaClientFailoverTests
+{
+    [Fact]
+    public void ResolveEndpointCandidates_prefers_EndpointUrls_when_provided()
+    {
+        var opts = new OpcUaClientDriverOptions
+        {
+            EndpointUrl = "opc.tcp://fallback:4840",
+            EndpointUrls = ["opc.tcp://primary:4840", "opc.tcp://backup:4841"],
+        };
+        var list = OpcUaClientDriver.ResolveEndpointCandidates(opts);
+        list.Count.ShouldBe(2);
+        list[0].ShouldBe("opc.tcp://primary:4840");
+        list[1].ShouldBe("opc.tcp://backup:4841");
+    }
+
+    [Fact]
+    public void ResolveEndpointCandidates_falls_back_to_single_EndpointUrl_when_list_empty()
+    {
+        var opts = new OpcUaClientDriverOptions { EndpointUrl = "opc.tcp://only:4840" };
+        var list = OpcUaClientDriver.ResolveEndpointCandidates(opts);
+        list.Count.ShouldBe(1);
+        list[0].ShouldBe("opc.tcp://only:4840");
+    }
+
+    [Fact]
+    public void ResolveEndpointCandidates_empty_list_treated_as_fallback_to_EndpointUrl()
+    {
+        // Explicit empty list should still fall back to the single-URL shortcut rather than
+        // producing a zero-candidate sweep that would immediately throw with no URLs tried.
+        var opts = new OpcUaClientDriverOptions
+        {
+            EndpointUrl = "opc.tcp://single:4840",
+            EndpointUrls = [],
+        };
+        OpcUaClientDriver.ResolveEndpointCandidates(opts).Count.ShouldBe(1);
+    }
+
+    [Fact]
+    public void HostName_uses_first_candidate_before_connect()
+    {
+        var opts = new OpcUaClientDriverOptions
+        {
+            EndpointUrls = ["opc.tcp://primary:4840", "opc.tcp://backup:4841"],
+        };
+        using var drv = new OpcUaClientDriver(opts, "opcua-host");
+        drv.HostName.ShouldBe("opc.tcp://primary:4840",
+            "pre-connect the dashboard should show the first candidate URL so operators can link back");
+    }
+
+    [Fact]
+    public async Task Initialize_against_all_unreachable_endpoints_throws_AggregateException_listing_each()
+    {
+        // Port 1 + port 2 + port 3 on loopback are all guaranteed closed (TCP RST immediate).
+        // Failover sweep should attempt all three and throw AggregateException naming each URL
+        // so operators see exactly which candidates were tried.
+        var opts = new OpcUaClientDriverOptions
+        {
+            EndpointUrls = ["opc.tcp://127.0.0.1:1", "opc.tcp://127.0.0.1:2", "opc.tcp://127.0.0.1:3"],
+            PerEndpointConnectTimeout = TimeSpan.FromMilliseconds(500),
+            Timeout = TimeSpan.FromMilliseconds(500),
+            AutoAcceptCertificates = true,
+        };
+        using var drv = new OpcUaClientDriver(opts, "opcua-failover");
+
+        var ex = await Should.ThrowAsync<AggregateException>(async () =>
+            await drv.InitializeAsync("{}", TestContext.Current.CancellationToken));
+
+        ex.Message.ShouldContain("127.0.0.1:1");
+        ex.Message.ShouldContain("127.0.0.1:2");
+        ex.Message.ShouldContain("127.0.0.1:3");
+        drv.GetHealth().State.ShouldBe(DriverState.Faulted);
+    }
+}