Merge ServiceLevel replay fixes for freshness integration

src/Core/ZB.MOM.WW.OtOpcUa.Commons/OpcUa/DeferredServiceLevelPublisher.cs

@@ -1,21 +1,38 @@
 namespace ZB.MOM.WW.OtOpcUa.Commons.OpcUa;
 
 /// <summary>
-/// Late-binding adapter that holds an inner <see cref="IServiceLevelPublisher"/> reference
-/// swappable at runtime. Mirrors <see cref="DeferredAddressSpaceSink"/>: Akka actors resolve
-/// the publisher at DI time, but the production <c>SdkServiceLevelPublisher</c> only exists
-/// after <c>StandardServer.Start</c>. The Host's hosted service swaps the inner once the SDK
-/// is up; until then writes route through <see cref="NullServiceLevelPublisher"/>.
+/// Late-binding adapter that accepts and retains the latest requested ServiceLevel until
+/// the SDK publisher is available. Binding synchronously replays that request, including zero.
 /// </summary>
+/// <remarks>
+/// Publishing and binding are serialized through the synchronous inner call. Inner publishers
+/// must not call back into this adapter. Acceptance while unbound is not an SDK write confirmation.
+/// A failed call retains both the requested level and the installed publisher for later recovery.
+/// </remarks>
 public sealed class DeferredServiceLevelPublisher : IServiceLevelPublisher
 {
-    private volatile IServiceLevelPublisher _inner = NullServiceLevelPublisher.Instance;
+    private readonly object _gate = new();
+    private IServiceLevelPublisher? _inner;
+    private byte? _latest;
 
-    /// <summary>Swap the underlying publisher. Pass null to revert to the Null no-op.</summary>
-    /// <param name="inner">The publisher implementation to use, or null to use the null publisher.</param>
-    public void SetInner(IServiceLevelPublisher? inner) =>
-        _inner = inner ?? NullServiceLevelPublisher.Instance;
+    /// <summary>Bind and replay the latest request, or unbind while retaining that request.</summary>
+    /// <param name="inner">The publisher implementation to use, or null to unbind.</param>
+    public void SetInner(IServiceLevelPublisher? inner)
+    {
+        lock (_gate)
+        {
+            _inner = inner;
+            if (_latest is { } latest) _inner?.Publish(latest);
+        }
+    }
 
     /// <inheritdoc />
-    public void Publish(byte serviceLevel) => _inner.Publish(serviceLevel);
+    public void Publish(byte serviceLevel)
+    {
+        lock (_gate)
+        {
+            _latest = serviceLevel;
+            _inner?.Publish(serviceLevel);
+        }
+    }
 }

src/Server/ZB.MOM.WW.OtOpcUa.Host/OpcUa/FailedServiceLevelBindingCleanup.cs

@@ -0,0 +1,38 @@
+using Microsoft.Extensions.Logging;
+
+namespace ZB.MOM.WW.OtOpcUa.Host.OpcUa;
+
+/// <summary>Coordinates endpoint cleanup after ServiceLevel binding fails during startup.</summary>
+internal static class FailedServiceLevelBindingCleanup
+{
+    /// <summary>Stops and disposes the endpoint, returning whether ownership can be released.</summary>
+    /// <param name="stop">The SDK stop operation.</param>
+    /// <param name="dispose">The application-host disposal operation.</param>
+    /// <param name="logger">The cleanup logger.</param>
+    /// <returns>Whether cleanup completed successfully.</returns>
+    internal static async Task<bool> StopAndDisposeAsync(
+        Func<Task> stop, Func<ValueTask> dispose, ILogger logger)
+    {
+        var completed = true;
+        try
+        {
+            await stop().ConfigureAwait(false);
+        }
+        catch (Exception ex)
+        {
+            completed = false;
+            logger.LogError(ex, "OtOpcUaServerHostedService: SDK stop failed after binding failure; retaining endpoint ownership for shutdown retry");
+        }
+        // Attempt application cleanup even when stopping faults; keep ownership on either failure.
+        try
+        {
+            await dispose().ConfigureAwait(false);
+        }
+        catch (Exception ex)
+        {
+            completed = false;
+            logger.LogError(ex, "OtOpcUaServerHostedService: endpoint disposal failed after binding failure; retaining endpoint ownership for shutdown retry");
+        }
+        return completed;
+    }
+}

src/Server/ZB.MOM.WW.OtOpcUa.Host/OpcUa/OtOpcUaServerHostedService.cs

@@ -339,11 +339,36 @@ public sealed class OtOpcUaServerHostedService : IHostedService, IAsyncDisposabl
         _server.NodeManager.Clock = historyUpdate.Clock;
 
         // ServiceLevel publisher needs IServerInternal — only available after Start.
-        if (_server.CurrentInstance is { } serverInternal)
+        try
         {
-            _deferredServiceLevel.SetInner(new SdkServiceLevelPublisher(
-                serverInternal,
-                _loggerFactory.CreateLogger<SdkServiceLevelPublisher>()));
+            if (_server.CurrentInstance is { } serverInternal)
+            {
+                _deferredServiceLevel.SetInner(new SdkServiceLevelPublisher(
+                    serverInternal,
+                    _loggerFactory.CreateLogger<SdkServiceLevelPublisher>()));
+            }
+        }
+        catch (Exception ex)
+        {
+            _logger.LogError(ex,
+                "OtOpcUaServerHostedService: ServiceLevel binding failed; stopping the OPC UA endpoint while the admin host continues");
+            try
+            {
+                await StopAsync(cancellationToken).ConfigureAwait(false);
+            }
+            catch (Exception cleanupError)
+            {
+                _logger.LogError(cleanupError, "OtOpcUaServerHostedService: failed to detach OPC UA bindings after startup failure");
+            }
+            var released = await FailedServiceLevelBindingCleanup.StopAndDisposeAsync(
+                async () => await _server.StopAsync(CancellationToken.None).ConfigureAwait(false),
+                () => _appHost.DisposeAsync(), _logger).ConfigureAwait(false);
+            if (released)
+            {
+                _appHost = null;
+                _server = null;
+            }
+            return;
         }
 
         // arch-review 03/S19 — LAST, after the node manager is known non-null and after the ACL gate is

src/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer/OpcUaApplicationHost.cs

@@ -560,10 +560,15 @@ public sealed class OpcUaApplicationHost : IAsyncDisposable
     /// <returns>A task that represents the asynchronous operation.</returns>
     public async ValueTask DisposeAsync()
     {
-        if (_impersonateHandler is not null && _server?.CurrentInstance?.SessionManager is { } sessionManager)
+        try
         {
-            try { sessionManager.ImpersonateUser -= _impersonateHandler; }
-            catch (Exception ex) { _logger.LogWarning(ex, "OpcUaApplicationHost: detaching ImpersonateUser threw"); }
+            // An explicit SDK stop may already have halted CurrentInstance; still continue disposal.
+            if (_impersonateHandler is not null && _server?.CurrentInstance?.SessionManager is { } sessionManager)
+                sessionManager.ImpersonateUser -= _impersonateHandler;
+        }
+        catch (Exception ex)
+        {
+            _logger.LogWarning(ex, "OpcUaApplicationHost: detaching ImpersonateUser threw");
         }
         _impersonateHandler = null;
 

src/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer/SdkServiceLevelPublisher.cs

@@ -38,8 +38,8 @@ public sealed class SdkServiceLevelPublisher : IServiceLevelPublisher
         var node = _serverInternal.ServerObject?.ServiceLevel;
         if (node is null)
         {
-            _logger.LogWarning("SdkServiceLevelPublisher: ServerObject.ServiceLevel unavailable; skipping write");
-            return;
+            _logger.LogWarning("SdkServiceLevelPublisher: ServerObject.ServiceLevel unavailable");
+            throw new InvalidOperationException("Server.ServiceLevel is unavailable.");
         }
 
         try
@@ -55,6 +55,7 @@ public sealed class SdkServiceLevelPublisher : IServiceLevelPublisher
         catch (Exception ex)
         {
             _logger.LogWarning(ex, "SdkServiceLevelPublisher: write to Server.ServiceLevel threw");
+            throw;
         }
     }
 }

src/Server/ZB.MOM.WW.OtOpcUa.Runtime/OpcUa/OpcUaPublishActor.cs

@@ -979,10 +979,10 @@ public sealed class OpcUaPublishActor : ReceiveActor, IWithTimers
         // node starting Detached/role-less (first level = 0) would be dedup'd away, leaving the SDK's
         // built-in default (255 = full service) standing — a degraded node wrongly advertising 255.
         if (_publishedAtLeastOnce && msg.ServiceLevel == _lastServiceLevel) return;
-        _lastServiceLevel = msg.ServiceLevel;
         try
         {
             _serviceLevel.Publish(msg.ServiceLevel);
+            _lastServiceLevel = msg.ServiceLevel;
             _publishedAtLeastOnce = true;
             OtOpcUaTelemetry.ServiceLevelChange.Add(1,
                 new KeyValuePair<string, object?>("level", msg.ServiceLevel));
@@ -990,6 +990,8 @@ public sealed class OpcUaPublishActor : ReceiveActor, IWithTimers
         }
         catch (Exception ex)
         {
+            // A failed write may be partially applied; even a previous successful value must retry.
+            _publishedAtLeastOnce = false;
             _log.Warning(ex, "OpcUaPublish: ServiceLevel publisher threw at level {Level}", msg.ServiceLevel);
         }
     }

tests/Core/ZB.MOM.WW.OtOpcUa.Commons.Tests/OpcUa/DeferredServiceLevelPublisherTests.cs

@@ -10,10 +10,10 @@ namespace ZB.MOM.WW.OtOpcUa.Commons.Tests.OpcUa;
 public class DeferredServiceLevelPublisherTests
 {
     [Fact]
-    public void Before_SetInner_Publish_is_a_noop()
+    public void Before_SetInner_Publish_accepts_a_cached_request()
     {
         var publisher = new DeferredServiceLevelPublisher();
-        // Must not throw; the NullServiceLevelPublisher absorbs it.
+        // Accept the request before an SDK publisher exists.
         publisher.Publish(200);
     }
 
@@ -30,7 +30,7 @@ public class DeferredServiceLevelPublisherTests
     }
 
     [Fact]
-    public void SetInner_null_reverts_to_null_publisher()
+    public void SetInner_null_detaches_the_previous_publisher()
     {
         var spy = new SpyPublisher();
         var publisher = new DeferredServiceLevelPublisher();
@@ -39,8 +39,139 @@ public class DeferredServiceLevelPublisherTests
 
         publisher.Publish(100);
 
-        // The spy should NOT have received the publish after revert.
-        spy.LastPublished.ShouldBe((byte)0, "spy should not have been updated after revert to null");
+        // The previous publisher must not receive an unbound request.
+        spy.LastPublished.ShouldBe((byte)0, "the previous publisher should not be updated after unbinding");
+    }
+
+    [Fact]
+    public void Binding_without_a_request_does_not_invent_a_level()
+    {
+        var received = new List<byte>();
+        new DeferredServiceLevelPublisher().SetInner(new CallbackPublisher(received.Add));
+        received.ShouldBeEmpty();
+    }
+
+    [Theory]
+    [InlineData((byte)100)]
+    [InlineData((byte)0)]
+    public void Binding_replays_the_latest_request_including_zero(byte latest)
+    {
+        var publisher = new DeferredServiceLevelPublisher();
+        publisher.Publish(240);
+        publisher.Publish(latest);
+        var received = new List<byte>();
+        publisher.SetInner(new CallbackPublisher(received.Add));
+        received.ShouldBe(new[] { latest });
+    }
+
+    [Fact]
+    public void Unbinding_retains_latest_for_rebinding()
+    {
+        var publisher = new DeferredServiceLevelPublisher();
+        var old = new List<byte>();
+        publisher.SetInner(new CallbackPublisher(old.Add));
+        publisher.Publish(240);
+        publisher.SetInner(null);
+        publisher.Publish(0);
+        var replacement = new List<byte>();
+        publisher.SetInner(new CallbackPublisher(replacement.Add));
+        old.ShouldBe(new byte[] { 240 });
+        replacement.ShouldBe(new byte[] { 0 });
+    }
+
+    [Fact]
+    public void Failed_publish_retains_the_requested_value_for_replay()
+    {
+        var publisher = new DeferredServiceLevelPublisher();
+        publisher.SetInner(new CallbackPublisher(_ => throw new InvalidOperationException()));
+        Should.Throw<InvalidOperationException>(() => publisher.Publish(0));
+        var received = new List<byte>();
+        publisher.SetInner(new CallbackPublisher(received.Add));
+        received.ShouldBe(new byte[] { 0 });
+    }
+
+    [Fact]
+    public void Failed_replay_keeps_candidate_bound_for_later_publish()
+    {
+        var publisher = new DeferredServiceLevelPublisher();
+        publisher.Publish(100);
+        var fail = true;
+        var received = new List<byte>();
+        var candidate = new CallbackPublisher(value =>
+        {
+            if (fail) throw new InvalidOperationException();
+            received.Add(value);
+        });
+        Should.Throw<InvalidOperationException>(() => publisher.SetInner(candidate));
+        fail = false;
+        publisher.Publish(0);
+        publisher.SetInner(null);
+        publisher.SetInner(candidate);
+        received.ShouldBe(new byte[] { 0, 0 });
+    }
+
+    [Fact]
+    public void Failed_replay_retains_latest_for_a_replacement_publisher()
+    {
+        var publisher = new DeferredServiceLevelPublisher();
+        publisher.Publish(100);
+        Should.Throw<InvalidOperationException>(() => publisher.SetInner(
+            new CallbackPublisher(_ => throw new InvalidOperationException())));
+        var received = new List<byte>();
+        publisher.SetInner(new CallbackPublisher(received.Add));
+        received.ShouldBe(new byte[] { 100 });
+    }
+
+    [Fact]
+    public async Task Binding_waits_for_inflight_publish_then_replays_latest()
+    {
+        var publisher = new DeferredServiceLevelPublisher();
+        using var entered = new ManualResetEventSlim();
+        using var release = new ManualResetEventSlim();
+        publisher.SetInner(new CallbackPublisher(_ =>
+        {
+            entered.Set();
+            release.Wait(TimeSpan.FromSeconds(5), TestContext.Current.CancellationToken).ShouldBeTrue();
+        }));
+        var publish = Task.Run(() => publisher.Publish(100), TestContext.Current.CancellationToken);
+        var received = new List<byte>();
+        using var bindingStarted = new ManualResetEventSlim();
+        Exception? bindingError = null;
+        var binding = new Thread(() =>
+        {
+            bindingStarted.Set();
+            try { publisher.SetInner(new CallbackPublisher(received.Add)); }
+            catch (Exception ex) { bindingError = ex; }
+        });
+        var bindingWasStarted = false;
+        try
+        {
+            entered.Wait(TimeSpan.FromSeconds(5), TestContext.Current.CancellationToken).ShouldBeTrue();
+            binding.Start();
+            bindingWasStarted = true;
+            bindingStarted.Wait(TimeSpan.FromSeconds(5), TestContext.Current.CancellationToken).ShouldBeTrue();
+            // Past the marker, SetInner is the thread's only potentially blocking operation.
+            // Observe actual contention rather than assuming Task.Run has entered the adapter.
+            SpinWait.SpinUntil(() => !binding.IsAlive ||
+                (binding.ThreadState & System.Threading.ThreadState.WaitSleepJoin) != 0,
+                TimeSpan.FromSeconds(3)).ShouldBeTrue();
+            (binding.ThreadState & System.Threading.ThreadState.WaitSleepJoin).ShouldNotBe(
+                (System.Threading.ThreadState)0, "binding must wait for the in-flight delivery");
+        }
+        finally
+        {
+            release.Set();
+            if (bindingWasStarted) binding.Join(TimeSpan.FromSeconds(5)).ShouldBeTrue();
+            await publish.WaitAsync(TimeSpan.FromSeconds(5), TestContext.Current.CancellationToken);
+        }
+        bindingError.ShouldBeNull();
+        publisher.Publish(0);
+        received.ShouldBe(new byte[] { 100, 0 });
+    }
+
+    private sealed class CallbackPublisher(Action<byte> publish) : IServiceLevelPublisher
+    {
+        public void Publish(byte serviceLevel) => publish(serviceLevel);
     }
 
     // ---- test double ----

tests/Server/ZB.MOM.WW.OtOpcUa.Host.Tests/OpcUa/FailedServiceLevelBindingCleanupTests.cs

@@ -0,0 +1,45 @@
+using Microsoft.Extensions.Logging.Abstractions;
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Host.OpcUa;
+
+namespace ZB.MOM.WW.OtOpcUa.Host.Tests.OpcUa;
+
+public sealed class FailedServiceLevelBindingCleanupTests
+{
+    [Fact]
+    public async Task Successful_cleanup_observes_stop_before_disposal_and_releases_ownership()
+    {
+        var events = new List<string>();
+        var released = await FailedServiceLevelBindingCleanup.StopAndDisposeAsync(
+            () => { events.Add("stop"); return Task.CompletedTask; },
+            () => { events.Add("dispose"); return ValueTask.CompletedTask; },
+            NullLogger.Instance);
+        events.ShouldBe(new[] { "stop", "dispose" });
+        released.ShouldBeTrue();
+    }
+
+    [Fact]
+    public async Task Failed_stop_still_disposes_and_retains_ownership_for_retry()
+    {
+        var events = new List<string>();
+        var released = await FailedServiceLevelBindingCleanup.StopAndDisposeAsync(
+            () => { events.Add("stop"); throw new InvalidOperationException("controlled stop failure"); },
+            () => { events.Add("dispose"); return ValueTask.CompletedTask; },
+            NullLogger.Instance);
+        events.ShouldBe(new[] { "stop", "dispose" });
+        released.ShouldBeFalse();
+    }
+
+    [Fact]
+    public async Task Failed_disposal_retains_ownership_without_escaping_the_binding_failure_handler()
+    {
+        var events = new List<string>();
+        var released = await FailedServiceLevelBindingCleanup.StopAndDisposeAsync(
+            () => { events.Add("stop"); return Task.CompletedTask; },
+            () => { events.Add("dispose"); throw new InvalidOperationException("controlled disposal failure"); },
+            NullLogger.Instance);
+        events.ShouldBe(new[] { "stop", "dispose" });
+        released.ShouldBeFalse();
+    }
+}

tests/Server/ZB.MOM.WW.OtOpcUa.Host.Tests/OpcUa/OtOpcUaServerHostedServiceBindingFailureTests.cs

@@ -0,0 +1,103 @@
+using System.Net;
+using System.Net.Sockets;
+using Akka.Hosting;
+using Microsoft.Extensions.Configuration;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Hosting;
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Logging.Abstractions;
+using Microsoft.Extensions.Options;
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Commons.OpcUa;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+using ZB.MOM.WW.OtOpcUa.Core.Authorization;
+using ZB.MOM.WW.OtOpcUa.Host.OpcUa;
+using ZB.MOM.WW.OtOpcUa.OpcUaServer;
+using ZB.MOM.WW.OtOpcUa.OpcUaServer.Security;
+
+namespace ZB.MOM.WW.OtOpcUa.Host.Tests.OpcUa;
+
+public sealed class OtOpcUaServerHostedServiceBindingFailureTests
+{
+    [Fact]
+    public async Task Binding_failure_closes_started_endpoint_and_allows_other_host_services_to_start()
+    {
+        using var deadline = new CancellationTokenSource(TimeSpan.FromSeconds(15));
+        using var probe = new TcpListener(IPAddress.Loopback, 0);
+        probe.Start();
+        var port = ((IPEndPoint)probe.LocalEndpoint).Port;
+        probe.Stop();
+        var pki = Path.Combine(Path.GetTempPath(), $"otopcua-binding-{Guid.NewGuid():N}");
+        var deferred = new DeferredServiceLevelPublisher();
+        deferred.Publish(0);
+        var bindingReached = false;
+        var listenerObserved = false;
+        OtOpcUaServerHostedService? service = null;
+        var factory = new FaultInjectingLoggerFactory(() =>
+        {
+            using var connected = new TcpClient();
+            connected.Connect(IPAddress.Loopback, port);
+            listenerObserved = connected.Connected;
+            bindingReached = true;
+            throw new InvalidOperationException("controlled binding factory failure");
+        });
+        service = new OtOpcUaServerHostedService(
+            Options.Create(new OpcUaApplicationHostOptions
+            {
+                ApplicationName = "OtOpcUa.BindingFailure",
+                ApplicationUri = $"urn:OtOpcUa.BindingFailure:{Guid.NewGuid():N}",
+                OpcUaPort = port, PublicHostname = "localhost", PkiStoreRoot = pki,
+            }), new DeferredAddressSpaceSink(), deferred,
+            NullOpcUaUserAuthenticator.Instance, NullHistorianDataSource.Instance,
+            EmptyDataPlaneAuthoritySource.Instance, new ConfigurationBuilder().Build(),
+            () => throw new NotSupportedException("no actor system required"), new ActorRegistry(),
+            null, Options.Create(new AclOptions()), TimeProvider.System, factory);
+        var admin = new AdminBootSentinel();
+        using var host = new HostBuilder().ConfigureServices(services =>
+        {
+            services.AddSingleton<IHostedService>(service);
+            services.AddSingleton<IHostedService>(admin);
+        }).Build();
+        try
+        {
+            await host.StartAsync(deadline.Token);
+            listenerObserved.ShouldBeTrue("the SDK endpoint was actually listening before replay");
+            bindingReached.ShouldBeTrue();
+            admin.Started.ShouldBeTrue();
+            using var client = new TcpClient();
+            await Should.ThrowAsync<SocketException>(async () =>
+                await client.ConnectAsync(IPAddress.Loopback, port, deadline.Token));
+            // After construction failure, subsequent requests are accepted into the unbound cache.
+            deferred.Publish(100);
+            await host.StopAsync(deadline.Token);
+        }
+        finally
+        {
+            await service.DisposeAsync();
+            if (Directory.Exists(pki)) Directory.Delete(pki, recursive: true);
+        }
+    }
+
+    private sealed class FaultInjectingLoggerFactory(Action installFault) : ILoggerFactory
+    {
+        public ILogger CreateLogger(string categoryName)
+        {
+            if (categoryName == typeof(SdkServiceLevelPublisher).FullName) installFault();
+            return NullLogger.Instance;
+        }
+        public void AddProvider(ILoggerProvider provider) { }
+        public void Dispose() { }
+    }
+
+    private sealed class AdminBootSentinel : IHostedService
+    {
+        public bool Started { get; private set; }
+        public Task StartAsync(CancellationToken cancellationToken)
+        {
+            Started = true;
+            return Task.CompletedTask;
+        }
+        public Task StopAsync(CancellationToken cancellationToken) => Task.CompletedTask;
+    }
+}

tests/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer.IntegrationTests/ServiceLevelBindingReplayTests.cs

@@ -0,0 +1,71 @@
+using Microsoft.Extensions.Logging.Abstractions;
+using Opc.Ua;
+using Opc.Ua.Client;
+using Opc.Ua.Server;
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Commons.OpcUa;
+
+namespace ZB.MOM.WW.OtOpcUa.OpcUaServer.IntegrationTests;
+
+/// <summary>Local loopback evidence that late binding replaces the SDK default on the wire.</summary>
+public sealed class ServiceLevelBindingReplayTests
+{
+    [Theory]
+    [InlineData((byte)100)]
+    [InlineData((byte)0)]
+    public async Task Late_binding_replays_latest_request_to_standard_ServiceLevel_read(byte latest)
+    {
+        using var deadline = CancellationTokenSource.CreateLinkedTokenSource(TestContext.Current.CancellationToken);
+        deadline.CancelAfter(TimeSpan.FromSeconds(15));
+        var ct = deadline.Token;
+        var pki = Path.Combine(Path.GetTempPath(), $"otopcua-replay-{Guid.NewGuid():N}");
+        var clientPki = TestClientSecurity.AllocatePkiRoot();
+        var port = 0;
+        try
+        {
+            var deferred = new DeferredServiceLevelPublisher();
+            deferred.Publish(240);
+            deferred.Publish(latest);
+            var started = await OpcUaHostStart.StartAsync(p =>
+            {
+                port = p;
+                return new OpcUaApplicationHost(new OpcUaApplicationHostOptions
+                {
+                    ApplicationName = "OtOpcUa.Replay",
+                    ApplicationUri = $"urn:OtOpcUa.Replay:{Path.GetFileName(pki)}",
+                    OpcUaPort = p, PublicHostname = "localhost", PkiStoreRoot = pki,
+                }, NullLogger<OpcUaApplicationHost>.Instance);
+            }, () => new StandardServer(), ct);
+            await using var host = started.Host;
+            deferred.SetInner(new SdkServiceLevelPublisher(started.Server.CurrentInstance,
+                NullLogger<SdkServiceLevelPublisher>.Instance));
+            var config = new ApplicationConfiguration
+            {
+                ApplicationName = "OtOpcUa.ReplayClient",
+                ApplicationUri = $"urn:OtOpcUa.ReplayClient:{Guid.NewGuid():N}",
+                ApplicationType = ApplicationType.Client,
+                SecurityConfiguration = TestClientSecurity.Build(clientPki),
+                ClientConfiguration = new ClientConfiguration { DefaultSessionTimeout = 10_000 },
+                TransportQuotas = new TransportQuotas { OperationTimeout = 5_000 },
+            };
+            await config.ValidateAsync(ApplicationType.Client, ct);
+            config.CertificateValidator.CertificateValidation += (_, e) => e.Accept = true;
+            var endpoint = await CoreClientUtils.SelectEndpointAsync(config,
+                $"opc.tcp://localhost:{port}/OtOpcUa", false, DefaultTelemetry.Create(_ => { }), ct);
+            using var session = await new DefaultSessionFactory(DefaultTelemetry.Create(_ => { })).CreateAsync(
+                config, new ConfiguredEndpoint(null, endpoint, EndpointConfiguration.Create(config)),
+                updateBeforeConnect: false, checkDomain: false, sessionName: "ServiceLevelReplay",
+                sessionTimeout: 10_000, identity: new UserIdentity(new AnonymousIdentityToken()),
+                preferredLocales: null, ct: ct);
+            var read = await session.ReadValueAsync(new Opc.Ua.NodeId(2267u), ct);
+            read.StatusCode.ShouldBe(StatusCodes.Good);
+            read.Value.ShouldBe(latest);
+        }
+        finally
+        {
+            if (Directory.Exists(pki)) Directory.Delete(pki, recursive: true);
+            if (Directory.Exists(clientPki)) Directory.Delete(clientPki, recursive: true);
+        }
+    }
+}

tests/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer.Tests/DeferredServiceLevelPublisherTests.cs

@@ -6,9 +6,9 @@ namespace ZB.MOM.WW.OtOpcUa.OpcUaServer.Tests;
 
 public sealed class DeferredServiceLevelPublisherTests
 {
-    /// <summary>Verifies that publish before SetInner is a safe no-op.</summary>
+    /// <summary>Verifies that publish before SetInner safely accepts a cached request.</summary>
     [Fact]
-    public void Publish_before_SetInner_is_a_safe_noop()
+    public void Publish_before_SetInner_accepts_a_cached_request()
     {
         var deferred = new DeferredServiceLevelPublisher();
 
@@ -28,9 +28,9 @@ public sealed class DeferredServiceLevelPublisherTests
         recording.LastValue.ShouldBe((byte)200);
     }
 
-    /// <summary>Verifies that SetInner with null reverts to the null publisher.</summary>
+    /// <summary>Verifies that SetInner with null detaches the previous publisher.</summary>
     [Fact]
-    public void SetInner_null_reverts_to_Null_publisher()
+    public void SetInner_null_detaches_the_previous_publisher()
     {
         var recording = new RecordingPublisher();
         var deferred = new DeferredServiceLevelPublisher();

tests/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer.Tests/SdkServiceLevelPublisherTests.cs

@@ -19,6 +19,40 @@ public sealed class SdkServiceLevelPublisherTests : IDisposable
         Path.GetTempPath(),
         $"otopcua-pki-{Guid.NewGuid():N}");
 
+    [Fact]
+    public void Missing_ServiceLevel_is_reported_as_failure()
+    {
+        var server = System.Reflection.DispatchProxy.Create<IServerInternal, FaultingServer>();
+        var publisher = new SdkServiceLevelPublisher(server, NullLogger<SdkServiceLevelPublisher>.Instance);
+        Should.Throw<InvalidOperationException>(() => publisher.Publish(0));
+    }
+
+    [Fact]
+    public void Sdk_write_failure_propagates_the_original_exception()
+    {
+        var server = System.Reflection.DispatchProxy.Create<IServerInternal, FaultingServer>();
+        var proxy = (FaultingServer)(object)server;
+        proxy.Node = new Opc.Ua.ServerObjectState(null)
+        {
+            ServiceLevel = new Opc.Ua.PropertyState<byte>(null),
+        };
+        proxy.Fault = new InvalidOperationException("controlled diagnostics failure");
+        var publisher = new SdkServiceLevelPublisher(server, NullLogger<SdkServiceLevelPublisher>.Instance);
+        Should.Throw<InvalidOperationException>(() => publisher.Publish(0)).ShouldBeSameAs(proxy.Fault);
+    }
+
+    public class FaultingServer : System.Reflection.DispatchProxy
+    {
+        public Opc.Ua.ServerObjectState? Node { get; set; }
+        public Exception? Fault { get; set; }
+        protected override object? Invoke(System.Reflection.MethodInfo? method, object?[]? args)
+        {
+            if (method?.Name == "get_ServerObject") return Node;
+            if (method?.Name == "get_DiagnosticsLock") throw Fault!;
+            throw new NotSupportedException(method?.Name);
+        }
+    }
+
     /// <summary>Verifies that the publisher writes values to the standard Server.ServiceLevel variable.</summary>
     [Fact]
     public async Task Publish_writes_value_to_Server_ServiceLevel_variable()

tests/Server/ZB.MOM.WW.OtOpcUa.Runtime.Tests/OpcUa/OpcUaPublishActorTests.cs

@@ -15,6 +15,49 @@ namespace ZB.MOM.WW.OtOpcUa.Runtime.Tests.OpcUa;
 
 public sealed class OpcUaPublishActorTests : RuntimeActorTestBase
 {
+    [Fact]
+    public void Failed_value_is_retried_and_last_level_only_tracks_success()
+    {
+        var sink = new RecoveringPublisher { Fail = false };
+        var actor = ActorOfAsTestActorRef<OpcUaPublishActor>(OpcUaPublishActor.PropsForTests(serviceLevel: sink));
+        actor.Tell(new OpcUaPublishActor.ServiceLevelChanged(100));
+        sink.Fail = true;
+        actor.Tell(new OpcUaPublishActor.ServiceLevelChanged(0));
+        actor.UnderlyingActor.LastServiceLevel.ShouldBe((byte)100);
+        sink.Fail = false;
+        actor.Tell(new OpcUaPublishActor.ServiceLevelChanged(0));
+        sink.Delivered.ShouldBe(new byte[] { 100, 0 });
+    }
+
+    [Fact]
+    public void Requesting_prior_success_after_failure_updates_delivery_and_replay()
+    {
+        var sink = new RecoveringPublisher();
+        var deferred = new DeferredServiceLevelPublisher();
+        deferred.SetInner(sink);
+        var actor = ActorOfAsTestActorRef<OpcUaPublishActor>(OpcUaPublishActor.PropsForTests(serviceLevel: deferred));
+        actor.Tell(new OpcUaPublishActor.ServiceLevelChanged(100));
+        sink.Fail = true;
+        actor.Tell(new OpcUaPublishActor.ServiceLevelChanged(0));
+        sink.Fail = false;
+        actor.Tell(new OpcUaPublishActor.ServiceLevelChanged(100));
+        sink.Delivered.ShouldBe(new byte[] { 100, 100 });
+        var rebound = new RecoveringPublisher();
+        deferred.SetInner(rebound);
+        rebound.Delivered.ShouldBe(new byte[] { 100 });
+    }
+
+    private sealed class RecoveringPublisher : IServiceLevelPublisher
+    {
+        public bool Fail { get; set; }
+        public List<byte> Delivered { get; } = new();
+        public void Publish(byte level)
+        {
+            if (Fail) throw new InvalidOperationException("controlled failure");
+            Delivered.Add(level);
+        }
+    }
+
     /// <summary>Verifies that message contracts are accepted without a pinned dispatcher in tests.</summary>
     [Fact]
     public void Accepts_message_contracts_without_pinned_dispatcher_in_tests()