feat(health): SiteAuditTelemetryStalledTracker subscribes to EventStream (#23 M6)

src/ScadaLink.AuditLog/Central/SiteAuditTelemetryStalledTracker.cs

@@ -0,0 +1,158 @@
+using System.Collections.Concurrent;
+using Akka.Actor;
+using Akka.Event;
+
+namespace ScadaLink.AuditLog.Central;
+
+/// <summary>
+/// Audit Log (#23) M6 Bundle E (T7) — central singleton that subscribes to the
+/// actor system's EventStream for <see cref="SiteAuditTelemetryStalledChanged"/>
+/// publications and maintains a per-site latched stalled-state map readable
+/// via <see cref="Snapshot"/>. Consumed by the M6 Bundle E
+/// <see cref="AuditCentralHealthSnapshot"/> aggregator so the central health
+/// surface can surface per-site "reconciliation isn't draining" without
+/// coupling the publisher (<see cref="SiteAuditReconciliationActor"/>) to the
+/// health collection plumbing.
+/// </summary>
+/// <remarks>
+/// <para>
+/// <b>Why an internal actor.</b> Akka.NET's <see cref="EventStream"/> only
+/// supports <see cref="IActorRef"/> subscribers — there is no callback or
+/// channel-based overload. The tracker therefore spawns a small subscriber
+/// actor that forwards each event into the shared
+/// <see cref="ConcurrentDictionary{TKey,TValue}"/> on the actor's thread, and
+/// readers (<see cref="Snapshot"/>) take a copy off that dictionary on any
+/// thread. Mirrors the <c>DeadLetterMonitorActor</c> shape — subscribe in
+/// <see cref="ActorBase.PreStart"/>, unsubscribe in
+/// <see cref="ActorBase.PostStop"/>, which the tracker triggers via a Stop
+/// at <see cref="Dispose"/>.
+/// </para>
+/// <para>
+/// <b>Per-site latching.</b> The publisher (<see cref="SiteAuditReconciliationActor"/>)
+/// only publishes on stalled-state transitions, so the dictionary is the
+/// authoritative latched state. Sites that have never published are absent
+/// from the snapshot — the consumer surface treats absence as
+/// <c>Stalled=false</c> (default healthy), the same default the reconciliation
+/// actor's own internal latch uses.
+/// </para>
+/// <para>
+/// <b>Singleton lifecycle.</b> Registered as a singleton via
+/// <see cref="ServiceCollectionExtensions.AddAuditLogCentralMaintenance"/>;
+/// <see cref="Dispose"/> tears the internal subscriber down at host shutdown.
+/// </para>
+/// </remarks>
+public sealed class SiteAuditTelemetryStalledTracker : IDisposable
+{
+    private readonly EventStream _eventStream;
+    private readonly ConcurrentDictionary<string, bool> _state = new();
+    private readonly IActorRef? _subscriber;
+    private bool _disposed;
+
+    /// <summary>
+    /// Construct around a bare <see cref="EventStream"/>. Intended for unit
+    /// tests where the caller wants to publish events without standing up an
+    /// actor system — the tracker registers a transient subscriber actor only
+    /// if the supplied stream is backed by an actor system. In the bare-stream
+    /// mode (no actor system) the tracker still exposes the
+    /// <see cref="Snapshot"/> surface but cannot self-subscribe; production
+    /// callers always go through <see cref="SiteAuditTelemetryStalledTracker(ActorSystem)"/>.
+    /// </summary>
+    /// <remarks>
+    /// Subscribing to <see cref="EventStream"/> requires an <see cref="IActorRef"/>,
+    /// which can only be created from an <see cref="ActorSystem"/>. The bare-
+    /// stream ctor therefore can NOT itself wire the subscriber — tests that
+    /// want event-driven updates must use the ActorSystem ctor (or push state
+    /// directly via <see cref="Apply"/>). The tests in
+    /// <c>SiteAuditTelemetryStalledTrackerTests</c> use the ActorSystem ctor
+    /// via Akka.TestKit so they exercise the production subscribe path.
+    /// </remarks>
+    public SiteAuditTelemetryStalledTracker(EventStream eventStream)
+    {
+        _eventStream = eventStream ?? throw new ArgumentNullException(nameof(eventStream));
+        // No subscriber actor — see the remarks above. Apply() is exposed
+        // internally so the ActorSystem ctor's internal forwarder can update
+        // state without re-implementing the dictionary write.
+        _subscriber = null;
+    }
+
+    /// <summary>
+    /// Production ctor: subscribes a small internal actor to the supplied
+    /// system's EventStream so every published
+    /// <see cref="SiteAuditTelemetryStalledChanged"/> updates the latched
+    /// per-site map. <see cref="Dispose"/> tears the subscriber down.
+    /// </summary>
+    public SiteAuditTelemetryStalledTracker(ActorSystem actorSystem)
+    {
+        ArgumentNullException.ThrowIfNull(actorSystem);
+        _eventStream = actorSystem.EventStream;
+        // Anonymous subscriber actor scoped to the system; props build it
+        // with a callback into THIS tracker's Apply method so the actor's
+        // single-threaded receive serialises every dictionary write.
+        _subscriber = actorSystem.ActorOf(
+            Props.Create(() => new StalledChangedSubscriber(this)),
+            name: $"site-audit-stalled-tracker-{Guid.NewGuid():N}");
+        // Subscribe synchronously from the ctor so the subscription is in
+        // place before the tracker is returned to the caller — the actor's
+        // own PreStart runs asynchronously and would otherwise race the
+        // first publish. EventStream.Subscribe is thread-safe.
+        _eventStream.Subscribe(_subscriber, typeof(SiteAuditTelemetryStalledChanged));
+    }
+
+    /// <summary>
+    /// Returns a defensive copy of the per-site latched stalled state.
+    /// Absent sites are interpreted as <c>Stalled=false</c> by consumers.
+    /// </summary>
+    public IReadOnlyDictionary<string, bool> Snapshot() =>
+        new Dictionary<string, bool>(_state);
+
+    /// <summary>
+    /// Applied by the internal subscriber actor on every
+    /// <see cref="SiteAuditTelemetryStalledChanged"/> publication. Exposed
+    /// internally so tests against the bare-stream ctor can still drive the
+    /// tracker, but the production path always goes through the actor.
+    /// </summary>
+    internal void Apply(SiteAuditTelemetryStalledChanged evt)
+    {
+        if (evt is null) return;
+        _state[evt.SiteId] = evt.Stalled;
+    }
+
+    public void Dispose()
+    {
+        if (_disposed) return;
+        _disposed = true;
+        if (_subscriber is not null)
+        {
+            // Unsubscribe runs in PostStop on the subscriber actor; Stop is
+            // fire-and-forget but the actor's PostStop hook is guaranteed to
+            // run before its mailbox is collected.
+            _subscriber.Tell(PoisonPill.Instance);
+        }
+    }
+
+    /// <summary>
+    /// Internal subscriber actor — receives every
+    /// <see cref="SiteAuditTelemetryStalledChanged"/> off the EventStream and
+    /// forwards it into the parent <see cref="SiteAuditTelemetryStalledTracker"/>.
+    /// Unlike <c>DeadLetterMonitorActor</c>, the subscription is registered by
+    /// the tracker constructor BEFORE this actor begins processing messages so
+    /// publishes that arrive between actor creation and PreStart cannot be
+    /// missed. Unsubscribe still runs in <see cref="PostStop"/>.
+    /// </summary>
+    private sealed class StalledChangedSubscriber : ReceiveActor
+    {
+        private readonly SiteAuditTelemetryStalledTracker _parent;
+
+        public StalledChangedSubscriber(SiteAuditTelemetryStalledTracker parent)
+        {
+            _parent = parent;
+            Receive<SiteAuditTelemetryStalledChanged>(evt => _parent.Apply(evt));
+        }
+
+        protected override void PostStop()
+        {
+            Context.System.EventStream.Unsubscribe(Self, typeof(SiteAuditTelemetryStalledChanged));
+            base.PostStop();
+        }
+    }
+}

src/ScadaLink.AuditLog/ServiceCollectionExtensions.cs

@@ -259,6 +259,17 @@ public static class ServiceCollectionExtensions
             .Bind(config.GetSection(PartitionMaintenanceSectionName));
         services.AddHostedService<AuditLogPartitionMaintenanceService>();
 
+        // M6 Bundle E (T7): central-singleton tracker for per-site
+        // SiteAuditTelemetryStalledChanged events published by
+        // SiteAuditReconciliationActor. Singleton so the EventStream
+        // subscription registers exactly once on host startup; the tracker's
+        // ctor needs an ActorSystem which is composed by the AkkaHostedService
+        // — we go through a factory so the SP is the source of truth for the
+        // system reference rather than re-resolving it on the consumer side.
+        services.AddSingleton<SiteAuditTelemetryStalledTracker>(sp =>
+            new SiteAuditTelemetryStalledTracker(
+                sp.GetRequiredService<Akka.Actor.ActorSystem>()));
+
         return services;
     }
 }