Phase 2 PR 8 — wire gateway-level host-status push from MxAccessGalaxyBackend. PR 4 built the IPC infrastructure for OnHostStatusChanged (MessageKind.RuntimeStatusChange frame + ConnectionSink forwarding through FrameWriter) but no backend actually raised the event; the #pragma warning disable CS0067 around MxAccessGalaxyBackend.OnHostStatusChanged declared the event for interface symmetry while acknowledging the wire-up was Phase 2 follow-up. This PR closes the gateway-level signal: MxAccessClient.ConnectionStateChanged (already raised on false→true Register and true→false Unregister transitions, including the reconnect path in MonitorLoopAsync) now drives OnHostStatusChanged with a synthetic HostConnectivityStatus tagged HostName=MxAccessClient.ClientName, RuntimeStatus="Running" on reconnect + "Stopped" on disconnect, LastObservedUtcUnixMs set to the transition moment. The Admin UI's existing IHostConnectivityProbe subscriber on GalaxyProxyDriver (HostStatusChangedEventArgs) already handles the full translation — OnHostConnectivityUpdate parses "Running"/"Stopped"/"Faulted" into the Core.Abstractions HostState enum and fires OnHostStatusChanged downstream, so this single backend-side event wire-up produces an end-to-end signal with no further Proxy changes required. Per-platform and per-AppEngine ScanState probing (the 472 LOC GalaxyRuntimeProbeManager state machine in v1 that advises <Host>.ScanState on every deployed $WinPlatform + $AppEngine gobject, tracks Unknown → Running → Stopped transitions, handles the on-change-only delivery quirk of ScanState, and surfaces IsHostStopped(gobjectId) for the node manager's Read path to short-circuit on-demand reads against known-stopped runtimes) remains deferred to a follow-up PR — the gateway-level signal gives operators the top-level transport-health rung of the status ladder, which is what matters when the Galaxy COM proxy itself goes down (vs a specific platform going down). MxAccessClient.ClientName property exposes the previously-private _clientName field so the backend can tag its pushes with a stable gateway identity — operators configure this via OTOPCUA_GALAXY_CLIENT_NAME env var (default "OtOpcUa-Galaxy.Host" per Program.cs). MxAccessGalaxyBackend constructor subscribes the new _onConnectionStateChanged field before returning + Dispose unsubscribes it via _mx.ConnectionStateChanged -= _onConnectionStateChanged to prevent the backend's own dispose from leaving a dangling handler on the MxAccessClient (same shape as MxAccessClient.SubscriptionReplayFailed PR 6 dispose discipline). #pragma warning disable CS0067 removed from around OnHostStatusChanged since the event is now raised; the directive is narrowed to cover only OnAlarmEvent which stays unraised pending the alarm subsystem port (PR 9 candidate). Tests — HostStatusPushTests (new, 2 cases): ConnectionStateChanged_raises_OnHostStatusChanged_with_gateway_name fires mx.ConnectAsync → mx.DisconnectAsync and asserts two notifications in order with HostName="GatewayClient" (the clientName passed to MxAccessClient ctor), RuntimeStatus="Running" then "Stopped", LastObservedUtcUnixMs > 0; Dispose_unsubscribes_so_post_dispose_state_changes_do_not_fire_events asserts that after backend.Dispose() a subsequent mx.DisconnectAsync does not bump the count on a registered OnHostStatusChanged handler — guards against the subscription-leak regression where a lingering backend instance would accumulate cross-reconnect notifications for a dead writer. Host.Tests csproj gains a Reference to lib/ArchestrA.MxAccess.dll (identical to the reference PR 6 adds — conflict-free cherry-pick/merge since both PRs stage the same <Reference> node; git will collapse to one when either lands first). Full Galaxy.Host.Tests Unit suite: 26 pass / 0 fail (2 new host-status + 9 PR5 historian + 15 pre-existing PostMortemMmf/RecyclePolicy/StaPump/MemoryWatchdog/EndToEndIpc/Handshake). Galaxy.Host builds clean (0 errors, 0 warnings). Branch base — PR 8 is on phase-2-pr5-historian rather than phase-2-pr4-findings because the constructor path on MxAccessGalaxyBackend gained a new historian parameter in PR 5 and the Dispose implementation needs to coordinate the two unsubscribes; targeting the earlier base would leave a trivial conflict on Dispose.

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

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

@@ -34,16 +34,34 @@ public sealed class MxAccessGalaxyBackend : IGalaxyBackend, IDisposable
         _refToSubs = new(System.StringComparer.OrdinalIgnoreCase);
 
     public event System.EventHandler<OnDataChangeNotification>? OnDataChange;
-#pragma warning disable CS0067 // event not yet raised — alarm + host-status wire-up in PR #4 follow-up
+#pragma warning disable CS0067 // alarm wire-up deferred to PR 9
     public event System.EventHandler<GalaxyAlarmEvent>? OnAlarmEvent;
-    public event System.EventHandler<HostConnectivityStatus>? OnHostStatusChanged;
 #pragma warning restore CS0067
+    public event System.EventHandler<HostConnectivityStatus>? OnHostStatusChanged;
+
+    private readonly System.EventHandler<bool> _onConnectionStateChanged;
 
     public MxAccessGalaxyBackend(GalaxyRepository repository, MxAccessClient mx, IHistorianDataSource? historian = null)
     {
         _repository = repository;
         _mx = mx;
         _historian = historian;
+
+        // PR 8: gateway-level host-status push. When the MXAccess COM proxy transitions
+        // connected↔disconnected, raise OnHostStatusChanged with a synthetic host entry named
+        // after the Wonderware client identity so the Admin UI surfaces top-level transport
+        // health even before per-platform/per-engine probing lands (deferred to a later PR that
+        // ports v1's GalaxyRuntimeProbeManager with ScanState subscriptions).
+        _onConnectionStateChanged = (_, connected) =>
+        {
+            OnHostStatusChanged?.Invoke(this, new HostConnectivityStatus
+            {
+                HostName = _mx.ClientName,
+                RuntimeStatus = connected ? "Running" : "Stopped",
+                LastObservedUtcUnixMs = DateTimeOffset.UtcNow.ToUnixTimeMilliseconds(),
+            });
+        };
+        _mx.ConnectionStateChanged += _onConnectionStateChanged;
     }
 
     public async Task<OpenSessionResponse> OpenSessionAsync(OpenSessionRequest req, CancellationToken ct)
@@ -267,7 +285,11 @@ public sealed class MxAccessGalaxyBackend : IGalaxyBackend, IDisposable
     public Task<RecycleStatusResponse> RecycleAsync(RecycleHostRequest req, CancellationToken ct)
         => Task.FromResult(new RecycleStatusResponse { Accepted = true, GraceSeconds = 15 });
 
-    public void Dispose() => _historian?.Dispose();
+    public void Dispose()
+    {
+        _mx.ConnectionStateChanged -= _onConnectionStateChanged;
+        _historian?.Dispose();
+    }
 
     private static GalaxyDataValue ToWire(string reference, Vtq vtq) => new()
     {