perf(comms+audit): close phase-2 residuals — direct ingest path, monotonic timeouts, synthetic probe, not-reporting set, cursor-exact audit pull

docs/requirements/Component-AuditLog.md

@@ -343,21 +343,29 @@ mapping/unexpected fault collapses to empty without a second dial (the other
 node would hit the same fault). The same resolution + failover applies to Site
 Call Audit's `PullSiteCalls`.
 
-**Cursor keyset.** The site side of the composite `(OccurredAtUtc, EventId)`
+**Cursor keyset.** The composite `(OccurredAtUtc, EventId)` keyset is wired end to
-keyset now exists (WP2.3): `PullAuditEventsRequest.after_id` (additive field 3)
+end. `PullAuditEventsRequest.after_id` (additive field 3) mirrors
-mirrors `PullSiteCallsRequest.after_id`, and `ISiteAuditQueue.ReadPendingSinceAsync`
+`PullSiteCallsRequest.after_id`; `ISiteAuditQueue.ReadPendingSinceAsync` switches
-switches from the inclusive `OccurredAtUtc >= since` to a strict composite
+from the inclusive `OccurredAtUtc >= since` to a strict composite comparison when it
-comparison when it is set, so a burst sharing one instant drains via the id
+is set, so a burst sharing one instant drains via the id tiebreak instead of pinning
-tiebreak instead of pinning the cursor.
+the cursor; and `IPullAuditEventsClient` /`GrpcPullAuditEventsClient` /
+`SiteAuditReconciliationActor` populate it — the actor's per-site watermark is the
+`(timestamp, id)` pair of the highest row ingested, threaded back into the next pull,
+mirroring `SiteCallAuditActor`'s `PullSiteCalls` cursor exactly.
 
-> **Central still sends a bare timestamp (tracked follow-up).**
+That makes retirement **exact**: `MarkReconciledUpToAsync` is handed the id half, so
-> `IPullAuditEventsClient`/`SiteAuditReconciliationActor` do not yet populate
+the rows **at** the cursor instant are proven received and retired, where a bare
-> `after_id`, so today's cursor remains a single `sinceUtc` under the legacy
+timestamp could only prove receipt of rows strictly older than itself and left the
-> inclusive read. Two consequences, both benign: a window whose rows all share one
+boundary rows servable until a newer row moved the cursor. `after_id` is null only on
-> instant re-serves rather than advancing (idempotent on `EventId`, as before), and
+the first pull for a site (and against a site that predates the field), which keeps
-> the rows **at** the cursor instant cannot be proven received, so they stay
+the legacy inclusive `>=` read as the compatible fallback. The cursor stays in-memory
-> servable until a newer row moves the cursor. Wiring `after_id` at central makes
+and resets on singleton restart; idempotent `InsertIfNotExistsAsync` still absorbs any
-> the retirement exact; the site accepts it already.
+re-pulled duplicates, so exactness is an optimization, not a correctness dependency.
+
+Unlike Site Call Audit, this actor issues ONE pull per tick rather than paging within
+it — a lagging drain is meant to surface as the stalled signal. The id tiebreak is
+what makes that safe against a same-instant burst larger than one batch: the cursor
+advances on every tick even when the timestamp cannot.
 
 ### Central direct-write (central-originated events)
 

src/ZB.MOM.WW.ScadaBridge.AuditLog/Central/AuditLogIngestActor.cs

@@ -58,15 +58,21 @@ public class AuditLogIngestActor : ReceiveActor
     /// SHORTER than the gRPC Ask that wraps it.
     /// </summary>
     /// <remarks>
-    /// The path used to stack three identical 30 s budgets — the site's Ask
+    /// The path used to stack three identical 30 s budgets — the site's Ask, the
-    /// (<c>CommunicationOptions.NotificationForwardTimeout</c>), the central gRPC
+    /// central gRPC handler's Ask (<c>SiteStreamGrpcServer.AuditIngestAskTimeout</c>)
-    /// handler's Ask (<c>SiteStreamGrpcServer.AuditIngestAskTimeout</c>) and the
+    /// and the ADO.NET command default — so they all expired at the same instant. The
-    /// ADO.NET command default — so they all expired at the same instant. The
     /// caller therefore learned nothing except "it took 30 s": no partial ack, no
     /// distinction between a slow database and a wedged singleton. Making the
     /// innermost budget strictly smallest means a slow batch is abandoned by the
     /// actor FIRST, with the accepted-so-far ids still replied, while the outer
     /// Asks are still waiting.
+    /// <para>
+    /// The full ladder is now strictly monotonic end to end:
+    /// <c>CommunicationOptions.AuditForwardTimeout</c> (35 s, the site-side forward Ask) &gt;
+    /// <c>SiteStreamGrpcServer.AuditIngestAskTimeout</c> (30 s, the gRPC deadline AND central's
+    /// Ask of this singleton) &gt; <see cref="IngestBudget"/> (20 s) &gt;
+    /// <see cref="IngestSqlCommandTimeout"/> (15 s).
+    /// </para>
     /// </remarks>
     internal static readonly TimeSpan IngestBudget = TimeSpan.FromSeconds(20);
 

src/ZB.MOM.WW.ScadaBridge.AuditLog/Central/GrpcPullAuditEventsClient.cs

@@ -73,6 +73,7 @@ public sealed class GrpcPullAuditEventsClient : IPullAuditEventsClient
     public async Task<PullAuditEventsResponse> PullAsync(
         string siteId,
         DateTime sinceUtc,
+        string? afterId,
         int batchSize,
         CancellationToken ct)
     {
@@ -93,6 +94,10 @@ public sealed class GrpcPullAuditEventsClient : IPullAuditEventsClient
             // EnsureUtc keeps Timestamp.FromDateTime happy (it requires UTC kind).
             SinceUtc = Timestamp.FromDateTime(EnsureUtc(sinceUtc)),
             BatchSize = batchSize,
+            // Composite-keyset tiebreak (proto field 3), mirroring PullSiteCalls exactly.
+            // proto3 has no nullable string — an unset/empty AfterId is the site's signal to
+            // keep the legacy inclusive-timestamp contract (also what a first pull sends).
+            AfterId = afterId ?? string.Empty,
         };
 
         var (reply, transportFault) = await TryInvokeAsync(endpoint, request, siteId, ct)
@@ -121,10 +126,13 @@ public sealed class GrpcPullAuditEventsClient : IPullAuditEventsClient
 
         // Map proto DTOs to canonical AuditEvent records and order oldest-first
         // (the wire is already ordered by the site queue, but the
-        // IPullAuditEventsClient contract is explicit, so sort defensively).
+        // IPullAuditEventsClient contract is explicit, so sort defensively). The EventId
+        // tiebreak matches the site's own composite ordering — ordinal over the "D" GUID
+        // text, which is what SQLite's BINARY collation compares.
         var events = reply.Events
             .Select(AuditEventDtoMapper.FromDto)
             .OrderBy(e => e.OccurredAtUtc)
+            .ThenBy(e => e.EventId.ToString(), StringComparer.Ordinal)
             .ToList();
 
         return new PullAuditEventsResponse(events, reply.MoreAvailable);

src/ZB.MOM.WW.ScadaBridge.AuditLog/Central/IPullAuditEventsClient.cs

@@ -39,12 +39,24 @@ public interface IPullAuditEventsClient
     /// </summary>
     /// <param name="siteId">The identifier of the site to pull audit events from.</param>
     /// <param name="sinceUtc">Only events with an <c>OccurredAtUtc</c> at or after this cursor time are returned.</param>
+    /// <param name="afterId">
+    /// The composite-keyset tiebreak cursor, mirroring
+    /// <see cref="IPullSiteCallsClient.PullAsync"/>. When non-null it is the
+    /// <c>EventId</c> ("D" GUID form) of the last row already consumed at
+    /// <paramref name="sinceUtc"/>; the site returns only rows strictly greater than the
+    /// composite <c>(OccurredAtUtc, EventId)</c> pair, so a burst sharing one exact instant
+    /// drains via the id tiebreak instead of pinning the inclusive-timestamp cursor — and the
+    /// site's <c>MarkReconciledUpToAsync</c> can retire the rows AT that instant, which a bare
+    /// timestamp can never prove received. Null on the first pull (or against a legacy site)
+    /// preserves the inclusive <c>&gt;=</c> contract.
+    /// </param>
     /// <param name="batchSize">Maximum number of events to return per call.</param>
     /// <param name="ct">Cancellation token.</param>
     /// <returns>A task that resolves to the next reconciliation batch with a <c>MoreAvailable</c> flag.</returns>
     Task<PullAuditEventsResponse> PullAsync(
         string siteId,
         DateTime sinceUtc,
+        string? afterId,
         int batchSize,
         CancellationToken ct);
 }

src/ZB.MOM.WW.ScadaBridge.AuditLog/Central/SiteAuditReconciliationActor.cs

@@ -76,14 +76,28 @@ public class SiteAuditReconciliationActor : ReceiveActor
     private readonly ILogger<SiteAuditReconciliationActor> _logger;
 
     /// <summary>
-    /// Per-site reconciliation watermark — the highest
+    /// Per-site reconciliation watermark — the COMPOSITE
-    /// <see cref="AuditEvent.OccurredAtUtc"/> seen for that site on a previous
+    /// <c>(OccurredAtUtc, EventId)</c> of the highest row seen for that site on a previous tick,
-    /// tick. Asking for <c>OccurredAtUtc &gt;= cursor</c> rather than &gt;
+    /// mirroring <c>SiteCallAuditActor</c>'s <c>PullSiteCalls</c> cursor.
-    /// is the site contract (<see cref="ZB.MOM.WW.ScadaBridge.Commons.Interfaces.Services.ISiteAuditQueue.ReadPendingSinceAsync"/>);
-    /// duplicate-with-same-timestamp rows are filtered out by the idempotent
-    /// repository write.
     /// </summary>
-    private readonly Dictionary<string, DateTime> _cursors = new();
+    /// <remarks>
+    /// <para>
+    /// The site's
+    /// <see cref="ZB.MOM.WW.ScadaBridge.Commons.Interfaces.Services.ISiteAuditQueue.ReadPendingSinceAsync"/>
+    /// serves rows strictly after the pair when <c>AfterId</c> is set, and falls back to the
+    /// legacy inclusive <c>OccurredAtUtc &gt;= since</c> when it is null (the first pull).
+    /// Sending the id half matters twice over: a burst sharing one exact instant drains via the
+    /// tiebreak instead of pinning the timestamp forever, and
+    /// <see cref="ZB.MOM.WW.ScadaBridge.Commons.Interfaces.Services.ISiteAuditQueue.MarkReconciledUpToAsync"/>
+    /// can retire the rows AT the cursor instant — a bare timestamp can only prove receipt of
+    /// rows strictly older than itself, so the boundary rows stayed servable forever.
+    /// </para>
+    /// <para>
+    /// Duplicates that a re-pull does produce are still filtered by the idempotent repository
+    /// write, so the cursor is an optimization for exactness, never a correctness crutch.
+    /// </para>
+    /// </remarks>
+    private readonly Dictionary<string, (DateTime Since, string? AfterId)> _cursors = new();
 
     /// <summary>
     /// Per-site count of consecutive non-draining cycles. Resets to zero on the
@@ -239,20 +253,33 @@ public class SiteAuditReconciliationActor : ReceiveActor
     /// <summary>
     /// Issues one <c>PullAuditEvents</c> RPC against the site, ingests the
     /// returned rows idempotently into the central repository, and advances
-    /// the cursor based on the maximum <see cref="AuditEvent.OccurredAtUtc"/>
+    /// the composite <c>(<see cref="AuditEvent.OccurredAtUtc"/>, <see cref="AuditEvent.EventId"/>)</c>
-    /// observed. The brief's "saturate until backlog clears" intent is met by
+    /// cursor to the maximum row observed. The brief's "saturate until backlog clears" intent is
-    /// the natural cadence — each tick issues one pull, and a backed-up site
+    /// met by the natural cadence — each tick issues one pull, and a backed-up site
     /// drains across consecutive ticks. The stalled signal (two non-draining
     /// ticks in a row) surfaces when that drain isn't keeping up.
     /// </summary>
+    /// <remarks>
+    /// Unlike <c>SiteCallAuditActor</c> this does NOT page within a tick; one pull per tick is
+    /// deliberate (the stalled signal is how a lagging drain surfaces). The composite cursor is
+    /// what makes that safe against a same-instant burst larger than one batch: the id tiebreak
+    /// advances even when the timestamp cannot, so consecutive ticks make real progress instead
+    /// of re-serving the same window forever.
+    /// </remarks>
     private async Task PullSiteAsync(SiteEntry site, IAuditLogRepository repository, Akka.Event.EventStream eventStream)
     {
-        var since = _cursors.TryGetValue(site.SiteId, out var c) ? c : DateTime.MinValue;
+        var cursor = _cursors.TryGetValue(site.SiteId, out var c)
+            ? c
+            : (Since: DateTime.MinValue, AfterId: (string?)null);
+        var since = cursor.Since;
+        var afterId = cursor.AfterId;
+
         var response = await _client.PullAsync(
-            site.SiteId, since, _options.BatchSize, CancellationToken.None)
+            site.SiteId, since, afterId, _options.BatchSize, CancellationToken.None)
             .ConfigureAwait(false);
 
         var maxOccurred = since;
+        var maxAfterId = afterId;
         var hasUnresolvedFailure = false;
         var nowUtc = DateTime.UtcNow;
         foreach (var evt in response.Events)
@@ -310,10 +337,18 @@ public class SiteAuditReconciliationActor : ReceiveActor
             }
 
             // Canonical OccurredAtUtc is a DateTimeOffset; the cursor is a UTC DateTime.
+            // Advance the COMPOSITE max, exactly as SiteCallAuditActor does: a greater
+            // timestamp wins; on a tie the greater EventId (ordinal over the "D" GUID text,
+            // matching the site's SQLite BINARY collation) wins. CompareOrdinal handles the
+            // null seed — CompareOrdinal(x, null) > 0 for any non-null x.
             var occurredUtc = evt.OccurredAtUtc.UtcDateTime;
-            if (advanceForThisRow && occurredUtc > maxOccurred)
+            var rowId = evt.EventId.ToString();
+            if (advanceForThisRow &&
+                (occurredUtc > maxOccurred ||
+                 (occurredUtc == maxOccurred && string.CompareOrdinal(rowId, maxAfterId) > 0)))
             {
                 maxOccurred = occurredUtc;
+                maxAfterId = rowId;
             }
         }
 
@@ -322,8 +357,11 @@ public class SiteAuditReconciliationActor : ReceiveActor
         // the whole batch next tick — successful rows are no-ops thanks to
         // InsertIfNotExistsAsync's idempotency, and the failing row gets
         // another attempt. Once it succeeds (or hits the permanent-abandon
-        // threshold) the cursor unblocks naturally.
+        // threshold) the cursor unblocks naturally. Both halves move together: holding the
-        _cursors[site.SiteId] = hasUnresolvedFailure ? since : maxOccurred;
+        // timestamp back while advancing the id would skip the very rows being retried.
+        _cursors[site.SiteId] = hasUnresolvedFailure
+            ? (since, afterId)
+            : (maxOccurred, maxAfterId);
 
         var nonDraining = response.MoreAvailable && response.Events.Count > 0;
         UpdateStalledState(site.SiteId, draining: !nonDraining, eventStream);

src/ZB.MOM.WW.ScadaBridge.CentralUI/Components/Pages/Monitoring/AlarmSummary.razor

@@ -331,8 +331,10 @@
                 return;
             }
 
-            // _notReporting is the poll's unique authority — the alarm-only live cache
+            // _notReporting always comes from this call, live or not. While the cache is
-            // cannot compute it — so it is always refreshed.
+            // serving the site the façade sources it from the aggregator's own fan-out (no
+            // second fan-out is run); while it is cold the poll computes it. Either way the
+            // shape and ordering are identical.
             _notReporting = result.NotReportingInstances;
 
             // While the cache is live, the live deltas own the row set: a poll whose fan-out

src/ZB.MOM.WW.ScadaBridge.CentralUI/Services/AlarmSummaryService.cs

@@ -154,9 +154,11 @@ public sealed class AlarmSummaryService : IAlarmSummaryService
             .ThenBy(r => r.Alarm.AlarmName, StringComparer.OrdinalIgnoreCase)
             .ToList();
 
-        // The live cache is alarm-only, so it cannot enumerate "not reporting"
+        // Alarm rows alone cannot name the instances that failed to answer, so this pure
-        // instances — left empty here; the periodic poll (GetSiteAlarmsAsync)
+        // flattening leaves the list empty. The names come from the fan-out that produced the
-        // remains the authority for that list.
+        // snapshot: while the live cache is serving a site, SharedAlarmSummaryService pairs
+        // these rows with ISiteAlarmLiveCache.GetNotReportingInstances (the aggregator's own
+        // seed/reconcile result); while it is cold, GetSiteAlarmsAsync's poll supplies both.
         return new AlarmSummaryResult(orderedRows, Array.Empty<string>());
     }
 

src/ZB.MOM.WW.ScadaBridge.CentralUI/Services/SharedAlarmSummaryService.cs

@@ -1,6 +1,5 @@
 using System.Collections.Concurrent;
 using Microsoft.Extensions.DependencyInjection;
-using Microsoft.Extensions.Options;
 using ZB.MOM.WW.ScadaBridge.Commons.Messages.Streaming;
 using ZB.MOM.WW.ScadaBridge.Communication;
 
@@ -18,12 +17,25 @@ namespace ZB.MOM.WW.ScadaBridge.CentralUI.Services;
 /// viewers are watching.
 /// </para>
 /// <para>
-/// <b>The freshness window follows the live cache.</b> While
+/// <b>While the live cache is serving the site there is NO fan-out at all.</b> The page already
-/// <see cref="ISiteAlarmLiveCache.IsLive"/> is true the page deliberately ignores the poll's
+/// ignores the poll's alarm rows in that state (the live deltas own them — arch-review R2 N5),
-/// alarm rows (the live deltas own them — arch-review R2 N5), so the only thing the poll
+/// and the aggregator's own seed/reconcile fan-out now publishes the not-reporting set through
-/// still supplies is the not-reporting list, and the window widens to the aggregator's own
+/// <see cref="ISiteAlarmLiveCache.GetNotReportingInstances"/> — the one thing the poll used to
-/// reconcile interval. While the cache is cold the poll is the page's full-rebuild safety
+/// still supply. So the live path is answered entirely from the cache: rows flattened from
-/// net, so the window stays just under the page's 15s tick and every tick gets fresh data.
+/// <see cref="ISiteAlarmLiveCache.GetCurrentAlarms"/> through the same
+/// <c>AlarmSummaryService.BuildFromLiveAlarmsCore</c> the page's live subscription uses, plus the
+/// cached not-reporting names. One fan-out per site per reconcile window, run by the aggregator,
+/// instead of two.
+/// </para>
+/// <para>
+/// The rows are returned rather than left empty on purpose: liveness can flip to false in the
+/// window between this call and the page's own <c>IsLive</c> re-check, and the page would then
+/// adopt whatever came back. Handing it the live snapshot makes that race land on last-known
+/// state instead of a blank grid.
+/// </para>
+/// <para>
+/// While the cache is cold the poll is the page's full-rebuild safety net, so it still fans out,
+/// memoized with a window just under the page's 15s tick so every tick gets fresh data.
 /// </para>
 /// </summary>
 public sealed class SharedAlarmSummaryService : IAlarmSummaryService
@@ -36,7 +48,6 @@ public sealed class SharedAlarmSummaryService : IAlarmSummaryService
 
     private readonly IServiceScopeFactory _scopeFactory;
     private readonly ISiteAlarmLiveCache _liveCache;
-    private readonly TimeSpan _liveCacheTtl;
     private readonly Func<DateTimeOffset>? _clock;
 
     private readonly ConcurrentDictionary<int, SingleFlightMemo<AlarmSummaryResult>> _bySite = new();
@@ -44,36 +55,34 @@ public sealed class SharedAlarmSummaryService : IAlarmSummaryService
     /// <summary>
     /// Initializes the shared alarm summary façade.
     /// </summary>
+    /// <remarks>
+    /// There is no live-cache freshness window any more: the live path reads the aggregator's
+    /// cache directly and never fans out, so the only window left is <see cref="ColdCacheTtl"/>.
+    /// The reconcile interval this ctor used to read from <c>CommunicationOptions</c> is now
+    /// implicit — it IS the cadence at which the aggregator refreshes what the live path returns.
+    /// </remarks>
     /// <param name="scopeFactory">Opens a fresh DI scope per fan-out (fresh repositories, off any circuit scope).</param>
-    /// <param name="liveCache">The shared live alarm cache, consulted only for its per-site liveness.</param>
+    /// <param name="liveCache">The shared live alarm cache: liveness, alarm rows, and not-reporting names.</param>
-    /// <param name="options">Communication options; supplies the aggregator reconcile interval.</param>
     public SharedAlarmSummaryService(
         IServiceScopeFactory scopeFactory,
-        ISiteAlarmLiveCache liveCache,
+        ISiteAlarmLiveCache liveCache)
-        IOptions<CommunicationOptions> options)
+        : this(scopeFactory, liveCache, clock: null)
-        : this(scopeFactory, liveCache, (options ?? throw new ArgumentNullException(nameof(options)))
-            .Value.LiveAlarmCacheReconcileInterval, clock: null)
     {
     }
 
     /// <summary>
-    /// Test seam: same façade with an explicit live-cache window and clock.
+    /// Test seam: same façade with an explicit clock.
     /// </summary>
     /// <param name="scopeFactory">Opens a fresh DI scope per fan-out.</param>
     /// <param name="liveCache">The shared live alarm cache.</param>
-    /// <param name="liveCacheTtl">Freshness window used while the live cache is serving the site.</param>
     /// <param name="clock">Clock used for freshness.</param>
     internal SharedAlarmSummaryService(
         IServiceScopeFactory scopeFactory,
         ISiteAlarmLiveCache liveCache,
-        TimeSpan liveCacheTtl,
         Func<DateTimeOffset>? clock)
     {
         _scopeFactory = scopeFactory ?? throw new ArgumentNullException(nameof(scopeFactory));
         _liveCache = liveCache ?? throw new ArgumentNullException(nameof(liveCache));
-        // Never shorter than the cold window — a misconfigured reconcile interval must not
-        // silently make the memo useless.
-        _liveCacheTtl = liveCacheTtl > ColdCacheTtl ? liveCacheTtl : ColdCacheTtl;
         _clock = clock;
     }
 
@@ -81,17 +90,25 @@ public sealed class SharedAlarmSummaryService : IAlarmSummaryService
     public Task<AlarmSummaryResult> GetSiteAlarmsAsync(
         int siteId, CancellationToken cancellationToken = default)
     {
+        // Live: answer from the shared aggregator's cache and skip the fan-out entirely. Not
+        // memoized — there is nothing to amortize, and reading the cache directly keeps the
+        // answer as fresh as the aggregator itself.
+        if (_liveCache.IsLive(siteId))
+        {
+            var live = AlarmSummaryService.BuildFromLiveAlarmsCore(_liveCache.GetCurrentAlarms(siteId));
+            return Task.FromResult(new AlarmSummaryResult(
+                live.Alarms, _liveCache.GetNotReportingInstances(siteId)));
+        }
+
         var memo = _bySite.GetOrAdd(
             siteId,
             _ => new SingleFlightMemo<AlarmSummaryResult>(ColdCacheTtl, _clock));
 
-        var ttl = _liveCache.IsLive(siteId) ? _liveCacheTtl : ColdCacheTtl;
-
         return memo.GetAsync(
             () => FanOutAsync(siteId),
             forceRefresh: false,
             cancellationToken,
-            ttlOverride: ttl);
+            ttlOverride: ColdCacheTtl);
     }
 
     /// <inheritdoc/>

src/ZB.MOM.WW.ScadaBridge.Commons/Messages/Health/HeartbeatMessage.cs

@@ -1,7 +1,27 @@
 namespace ZB.MOM.WW.ScadaBridge.Commons.Messages.Health;
 
+/// <summary>
+/// Application-level heartbeat a site node sends to central on its heartbeat interval.
+/// Central's health aggregator stamps the site's last-seen time from it, and the offline
+/// timeout fires when they stop landing.
+/// </summary>
+/// <param name="SiteId">The reporting site's identifier.</param>
+/// <param name="NodeHostname">The reporting node's hostname.</param>
+/// <param name="IsActive">Whether the reporting node is the site's active (oldest-Up) node.</param>
+/// <param name="Timestamp">When the heartbeat was produced (UTC).</param>
+/// <param name="Synthetic">
+/// <c>true</c> marks a heartbeat that carries NO liveness meaning and that central must skip for
+/// all health bookkeeping — it neither stamps a last-seen time nor replicates to the peer node.
+/// The only producer today is <c>CentralChannelProvider</c>'s failback probe, which reuses the
+/// heartbeat RPC purely to ask "does the preferred central endpoint answer again?"; it is emitted
+/// by a SITE's transport layer, not by any node's heartbeat timer, and counting it as liveness
+/// would keep a site looking alive on the health dashboard from the probe alone. Additive with a
+/// <c>false</c> default (and proto field 5 on <c>HeartbeatDto</c>), so every real heartbeat —
+/// including one from a node that predates the flag — is unaffected.
+/// </param>
 public record HeartbeatMessage(
     string SiteId,
     string NodeHostname,
     bool IsActive,
-    DateTimeOffset Timestamp);
+    DateTimeOffset Timestamp,
+    bool Synthetic = false);

src/ZB.MOM.WW.ScadaBridge.Communication/Actors/CentralCommunicationActor.cs

@@ -67,36 +67,6 @@ public class CentralCommunicationActor : ReceiveActor
     /// </summary>
     private IActorRef? _notificationOutboxProxy;
 
-    /// <summary>
-    /// Proxy <see cref="IActorRef"/> for the central AuditLogIngestActor cluster
-    /// singleton. Set via <see cref="RegisterAuditIngest"/> — the Host creates the
-    /// singleton proxy after this actor and registers it (mirrors
-    /// <see cref="_notificationOutboxProxy"/>). Null until registration completes;
-    /// an audit ingest command arriving before then is answered with an empty
-    /// reply so the site keeps its rows Pending and retries.
-    ///
-    /// Once registered, the handler Asks this proxy and pipes the reply straight
-    /// back to the caller. On an Ask timeout or a faulted reply, PipeTo forwards a
-    /// <see cref="Status.Failure"/> to the caller — the fault propagates rather
-    /// than being swallowed. This differs from the gRPC handler
-    /// (<c>SiteStreamGrpcServer</c>), which catches the exception and returns an
-    /// empty ack; here the faulted Ask is the transient signal the site relies on
-    /// (see <see cref="HandleIngestAuditEvents"/>).
-    /// </summary>
-    private IActorRef? _auditIngestProxy;
-
-    /// <summary>
-    /// Default Ask timeout for routing audit ingest commands to the
-    /// Effective Ask timeout for audit ingest routing. Defaults to
-    /// <see cref="Grpc.SiteStreamGrpcServer.AuditIngestAskTimeout"/> (30 s) — the two
-    /// audit-ingest entry points (the site stream server and the control plane) share one source of truth
-    /// for the timeout. Overridable via the constructor so tests can exercise the
-    /// timeout/fault path without waiting 30 s. When the window is exceeded the Ask
-    /// faults and that fault is piped back to the caller as a
-    /// <see cref="Status.Failure"/> (see <see cref="HandleIngestAuditEvents"/>).
-    /// </summary>
-    private readonly TimeSpan _auditIngestAskTimeout;
-
     /// <summary>
     /// DistributedPubSub topic used to fan health reports out to the peer
     /// central node so both per-node aggregators stay in sync. See
@@ -111,12 +81,10 @@ public class CentralCommunicationActor : ReceiveActor
     /// </summary>
     /// <param name="serviceProvider">DI service provider for scoped repository and aggregator access.</param>
     /// <param name="transport">The central→site command transport to route every <see cref="SiteEnvelope"/> through.</param>
-    /// <param name="auditIngestAskTimeout">Optional override for the audit-ingest Ask timeout (test hook).</param>
     public CentralCommunicationActor(
         IServiceProvider serviceProvider,
-        ISiteCommandTransport transport,
+        ISiteCommandTransport transport)
-        TimeSpan? auditIngestAskTimeout = null)
+        : this(serviceProvider)
-        : this(serviceProvider, auditIngestAskTimeout)
     {
         _transport = transport ?? throw new ArgumentNullException(nameof(transport));
     }
@@ -125,13 +93,9 @@ public class CentralCommunicationActor : ReceiveActor
     /// <see cref="_transport"/> is assigned by the delegating public constructor before any message
     /// is dispatched.</summary>
     /// <param name="serviceProvider">DI service provider.</param>
-    /// <param name="auditIngestAskTimeout">Optional audit-ingest Ask timeout override.</param>
+    private CentralCommunicationActor(IServiceProvider serviceProvider)
-    private CentralCommunicationActor(
-        IServiceProvider serviceProvider,
-        TimeSpan? auditIngestAskTimeout)
     {
         _serviceProvider = serviceProvider;
-        _auditIngestAskTimeout = auditIngestAskTimeout ?? Grpc.SiteStreamGrpcServer.AuditIngestAskTimeout;
 
         // Site address cache loaded from database
         Receive<SiteAddressCacheLoaded>(HandleSiteAddressCacheLoaded);
@@ -176,24 +140,11 @@ public class CentralCommunicationActor : ReceiveActor
         // so the NotificationStatusResponse routes back to the querying site.
         Receive<NotificationStatusQuery>(HandleNotificationStatusQuery);
 
-        // Audit Log: the Host registers the AuditLogIngestActor singleton
+        // Audit Log site→central ingest is NOT relayed here. The central-hosted
-        // proxy after this actor is created (the proxy cannot exist before this
+        // CentralControlGrpcService Asks the audit-log-ingest singleton proxy directly
-        // actor's construction).
+        // (as SiteStreamGrpcServer always has), so this actor no longer sits in that
-        Receive<RegisterAuditIngest>(msg =>
+        // path — the relay was a second hop with an identical 30 s Ask timeout that
-        {
+        // could only add latency. Do not reintroduce it.
-            _auditIngestProxy = msg.AuditIngestActor;
-            _log.Info("Registered audit ingest proxy");
-        });
-
-        // Audit Log site→central ingest: a site forwards a batch of audit
-        // events to the central cluster. Ask the ingest proxy
-        // and pipe the IngestAuditEventsReply back to the original Sender (the
-        // site's transport path) so the site can flip its rows to Forwarded.
-        Receive<IngestAuditEventsCommand>(HandleIngestAuditEvents);
-
-        // Audit Log combined-telemetry ingest: routes to the same proxy
-        // the same way; the proxy replies with an IngestCachedTelemetryReply.
-        Receive<IngestCachedTelemetryCommand>(HandleIngestCachedTelemetry);
 
         // Startup reconciliation: a site node forwards its local deployed inventory on
         // startup. Resolve the scoped ReconcileService, diff the
@@ -239,51 +190,6 @@ public class CentralCommunicationActor : ReceiveActor
         _notificationOutboxProxy.Forward(msg);
     }
 
-    private void HandleIngestAuditEvents(IngestAuditEventsCommand msg)
-    {
-        if (_auditIngestProxy == null)
-        {
-            // No ingest proxy registered yet (host startup race). Reply with an
-            // empty IngestAuditEventsReply so the site keeps its rows Pending and
-            // retries — the same behaviour as the gRPC handler's wiring-race path.
-            _log.Warning(
-                "Cannot route IngestAuditEventsCommand ({0} events) — audit ingest not available",
-                msg.Events.Count);
-            Sender.Tell(new IngestAuditEventsReply(Array.Empty<Guid>()));
-            return;
-        }
-
-        // Capture Sender before the async/PipeTo — Akka resets Sender between
-        // dispatches. The reply is piped straight back to the calling site node.
-        // On an Ask timeout or a faulted reply, PipeTo delivers a Status.Failure to
-        // replyTo: the fault propagates to the caller rather than being swallowed.
-        // The site's own Ask through this path then faults, and the site drain loop
-        // treats that as a transient failure — rows stay Pending and are retried on
-        // the next tick. (The gRPC handler instead returns an empty ack on fault;
-        // propagating the fault here is the cleaner transient signal.)
-        var replyTo = Sender;
-        _log.Debug("Routing IngestAuditEventsCommand ({0} events) to the audit ingest actor", msg.Events.Count);
-        _auditIngestProxy.Ask<IngestAuditEventsReply>(msg, _auditIngestAskTimeout)
-            .PipeTo(replyTo);
-    }
-
-    private void HandleIngestCachedTelemetry(IngestCachedTelemetryCommand msg)
-    {
-        if (_auditIngestProxy == null)
-        {
-            _log.Warning(
-                "Cannot route IngestCachedTelemetryCommand ({0} entries) — audit ingest not available",
-                msg.Entries.Count);
-            Sender.Tell(new IngestCachedTelemetryReply(Array.Empty<Guid>()));
-            return;
-        }
-
-        var replyTo = Sender;
-        _log.Debug("Routing IngestCachedTelemetryCommand ({0} entries) to the audit ingest actor", msg.Entries.Count);
-        _auditIngestProxy.Ask<IngestCachedTelemetryReply>(msg, _auditIngestAskTimeout)
-            .PipeTo(replyTo);
-    }
-
     /// <summary>
     /// Startup reconciliation (site→central): resolve the scoped
     /// <see cref="ReconcileService"/> in a DI scope, diff the node's reported inventory
@@ -291,7 +197,7 @@ public class CentralCommunicationActor : ReceiveActor
     /// back to the site node's transport path. The actor stays thin — all the diff
     /// and staging logic lives in the service. Mirrors the DB-access pattern used by
     /// <see cref="LoadSiteAddressesFromDb"/> (Task.Run + CreateScope + PipeTo) and the
-    /// Sender-preservation pattern of <see cref="HandleIngestAuditEvents"/>.
+    /// Sender-preservation pattern of <see cref="HandleNotificationSubmit"/>.
     ///
     /// On a faulted task PipeTo delivers a <see cref="Status.Failure"/> to the node; its
     /// Ask faults and it simply retries reconcile on the next startup — reconcile is
@@ -328,6 +234,21 @@ public class CentralCommunicationActor : ReceiveActor
 
     private void HandleHeartbeat(HeartbeatMessage heartbeat)
     {
+        // Synthetic heartbeats carry no liveness meaning — today only
+        // CentralChannelProvider's failback probe, which reuses this RPC to ask whether the
+        // preferred central endpoint answers again. It is emitted by a site's TRANSPORT layer,
+        // not by any node's heartbeat timer, so stamping it would let a site whose real
+        // heartbeats had stopped keep looking alive on the health dashboard for as long as its
+        // transport kept probing. Dropped before the local mark AND before the peer fan-out,
+        // so neither central node's aggregator ever sees it.
+        if (heartbeat.Synthetic)
+        {
+            _log.Debug(
+                "Ignoring synthetic heartbeat from site {0} (node '{1}') — probe traffic, not liveness",
+                heartbeat.SiteId, heartbeat.NodeHostname);
+            return;
+        }
+
         MarkHeartbeatLocally(heartbeat);
 
         // Fan the heartbeat out to the peer central node so BOTH aggregators mark
@@ -350,9 +271,17 @@ public class CentralCommunicationActor : ReceiveActor
     /// <summary>
     /// Marks a site heartbeat on the local aggregator without re-broadcasting.
     /// Used for both site-originated heartbeats and peer-replicated ones.
+    /// A synthetic heartbeat is never marked — <see cref="HandleHeartbeat"/> already drops it
+    /// before the fan-out, so a replica can only carry one if a peer node predates the flag;
+    /// the guard here is belt-and-braces on the last hop before the aggregator.
     /// </summary>
     private void MarkHeartbeatLocally(HeartbeatMessage heartbeat)
     {
+        if (heartbeat.Synthetic)
+        {
+            return;
+        }
+
         var aggregator = _serviceProvider.GetService<ICentralHealthAggregator>();
         aggregator?.MarkHeartbeat(heartbeat.SiteId, heartbeat.Timestamp);
     }
@@ -623,13 +552,7 @@ public record DebugStreamTerminated(string SiteId, string CorrelationId);
 /// </summary>
 public record RegisterNotificationOutbox(IActorRef OutboxProxy);
 
-/// <summary>
+// NOTE: there is deliberately no RegisterAuditIngest counterpart. The audit-log-ingest
-/// Registers the central AuditLogIngestActor singleton proxy with the
+// singleton proxy goes STRAIGHT to the two gRPC servers that need it
-/// <see cref="CentralCommunicationActor"/> so site-forwarded
+// (CentralControlGrpcService.SetAuditIngestActor / SiteStreamGrpcServer.SetAuditIngestActor);
-/// <see cref="IngestAuditEventsCommand"/> and <see cref="IngestCachedTelemetryCommand"/>
+// routing audit batches through this actor was a redundant hop and was removed.
-/// messages can be routed to it. Sent by the Host after the audit-ingest
-/// singleton proxy is created. Lives here (not in Commons) because
-/// <c>ZB.MOM.WW.ScadaBridge.Commons</c> has no Akka package reference and cannot hold an
-/// <see cref="IActorRef"/> field.
-/// </summary>
-public sealed record RegisterAuditIngest(IActorRef AuditIngestActor);

src/ZB.MOM.WW.ScadaBridge.Communication/CentralControlGrpc/CentralControl.cs

@@ -114,33 +114,34 @@ namespace ZB.MOM.WW.ScadaBridge.Communication.Grpc {
             "c0R0bzoCOAEaQgogU3RvcmVBbmRGb3J3YXJkQnVmZmVyRGVwdGhzRW50cnkS",
             "CwoDa2V5GAEgASgJEg0KBXZhbHVlGAIgASgFOgI4ASJjChZTaXRlSGVhbHRo",
             "UmVwb3J0QWNrRHRvEg8KB3NpdGVfaWQYASABKAkSFwoPc2VxdWVuY2VfbnVt",
-            "YmVyGAIgASgDEhAKCGFjY2VwdGVkGAMgASgIEg0KBWVycm9yGAQgASgJIngK",
+            "YmVyGAIgASgDEhAKCGFjY2VwdGVkGAMgASgIEg0KBWVycm9yGAQgASgJIosB",
-            "DEhlYXJ0YmVhdER0bxIPCgdzaXRlX2lkGAEgASgJEhUKDW5vZGVfaG9zdG5h",
+            "CgxIZWFydGJlYXREdG8SDwoHc2l0ZV9pZBgBIAEoCRIVCg1ub2RlX2hvc3Ru",
-            "bWUYAiABKAkSEQoJaXNfYWN0aXZlGAMgASgIEi0KCXRpbWVzdGFtcBgEIAEo",
+            "YW1lGAIgASgJEhEKCWlzX2FjdGl2ZRgDIAEoCBItCgl0aW1lc3RhbXAYBCAB",
-            "CzIaLmdvb2dsZS5wcm90b2J1Zi5UaW1lc3RhbXAqvQEKFENvbm5lY3Rpb25I",
+            "KAsyGi5nb29nbGUucHJvdG9idWYuVGltZXN0YW1wEhEKCXN5bnRoZXRpYxgF",
-            "ZWFsdGhFbnVtEiEKHUNPTk5FQ1RJT05fSEVBTFRIX1VOU1BFQ0lGSUVEEAAS",
+            "IAEoCCq9AQoUQ29ubmVjdGlvbkhlYWx0aEVudW0SIQodQ09OTkVDVElPTl9I",
-            "HwobQ09OTkVDVElPTl9IRUFMVEhfQ09OTkVDVEVEEAESIgoeQ09OTkVDVElP",
+            "RUFMVEhfVU5TUEVDSUZJRUQQABIfChtDT05ORUNUSU9OX0hFQUxUSF9DT05O",
-            "Tl9IRUFMVEhfRElTQ09OTkVDVEVEEAISIAocQ09OTkVDVElPTl9IRUFMVEhf",
+            "RUNURUQQARIiCh5DT05ORUNUSU9OX0hFQUxUSF9ESVNDT05ORUNURUQQAhIg",
-            "Q09OTkVDVElORxADEhsKF0NPTk5FQ1RJT05fSEVBTFRIX0VSUk9SEAQyoQYK",
+            "ChxDT05ORUNUSU9OX0hFQUxUSF9DT05ORUNUSU5HEAMSGwoXQ09OTkVDVElP",
-            "FUNlbnRyYWxDb250cm9sU2VydmljZRKDAQoSU3VibWl0Tm90aWZpY2F0aW9u",
+            "Tl9IRUFMVEhfRVJST1IQBDKhBgoVQ2VudHJhbENvbnRyb2xTZXJ2aWNlEoMB",
-            "EjQuc2NhZGFicmlkZ2UuY2VudHJhbGNvbnRyb2wudjEuTm90aWZpY2F0aW9u",
+            "ChJTdWJtaXROb3RpZmljYXRpb24SNC5zY2FkYWJyaWRnZS5jZW50cmFsY29u",
-            "U3VibWl0RHRvGjcuc2NhZGFicmlkZ2UuY2VudHJhbGNvbnRyb2wudjEuTm90",
+            "dHJvbC52MS5Ob3RpZmljYXRpb25TdWJtaXREdG8aNy5zY2FkYWJyaWRnZS5j",
-            "aWZpY2F0aW9uU3VibWl0QWNrRHRvEpIBChdRdWVyeU5vdGlmaWNhdGlvblN0",
+            "ZW50cmFsY29udHJvbC52MS5Ob3RpZmljYXRpb25TdWJtaXRBY2tEdG8SkgEK",
-            "YXR1cxI5LnNjYWRhYnJpZGdlLmNlbnRyYWxjb250cm9sLnYxLk5vdGlmaWNh",
+            "F1F1ZXJ5Tm90aWZpY2F0aW9uU3RhdHVzEjkuc2NhZGFicmlkZ2UuY2VudHJh",
-            "dGlvblN0YXR1c1F1ZXJ5RHRvGjwuc2NhZGFicmlkZ2UuY2VudHJhbGNvbnRy",
+            "bGNvbnRyb2wudjEuTm90aWZpY2F0aW9uU3RhdHVzUXVlcnlEdG8aPC5zY2Fk",
-            "b2wudjEuTm90aWZpY2F0aW9uU3RhdHVzUmVzcG9uc2VEdG8SRwoRSW5nZXN0",
+            "YWJyaWRnZS5jZW50cmFsY29udHJvbC52MS5Ob3RpZmljYXRpb25TdGF0dXNS",
-            "QXVkaXRFdmVudHMSGy5zaXRlc3RyZWFtLkF1ZGl0RXZlbnRCYXRjaBoVLnNp",
+            "ZXNwb25zZUR0bxJHChFJbmdlc3RBdWRpdEV2ZW50cxIbLnNpdGVzdHJlYW0u",
-            "dGVzdHJlYW0uSW5nZXN0QWNrElAKFUluZ2VzdENhY2hlZFRlbGVtZXRyeRIg",
+            "QXVkaXRFdmVudEJhdGNoGhUuc2l0ZXN0cmVhbS5Jbmdlc3RBY2sSUAoVSW5n",
-            "LnNpdGVzdHJlYW0uQ2FjaGVkVGVsZW1ldHJ5QmF0Y2gaFS5zaXRlc3RyZWFt",
+            "ZXN0Q2FjaGVkVGVsZW1ldHJ5EiAuc2l0ZXN0cmVhbS5DYWNoZWRUZWxlbWV0",
-            "LkluZ2VzdEFjaxKAAQoNUmVjb25jaWxlU2l0ZRI2LnNjYWRhYnJpZGdlLmNl",
+            "cnlCYXRjaBoVLnNpdGVzdHJlYW0uSW5nZXN0QWNrEoABCg1SZWNvbmNpbGVT",
-            "bnRyYWxjb250cm9sLnYxLlJlY29uY2lsZVNpdGVSZXF1ZXN0RHRvGjcuc2Nh",
+            "aXRlEjYuc2NhZGFicmlkZ2UuY2VudHJhbGNvbnRyb2wudjEuUmVjb25jaWxl",
-            "ZGFicmlkZ2UuY2VudHJhbGNvbnRyb2wudjEuUmVjb25jaWxlU2l0ZVJlc3Bv",
+            "U2l0ZVJlcXVlc3REdG8aNy5zY2FkYWJyaWRnZS5jZW50cmFsY29udHJvbC52",
-            "bnNlRHRvEn0KEFJlcG9ydFNpdGVIZWFsdGgSMi5zY2FkYWJyaWRnZS5jZW50",
+            "MS5SZWNvbmNpbGVTaXRlUmVzcG9uc2VEdG8SfQoQUmVwb3J0U2l0ZUhlYWx0",
-            "cmFsY29udHJvbC52MS5TaXRlSGVhbHRoUmVwb3J0RHRvGjUuc2NhZGFicmlk",
+            "aBIyLnNjYWRhYnJpZGdlLmNlbnRyYWxjb250cm9sLnYxLlNpdGVIZWFsdGhS",
-            "Z2UuY2VudHJhbGNvbnRyb2wudjEuU2l0ZUhlYWx0aFJlcG9ydEFja0R0bxJQ",
+            "ZXBvcnREdG8aNS5zY2FkYWJyaWRnZS5jZW50cmFsY29udHJvbC52MS5TaXRl",
-            "CglIZWFydGJlYXQSKy5zY2FkYWJyaWRnZS5jZW50cmFsY29udHJvbC52MS5I",
+            "SGVhbHRoUmVwb3J0QWNrRHRvElAKCUhlYXJ0YmVhdBIrLnNjYWRhYnJpZGdl",
-            "ZWFydGJlYXREdG8aFi5nb29nbGUucHJvdG9idWYuRW1wdHlCK6oCKFpCLk1P",
+            "LmNlbnRyYWxjb250cm9sLnYxLkhlYXJ0YmVhdER0bxoWLmdvb2dsZS5wcm90",
-            "TS5XVy5TY2FkYUJyaWRnZS5Db21tdW5pY2F0aW9uLkdycGNiBnByb3RvMw=="));
+            "b2J1Zi5FbXB0eUIrqgIoWkIuTU9NLldXLlNjYWRhQnJpZGdlLkNvbW11bmlj",
+            "YXRpb24uR3JwY2IGcHJvdG8z"));
       descriptor = pbr::FileDescriptor.FromGeneratedCode(descriptorData,
           new pbr::FileDescriptor[] { global::Google.Protobuf.WellKnownTypes.EmptyReflection.Descriptor, global::Google.Protobuf.WellKnownTypes.TimestampReflection.Descriptor, global::Google.Protobuf.WellKnownTypes.WrappersReflection.Descriptor, global::ZB.MOM.WW.ScadaBridge.Communication.Grpc.SitestreamReflection.Descriptor, },
           new pbr::GeneratedClrTypeInfo(new[] {typeof(global::ZB.MOM.WW.ScadaBridge.Communication.Grpc.ConnectionHealthEnum), }, null, new pbr::GeneratedClrTypeInfo[] {
@@ -160,7 +161,7 @@ namespace ZB.MOM.WW.ScadaBridge.Communication.Grpc {
             new pbr::GeneratedClrTypeInfo(typeof(global::ZB.MOM.WW.ScadaBridge.Communication.Grpc.NodeStatusListDto), global::ZB.MOM.WW.ScadaBridge.Communication.Grpc.NodeStatusListDto.Parser, new[]{ "Nodes" }, null, null, null, null),
             new pbr::GeneratedClrTypeInfo(typeof(global::ZB.MOM.WW.ScadaBridge.Communication.Grpc.SiteHealthReportDto), global::ZB.MOM.WW.ScadaBridge.Communication.Grpc.SiteHealthReportDto.Parser, new[]{ "SiteId", "SequenceNumber", "ReportTimestamp", "DataConnectionStatuses", "TagResolutionCounts", "ScriptErrorCount", "AlarmEvaluationErrorCount", "StoreAndForwardBufferDepths", "DeadLetterCount", "DeployedInstanceCount", "EnabledInstanceCount", "DisabledInstanceCount", "NodeRole", "NodeHostname", "DataConnectionEndpoints", "DataConnectionTagQuality", "ParkedMessageCount", "ClusterNodes", "SiteAuditWriteFailures", "AuditRedactionFailure", "SiteAuditBacklog", "SiteEventLogWriteFailures", "OldestParkedMessageAgeSeconds", "ScriptQueueDepth", "ScriptBusyThreads", "ScriptOldestBusyAgeSeconds", "LocalDbReplicationConnected", "LocalDbOplogBacklog" }, null, null, null, new pbr::GeneratedClrTypeInfo[] { null, null, null, }),
             new pbr::GeneratedClrTypeInfo(typeof(global::ZB.MOM.WW.ScadaBridge.Communication.Grpc.SiteHealthReportAckDto), global::ZB.MOM.WW.ScadaBridge.Communication.Grpc.SiteHealthReportAckDto.Parser, new[]{ "SiteId", "SequenceNumber", "Accepted", "Error" }, null, null, null, null),
-            new pbr::GeneratedClrTypeInfo(typeof(global::ZB.MOM.WW.ScadaBridge.Communication.Grpc.HeartbeatDto), global::ZB.MOM.WW.ScadaBridge.Communication.Grpc.HeartbeatDto.Parser, new[]{ "SiteId", "NodeHostname", "IsActive", "Timestamp" }, null, null, null, null)
+            new pbr::GeneratedClrTypeInfo(typeof(global::ZB.MOM.WW.ScadaBridge.Communication.Grpc.HeartbeatDto), global::ZB.MOM.WW.ScadaBridge.Communication.Grpc.HeartbeatDto.Parser, new[]{ "SiteId", "NodeHostname", "IsActive", "Timestamp", "Synthetic" }, null, null, null, null)
           }));
     }
     #endregion
@@ -5826,6 +5827,7 @@ namespace ZB.MOM.WW.ScadaBridge.Communication.Grpc {
       nodeHostname_ = other.nodeHostname_;
       isActive_ = other.isActive_;
       timestamp_ = other.timestamp_ != null ? other.timestamp_.Clone() : null;
+      synthetic_ = other.synthetic_;
       _unknownFields = pb::UnknownFieldSet.Clone(other._unknownFields);
     }
 
@@ -5883,6 +5885,27 @@ namespace ZB.MOM.WW.ScadaBridge.Communication.Grpc {
       }
     }
 
+    /// <summary>Field number for the "synthetic" field.</summary>
+    public const int SyntheticFieldNumber = 5;
+    private bool synthetic_;
+    /// <summary>
+    /// Additive (field 5): true marks a SYNTHETIC heartbeat that carries no liveness
+    /// meaning — today only CentralChannelProvider's failback probe, which reuses this
+    /// RPC purely to test whether the preferred central endpoint answers again. Central
+    /// must skip ALL liveness/health bookkeeping for such a heartbeat: it does not come
+    /// from a site node's heartbeat timer, and treating it as one would keep a site
+    /// looking alive on the health dashboard from the probe alone. proto3 defaults it to
+    /// false, so a pre-existing site's heartbeat is a real one, as before.
+    /// </summary>
+    [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
+    [global::System.CodeDom.Compiler.GeneratedCode("protoc", null)]
+    public bool Synthetic {
+      get { return synthetic_; }
+      set {
+        synthetic_ = value;
+      }
+    }
+
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
     [global::System.CodeDom.Compiler.GeneratedCode("protoc", null)]
     public override bool Equals(object other) {
@@ -5902,6 +5925,7 @@ namespace ZB.MOM.WW.ScadaBridge.Communication.Grpc {
       if (NodeHostname != other.NodeHostname) return false;
       if (IsActive != other.IsActive) return false;
       if (!object.Equals(Timestamp, other.Timestamp)) return false;
+      if (Synthetic != other.Synthetic) return false;
       return Equals(_unknownFields, other._unknownFields);
     }
 
@@ -5913,6 +5937,7 @@ namespace ZB.MOM.WW.ScadaBridge.Communication.Grpc {
       if (NodeHostname.Length != 0) hash ^= NodeHostname.GetHashCode();
       if (IsActive != false) hash ^= IsActive.GetHashCode();
       if (timestamp_ != null) hash ^= Timestamp.GetHashCode();
+      if (Synthetic != false) hash ^= Synthetic.GetHashCode();
       if (_unknownFields != null) {
         hash ^= _unknownFields.GetHashCode();
       }
@@ -5947,6 +5972,10 @@ namespace ZB.MOM.WW.ScadaBridge.Communication.Grpc {
         output.WriteRawTag(34);
         output.WriteMessage(Timestamp);
       }
+      if (Synthetic != false) {
+        output.WriteRawTag(40);
+        output.WriteBool(Synthetic);
+      }
       if (_unknownFields != null) {
         _unknownFields.WriteTo(output);
       }
@@ -5973,6 +6002,10 @@ namespace ZB.MOM.WW.ScadaBridge.Communication.Grpc {
         output.WriteRawTag(34);
         output.WriteMessage(Timestamp);
       }
+      if (Synthetic != false) {
+        output.WriteRawTag(40);
+        output.WriteBool(Synthetic);
+      }
       if (_unknownFields != null) {
         _unknownFields.WriteTo(ref output);
       }
@@ -5995,6 +6028,9 @@ namespace ZB.MOM.WW.ScadaBridge.Communication.Grpc {
       if (timestamp_ != null) {
         size += 1 + pb::CodedOutputStream.ComputeMessageSize(Timestamp);
       }
+      if (Synthetic != false) {
+        size += 1 + 1;
+      }
       if (_unknownFields != null) {
         size += _unknownFields.CalculateSize();
       }
@@ -6022,6 +6058,9 @@ namespace ZB.MOM.WW.ScadaBridge.Communication.Grpc {
         }
         Timestamp.MergeFrom(other.Timestamp);
       }
+      if (other.Synthetic != false) {
+        Synthetic = other.Synthetic;
+      }
       _unknownFields = pb::UnknownFieldSet.MergeFrom(_unknownFields, other._unknownFields);
     }
 
@@ -6060,6 +6099,10 @@ namespace ZB.MOM.WW.ScadaBridge.Communication.Grpc {
             input.ReadMessage(Timestamp);
             break;
           }
+          case 40: {
+            Synthetic = input.ReadBool();
+            break;
+          }
         }
       }
     #endif
@@ -6098,6 +6141,10 @@ namespace ZB.MOM.WW.ScadaBridge.Communication.Grpc {
             input.ReadMessage(Timestamp);
             break;
           }
+          case 40: {
+            Synthetic = input.ReadBool();
+            break;
+          }
         }
       }
     }

src/ZB.MOM.WW.ScadaBridge.Communication/CommunicationOptions.cs

@@ -47,6 +47,36 @@ public class CommunicationOptions
     /// </summary>
     public TimeSpan NotificationForwardTimeout { get; set; } = TimeSpan.FromSeconds(30);
 
+    /// <summary>
+    /// Audit Log: timeout for the SITE-side Ask that forwards one audit-telemetry batch
+    /// (<c>IngestAuditEventsCommand</c> / <c>IngestCachedTelemetryCommand</c>) through the site's
+    /// <c>SiteCommunicationActor</c> and awaits central's ack. Deliberately the LONGEST rung of the
+    /// ingest timeout ladder.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// The ladder is strictly monotonic, outermost first:
+    /// <c>AuditForwardTimeout</c> (35 s, this option) &gt;
+    /// <c>SiteStreamGrpcServer.AuditIngestAskTimeout</c> (30 s — both the gRPC call deadline and
+    /// central's own Ask of the ingest singleton) &gt;
+    /// <c>AuditLogIngestActor.IngestBudget</c> (20 s) &gt;
+    /// <c>AuditLogIngestActor.IngestSqlCommandTimeout</c> (15 s).
+    /// </para>
+    /// <para>
+    /// Strictness matters: it used to reuse <see cref="NotificationForwardTimeout"/> (30 s), the
+    /// same value as the rung below it, so a slow-but-succeeding central write could be acked to a
+    /// caller whose Ask had already expired — the drain loop would treat the batch as unsent and
+    /// re-ship it. Central dedups on <c>EventId</c>, so the duplicate was harmless but the retry
+    /// traffic and the "stalled" telemetry signal were not. A timeout here is still transient: the
+    /// rows stay <c>Pending</c> and drain on the next tick.
+    /// </para>
+    /// <para>
+    /// <see cref="CommunicationOptionsValidator"/> enforces the outermost inequality — this value
+    /// must be strictly greater than the 30 s gRPC/central Ask rung.
+    /// </para>
+    /// </remarks>
+    public TimeSpan AuditForwardTimeout { get; set; } = TimeSpan.FromSeconds(35);
+
     /// <summary>
     /// Preshared key authenticating this node's gRPC control plane — the site↔central
     /// boundary. On a site node this is the key its inbound gate

src/ZB.MOM.WW.ScadaBridge.Communication/CommunicationOptionsValidator.cs

@@ -39,6 +39,15 @@ public sealed class CommunicationOptionsValidator : OptionsValidatorBase<Communi
         builder.RequireThat(options.NotificationForwardTimeout > TimeSpan.Zero,
             $"ScadaBridge:Communication:NotificationForwardTimeout must be a positive duration (was {options.NotificationForwardTimeout}).");
 
+        // Outermost rung of the audit-ingest timeout ladder. It must be strictly greater than the
+        // gRPC deadline / central Ask beneath it, or a slow-but-succeeding central write gets acked
+        // to a caller that already gave up and re-sent the batch. See CommunicationOptions.
+        builder.RequireThat(
+            options.AuditForwardTimeout > Grpc.SiteStreamGrpcServer.AuditIngestAskTimeout,
+            $"ScadaBridge:Communication:AuditForwardTimeout must be strictly greater than the "
+            + $"central audit-ingest Ask timeout ({Grpc.SiteStreamGrpcServer.AuditIngestAskTimeout}) "
+            + $"so the ingest timeout ladder stays monotonic (was {options.AuditForwardTimeout}).");
+
         builder.RequireThat(options.GrpcKeepAlivePingDelay > TimeSpan.Zero,
             $"ScadaBridge:Communication:GrpcKeepAlivePingDelay must be a positive duration (was {options.GrpcKeepAlivePingDelay}).");
 

src/ZB.MOM.WW.ScadaBridge.Communication/Grpc/CentralChannelProvider.cs

@@ -38,6 +38,15 @@ namespace ZB.MOM.WW.ScadaBridge.Communication.Grpc;
 /// </remarks>
 public sealed class CentralChannelProvider : IDisposable
 {
+    /// <summary>
+    /// Hostname stamped on the failback probe's heartbeat, so a central-side log line names it
+    /// for what it is. It is a LABEL only — the machine-readable contract is
+    /// <see cref="ZB.MOM.WW.ScadaBridge.Commons.Messages.Health.HeartbeatMessage.Synthetic"/>
+    /// (<c>HeartbeatDto.synthetic</c>, proto field 5), which central keys its skip on. Never
+    /// filter on this string.
+    /// </summary>
+    public const string SyntheticProbeHostname = "failback-probe";
+
     private static readonly TimeSpan DefaultBackoffBase = TimeSpan.FromSeconds(1);
     private static readonly TimeSpan DefaultBackoffCap = TimeSpan.FromSeconds(60);
     private static readonly TimeSpan DefaultProbeDeadline = TimeSpan.FromSeconds(5);
@@ -202,9 +211,16 @@ public sealed class CentralChannelProvider : IDisposable
                 new HeartbeatDto
                 {
                     SiteId = _siteId,
-                    NodeHostname = "failback-probe",
+                    NodeHostname = SyntheticProbeHostname,
                     IsActive = false,
                     Timestamp = Timestamp.FromDateTimeOffset(DateTimeOffset.UtcNow),
+                    // Explicit contract, not a hostname convention: central skips ALL
+                    // liveness/health bookkeeping for a synthetic heartbeat. Without it the
+                    // probe would stamp the site's last-seen time on the central health
+                    // aggregator every backoff tick, so a site whose real heartbeats had
+                    // stopped could still look alive purely because its transport layer was
+                    // probing for failback.
+                    Synthetic = true,
                 },
                 deadline: DateTime.UtcNow.Add(_probeDeadline)).ConfigureAwait(false);
 

src/ZB.MOM.WW.ScadaBridge.Communication/Grpc/CentralControlDtoMapper.cs

@@ -695,6 +695,7 @@ public static class CentralControlDtoMapper
             NodeHostname = msg.NodeHostname,
             IsActive = msg.IsActive,
             Timestamp = Timestamp.FromDateTimeOffset(msg.Timestamp),
+            Synthetic = msg.Synthetic,
         };
     }
 
@@ -709,7 +710,8 @@ public static class CentralControlDtoMapper
             SiteId: dto.SiteId,
             NodeHostname: dto.NodeHostname,
             IsActive: dto.IsActive,
-            Timestamp: dto.Timestamp.ToDateTimeOffset());
+            Timestamp: dto.Timestamp.ToDateTimeOffset(),
+            Synthetic: dto.Synthetic);
     }
 
     /// <summary>Projects a <see cref="ConnectionHealth"/> onto its wire enum.</summary>

src/ZB.MOM.WW.ScadaBridge.Communication/Grpc/CentralControlGrpcService.cs

@@ -33,13 +33,23 @@ namespace ZB.MOM.WW.ScadaBridge.Communication.Grpc;
 /// host's Akka bootstrap, not by the container.
 /// </para>
 /// <para>
+/// <b>The two audit-ingest RPCs are the exception: they Ask the ingest singleton DIRECTLY.</b>
+/// <c>CentralCommunicationActor</c> used to relay them — an extra hop that re-<c>Ask</c>ed the
+/// same <c>audit-log-ingest</c> proxy with the same 30 s
+/// <see cref="SiteStreamGrpcServer.AuditIngestAskTimeout"/>, so the inner Ask expired at the same
+/// instant as the outer one and could only add latency plus a
+/// <see cref="Akka.Actor.Status.Failure"/> repackaging. The proxy now arrives through
+/// <see cref="SetAuditIngestActor"/>, exactly as <see cref="SiteStreamGrpcServer"/> has always
+/// taken it, and the relay handlers were deleted.
+/// </para>
+/// <para>
 /// <b>Fault semantics deliberately differ from <see cref="SiteStreamGrpcServer"/>'s ingest
 /// RPCs.</b> That server answers a failed audit ingest with an EMPTY <c>IngestAck</c>; this one
 /// fails the call with a non-OK status. Both leave the site's rows <c>Pending</c> for the next
 /// drain, so the outcome is the same — but this service replaced the ClusterClient path, whose
-/// documented behaviour was to propagate the fault (<c>CentralCommunicationActor</c>'s
+/// documented behaviour was to propagate the fault, and preserving that keeps a lost batch
-/// <c>HandleIngestAuditEvents</c> pipes a <c>Status.Failure</c> back), and preserving that keeps
+/// visible as a failure rather than as a successful call that acked nothing. A proxy that has
-/// a lost batch visible as a failure rather than as a successful call that acked nothing.
+/// not been handed over yet is NOT a fault and still answers with an empty ack.
 /// </para>
 /// <para>
 /// <b>Status mapping, and why it is not uniform.</b> A site transport may safely re-send a call
@@ -66,6 +76,14 @@ public sealed class CentralControlGrpcService : CentralControlService.CentralCon
     // Kestrel's thread pool.
     private volatile IActorRef? _central;
 
+    // Central-side audit-ingest singleton proxy (audit-log-ingest). Handed over by the host
+    // AFTER the singleton starts, so it arrives strictly later than _central. The two ingest
+    // RPCs Ask THIS proxy directly rather than relaying through CentralCommunicationActor —
+    // see the class remarks. Null until registration completes; an ingest arriving before then
+    // gets an empty ack (leave the site's rows Pending, retry next drain), which is exactly
+    // what the removed relay replied in the same window.
+    private volatile IActorRef? _auditIngest;
+
     /// <summary>
     /// Creates the service. <b>This must remain the only public constructor</b> — see
     /// <see cref="SetReady"/> for how the actor arrives, and the Host's
@@ -105,9 +123,32 @@ public sealed class CentralControlGrpcService : CentralControlService.CentralCon
         _central = centralCommunicationActor;
     }
 
+    /// <summary>
+    /// Hands the central <c>AuditLogIngestActor</c> singleton proxy to the service so the two
+    /// ingest RPCs can Ask it DIRECTLY. Mirrors
+    /// <see cref="SiteStreamGrpcServer.SetAuditIngestActor"/> — the site-stream server has always
+    /// dispatched straight to the singleton, and this service now matches it.
+    /// </summary>
+    /// <remarks>
+    /// Removing the <c>CentralCommunicationActor</c> relay removes an actor hop, a second Ask,
+    /// and a second timeout from every audit batch: the relay's own Ask was armed with the same
+    /// 30 s <see cref="SiteStreamGrpcServer.AuditIngestAskTimeout"/> as this service's, so the two
+    /// expired together and the inner one could only ever add latency and a
+    /// <see cref="Akka.Actor.Status.Failure"/> repackaging on the way out.
+    /// </remarks>
+    /// <param name="auditIngestProxy">The audit-log-ingest singleton proxy.</param>
+    public void SetAuditIngestActor(IActorRef auditIngestProxy)
+    {
+        ArgumentNullException.ThrowIfNull(auditIngestProxy);
+        _auditIngest = auditIngestProxy;
+    }
+
     /// <summary>Exposed for wiring assertions in tests.</summary>
     internal bool IsReady => _central is not null;
 
+    /// <summary>Exposed for wiring assertions in tests.</summary>
+    internal bool IsAuditIngestBound => _auditIngest is not null;
+
     /// <inheritdoc />
     public override async Task<NotificationSubmitAckDto> SubmitNotification(
         NotificationSubmitDto request, ServerCallContext context)
@@ -147,9 +188,16 @@ public sealed class CentralControlGrpcService : CentralControlService.CentralCon
             return new IngestAck();
         }
 
-        var central = RequireReady(context);
+        // Node-readiness gate first (the actor system must be up), then the ingest proxy.
+        RequireReady(context);
+        var ingest = RequireAuditIngest(request.Events.Count, nameof(IngestAuditEvents));
+        if (ingest is null)
+        {
+            return new IngestAck();
+        }
+
         var reply = await AskAsync<IngestAuditEventsReply>(
-            central,
+            ingest,
             CentralControlDtoMapper.FromDto(request),
             SiteStreamGrpcServer.AuditIngestAskTimeout,
             context).ConfigureAwait(false);
@@ -166,9 +214,15 @@ public sealed class CentralControlGrpcService : CentralControlService.CentralCon
             return new IngestAck();
         }
 
-        var central = RequireReady(context);
+        RequireReady(context);
+        var ingest = RequireAuditIngest(request.Packets.Count, nameof(IngestCachedTelemetry));
+        if (ingest is null)
+        {
+            return new IngestAck();
+        }
+
         var reply = await AskAsync<IngestCachedTelemetryReply>(
-            central,
+            ingest,
             CentralControlDtoMapper.FromDto(request),
             SiteStreamGrpcServer.AuditIngestAskTimeout,
             context).ConfigureAwait(false);
@@ -176,6 +230,30 @@ public sealed class CentralControlGrpcService : CentralControlService.CentralCon
         return CentralControlDtoMapper.ToIngestAck(reply.AcceptedEventIds);
     }
 
+    /// <summary>
+    /// Returns the audit-ingest singleton proxy, or <c>null</c> when the host has not handed it
+    /// over yet (it starts moments after <see cref="SetReady"/>). A null answer means the caller
+    /// must reply with an EMPTY <see cref="IngestAck"/> — nothing accepted, rows stay
+    /// <c>Pending</c> on the site and drain on the next tick. That is deliberately NOT
+    /// <see cref="StatusCode.Unavailable"/>: readiness here is intentionally narrow (see
+    /// <see cref="SetReady"/>), and this is the exact reply the removed
+    /// <c>CentralCommunicationActor</c> relay produced in the same window.
+    /// </summary>
+    private IActorRef? RequireAuditIngest(int itemCount, string rpc)
+    {
+        var ingest = _auditIngest;
+        if (ingest is not null)
+        {
+            return ingest;
+        }
+
+        _logger.LogWarning(
+            "{Rpc} received {Count} item(s) before SetAuditIngestActor was called; returning an "
+            + "empty ack so the site keeps its rows Pending and retries.",
+            rpc, itemCount);
+        return null;
+    }
+
     /// <inheritdoc />
     public override async Task<ReconcileSiteResponseDto> ReconcileSite(
         ReconcileSiteRequestDto request, ServerCallContext context)

src/ZB.MOM.WW.ScadaBridge.Communication/ISiteAlarmLiveCache.cs

@@ -62,4 +62,29 @@ public interface ISiteAlarmLiveCache
     /// <param name="siteId">The numeric site id.</param>
     /// <returns><c>true</c> while a <b>living</b> aggregator has seeded and published; reverts to false if the aggregator terminates (R2 N6) — a frozen snapshot is never reported as live.</returns>
     bool IsLive(int siteId);
+
+    /// <summary>
+    /// The Enabled instances on the site that failed to answer the aggregator's most recent
+    /// snapshot fan-out (seed or reconcile) — i.e. that reported <c>InstanceNotFound</c>, timed
+    /// out, or faulted. Deterministically ordered by instance name, and empty when the site has
+    /// no aggregator or has not yet completed a fan-out.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// Published alongside the alarm snapshot on every fan-out, because the aggregator already
+    /// knows it: the seed/reconcile is the SAME per-instance debug-snapshot fan-out the Alarm
+    /// Summary poll used to run a second time purely to compute this list. Exposing it here lets
+    /// the UI-side poll be skipped entirely while <see cref="IsLive"/> is true, which is the
+    /// whole point — one fan-out per site per reconcile window, not two.
+    /// </para>
+    /// <para>
+    /// It is refreshed at the aggregator's fan-out cadence (seed, then each reconcile), NOT on
+    /// every stream delta: an instance going silent is precisely what the stream cannot tell you.
+    /// Callers must therefore treat it as reconcile-fresh, not delta-fresh — which matches the
+    /// poll's own 15 s freshness, so nothing on screen changes.
+    /// </para>
+    /// </remarks>
+    /// <param name="siteId">The numeric site id.</param>
+    /// <returns>The not-reporting instance unique names (possibly empty), never <c>null</c>.</returns>
+    IReadOnlyList<string> GetNotReportingInstances(int siteId);
 }

src/ZB.MOM.WW.ScadaBridge.Communication/Protos/central_control.proto

@@ -244,4 +244,12 @@ message HeartbeatDto {
   string node_hostname = 2;
   bool is_active = 3;
   google.protobuf.Timestamp timestamp = 4;
+  // Additive (field 5): true marks a SYNTHETIC heartbeat that carries no liveness
+  // meaning — today only CentralChannelProvider's failback probe, which reuses this
+  // RPC purely to test whether the preferred central endpoint answers again. Central
+  // must skip ALL liveness/health bookkeeping for such a heartbeat: it does not come
+  // from a site node's heartbeat timer, and treating it as one would keep a site
+  // looking alive on the health dashboard from the probe alone. proto3 defaults it to
+  // false, so a pre-existing site's heartbeat is a real one, as before.
+  bool synthetic = 5;
 }

src/ZB.MOM.WW.ScadaBridge.Communication/SiteAlarmLiveCacheService.cs

@@ -30,6 +30,7 @@ namespace ZB.MOM.WW.ScadaBridge.Communication;
 public sealed class SiteAlarmLiveCacheService : ISiteAlarmLiveCache
 {
     private static readonly IReadOnlyList<AlarmStateChanged> Empty = Array.Empty<AlarmStateChanged>();
+    private static readonly IReadOnlyList<string> EmptyNames = Array.Empty<string>();
 
     private readonly IServiceProvider _serviceProvider;
     private readonly CommunicationService _communicationService;
@@ -135,6 +136,13 @@ public sealed class SiteAlarmLiveCacheService : ISiteAlarmLiveCache
             return _sites.TryGetValue(siteId, out var entry) && entry.HasPublished && entry.StreamLive;
     }
 
+    /// <inheritdoc />
+    public IReadOnlyList<string> GetNotReportingInstances(int siteId)
+    {
+        lock (_lock)
+            return _sites.TryGetValue(siteId, out var entry) ? entry.NotReporting : EmptyNames;
+    }
+
     // ── Subscriber teardown ─────────────────────────────────────────────────────
 
     private void Unsubscribe(Subscription subscription)
@@ -332,6 +340,15 @@ public sealed class SiteAlarmLiveCacheService : ISiteAlarmLiveCache
     /// <see cref="DebugViewSnapshot.InstanceNotFound"/> contributes no rows rather than
     /// failing the whole seed (mirrors <c>AlarmSummaryService</c>). Re-enumerates instances
     /// each call so a reconcile picks up enable/disable/deploy/delete drift.
+    /// <para>
+    /// It also records WHICH instances failed to answer into
+    /// <see cref="SiteEntry.NotReporting"/>, published through
+    /// <see cref="ISiteAlarmLiveCache.GetNotReportingInstances"/>. That set was previously
+    /// computed and thrown away here, forcing the Alarm Summary page to run a second, identical
+    /// fan-out through <c>AlarmSummaryService</c> purely to rebuild it. Same classification as
+    /// that service (<c>InstanceNotFound</c> or any non-cancellation fault), same
+    /// ordinal-ignore-case ordering, so the page renders identically from either source.
+    /// </para>
     /// </summary>
     private async Task<IReadOnlyList<AlarmStateChanged>> FanOutSnapshotsAsync(int siteId, CancellationToken ct)
     {
@@ -343,7 +360,10 @@ public sealed class SiteAlarmLiveCacheService : ISiteAlarmLiveCache
             var siteRepo = scope.ServiceProvider.GetRequiredService<ISiteRepository>();
             var site = await siteRepo.GetSiteByIdAsync(siteId, ct);
             if (site is null)
+            {
+                PublishNotReporting(siteId, EmptyNames);
                 return Empty;
+            }
             siteIdentifier = site.SiteIdentifier;
 
             var instanceRepo = scope.ServiceProvider.GetRequiredService<ITemplateEngineRepository>();
@@ -355,25 +375,48 @@ public sealed class SiteAlarmLiveCacheService : ISiteAlarmLiveCache
         }
 
         if (enabledInstanceNames.Count == 0)
+        {
+            PublishNotReporting(siteId, EmptyNames);
             return Empty;
+        }
 
         var rows = new ConcurrentBag<AlarmStateChanged>();
+        var notReporting = new ConcurrentBag<string>();
         using var gate = new SemaphoreSlim(
             Math.Max(1, _options.LiveAlarmCacheSeedConcurrency),
             Math.Max(1, _options.LiveAlarmCacheSeedConcurrency));
 
         var fetches = enabledInstanceNames.Select(name =>
-            FetchInstanceSnapshotAsync(siteIdentifier, name, gate, rows, ct));
+            FetchInstanceSnapshotAsync(siteIdentifier, name, gate, rows, notReporting, ct));
         await Task.WhenAll(fetches);
 
+        PublishNotReporting(
+            siteId,
+            notReporting.OrderBy(n => n, StringComparer.OrdinalIgnoreCase).ToList());
+
         return rows.ToList();
     }
 
+    /// <summary>
+    /// Stores the fan-out's not-reporting set on the site entry. Silently no-ops when the entry
+    /// has been torn down mid-fan-out (last viewer left), matching every other late-write path
+    /// in this service.
+    /// </summary>
+    private void PublishNotReporting(int siteId, IReadOnlyList<string> notReporting)
+    {
+        lock (_lock)
+        {
+            if (_sites.TryGetValue(siteId, out var entry))
+                entry.NotReporting = notReporting;
+        }
+    }
+
     private async Task FetchInstanceSnapshotAsync(
         string siteIdentifier,
         string instanceUniqueName,
         SemaphoreSlim gate,
         ConcurrentBag<AlarmStateChanged> rows,
+        ConcurrentBag<string> notReporting,
         CancellationToken ct)
     {
         await gate.WaitAsync(ct);
@@ -387,7 +430,10 @@ public sealed class SiteAlarmLiveCacheService : ISiteAlarmLiveCache
                 instanceUniqueName, Guid.NewGuid().ToString("N"), AlarmsOnly: true);
             var snapshot = await _communicationService.RequestDebugSnapshotAsync(siteIdentifier, request, ct);
             if (snapshot.InstanceNotFound)
+            {
+                notReporting.Add(instanceUniqueName);
                 return;
+            }
 
             foreach (var alarm in snapshot.AlarmStates)
                 rows.Add(alarm);
@@ -398,7 +444,9 @@ public sealed class SiteAlarmLiveCacheService : ISiteAlarmLiveCache
         }
         catch
         {
-            // Degrade this one instance to "no rows" rather than failing the whole seed.
+            // Degrade this one instance to "no rows, not reporting" rather than failing the
+            // whole seed. Same rule as AlarmSummaryService.FetchInstanceAsync.
+            notReporting.Add(instanceUniqueName);
         }
         finally
         {
@@ -474,6 +522,9 @@ public sealed class SiteAlarmLiveCacheService : ISiteAlarmLiveCache
             entry.HasPublished = false;
             entry.StreamLive = false;
             entry.Current = Empty;
+            // A dead aggregator's last fan-out result is stale too — clear it so the page's
+            // poll fallback (now authoritative again) owns the not-reporting list outright.
+            entry.NotReporting = EmptyNames;
 
             if (entry.Subscribers.Count > 0 && !entry.Starting)
             {
@@ -509,6 +560,13 @@ public sealed class SiteAlarmLiveCacheService : ISiteAlarmLiveCache
         /// <summary>True once the aggregator has seeded and published at least once.</summary>
         public bool HasPublished { get; set; }
 
+        /// <summary>
+        /// Enabled instances that failed to answer the most recent snapshot fan-out, ordered by
+        /// name. Refreshed on every seed/reconcile (see <c>FanOutSnapshotsAsync</c>) and cleared
+        /// when liveness resets, so a dead aggregator never leaves a stale list behind.
+        /// </summary>
+        public IReadOnlyList<string> NotReporting { get; set; } = EmptyNames;
+
         /// <summary>
         /// Liveness of the aggregator's site-wide gRPC stream as of the last publish. Both
         /// this and <see cref="HasPublished"/> must hold for <c>IsLive</c>: a published cache

src/ZB.MOM.WW.ScadaBridge.Host/Actors/AkkaHostedService.cs

@@ -534,12 +534,12 @@ akka {{
         _logger.LogInformation("NotificationOutbox singleton created and registered with CentralCommunicationActor");
 
         // Audit Log — central singleton mirrors the Notification Outbox
-        // pattern. The IngestAuditEvents gRPC handler lives on SiteStreamGrpcServer
+        // pattern. TWO gRPC servers can carry an IngestAuditEvents call and both Ask this
-        // (Communication.Grpc); a central node hosting that server (reconciliation
+        // proxy directly: the central-hosted CentralControlGrpcService (the production
-        // path) hands the proxy in via SetAuditIngestActor below. When the gRPC
+        // site→central path) and SiteStreamGrpcServer (Communication.Grpc). Each is handed
-        // server is not registered (current central topology), the host still
+        // the proxy below; either may be absent on a given node and the wiring no-ops. Even
-        // brings the singleton up so an in-process test (or a future
+        // with neither bound the host still brings the singleton up so an in-process test
-        // direct caller) can Ask the proxy without further wiring.
+        // (or a future direct caller) can Ask the proxy without further wiring.
         // IAuditLogRepository is a SCOPED EF Core service, so the singleton
         // actor takes the root IServiceProvider and creates a fresh scope per
         // message (mirroring NotificationOutboxActor). Pre-resolving the
@@ -555,10 +555,11 @@ akka {{
                 auditIngestLogger)),
             _logger);
 
-        // Hand the audit-ingest proxy to the CentralCommunicationActor so audit
+        // Hand the audit-ingest proxy to the central-hosted gRPC control plane so its two
-        // ingest commands forwarded by sites are routed to the
+        // ingest RPCs Ask the singleton DIRECTLY. There is deliberately no
-        // singleton. Mirrors the RegisterNotificationOutbox wiring above.
+        // CentralCommunicationActor relay any more: it re-Asked this same proxy with the same
-        centralCommActor.Tell(new RegisterAuditIngest(auditIngest.Proxy));
+        // 30 s timeout, so it could only add a hop and latency to every audit batch.
+        centralControlGrpc?.SetAuditIngestActor(auditIngest.Proxy);
 
         // Hand the proxy to the SiteStreamGrpcServer (if registered on this node)
         // so the IngestAuditEvents RPC routes incoming site batches to the singleton.
@@ -568,7 +569,9 @@ akka {{
         var grpcServer = _serviceProvider.GetService<ZB.MOM.WW.ScadaBridge.Communication.Grpc.SiteStreamGrpcServer>();
         grpcServer?.SetAuditIngestActor(auditIngest.Proxy);
         _logger.LogInformation(
-            "AuditLogIngestActor singleton created (gRPC server bound: {GrpcBound})",
+            "AuditLogIngestActor singleton created (control-plane bound: {ControlBound}, "
+            + "site-stream server bound: {GrpcBound})",
+            centralControlGrpc is not null,
             grpcServer is not null);
 
         // Subscribe the per-site stalled
@@ -1066,13 +1069,16 @@ akka {{
         // not the DI-resolved NoOpSiteStreamAuditClient. The NoOp default stays
         // correct for central/test composition roots (no SiteCommunicationActor);
         // a site role wires the real client here so the
-        // SQLite Pending backlog actually drains to central. The forward Ask
+        // SQLite Pending backlog actually drains to central. The forward Ask uses
-        // reuses NotificationForwardTimeout — the same site→central command
+        // AuditForwardTimeout (35 s) — the OUTERMOST rung of the ingest timeout
-        // forward bound notifications already use over this transport.
+        // ladder (35 > 30 gRPC/central Ask > 20 actor budget > 15 SQL), so a
+        // slow-but-succeeding central write is never acked to a caller that has
+        // already given up and re-sent. It used to reuse NotificationForwardTimeout
+        // (30 s), which tied it with the rung below.
         ZB.MOM.WW.ScadaBridge.AuditLog.Site.Telemetry.ISiteStreamAuditClient siteAuditClient =
             new ZB.MOM.WW.ScadaBridge.AuditLog.Site.Telemetry.SiteCommunicationAuditClient(
                 siteCommActor,
-                _communicationOptions.NotificationForwardTimeout);
+                _communicationOptions.AuditForwardTimeout);
         var siteAuditLogger = _serviceProvider.GetRequiredService<ILoggerFactory>()
             .CreateLogger<ZB.MOM.WW.ScadaBridge.AuditLog.Site.Telemetry.SiteAuditTelemetryActor>();
 

tests/ZB.MOM.WW.ScadaBridge.AuditLog.Tests/Central/GrpcPullAuditEventsClientTests.cs

@@ -117,7 +117,7 @@ public class GrpcPullAuditEventsClientTests
             invoker,
             NullLogger<GrpcPullAuditEventsClient>.Instance);
 
-        var result = await sut.PullAsync("site-a", BaseTime, batchSize: 256, CancellationToken.None);
+        var result = await sut.PullAsync("site-a", BaseTime, afterId: null, batchSize: 256, ct: CancellationToken.None);
 
         // Endpoint resolution + request shaping.
         Assert.Equal("http://site-a:8083", invoker.Endpoint);
@@ -141,7 +141,7 @@ public class GrpcPullAuditEventsClientTests
             invoker,
             NullLogger<GrpcPullAuditEventsClient>.Instance);
 
-        var result = await sut.PullAsync("site-a", BaseTime, batchSize: 256, CancellationToken.None);
+        var result = await sut.PullAsync("site-a", BaseTime, afterId: null, batchSize: 256, ct: CancellationToken.None);
 
         Assert.Empty(result.Events);
         Assert.False(result.MoreAvailable);
@@ -161,7 +161,7 @@ public class GrpcPullAuditEventsClientTests
             NullLogger<GrpcPullAuditEventsClient>.Instance);
 
         // MUST NOT throw — per the IPullAuditEventsClient contract.
-        var result = await sut.PullAsync("site-a", BaseTime, batchSize: 256, CancellationToken.None);
+        var result = await sut.PullAsync("site-a", BaseTime, afterId: null, batchSize: 256, ct: CancellationToken.None);
 
         Assert.Empty(result.Events);
         Assert.False(result.MoreAvailable);
@@ -178,7 +178,7 @@ public class GrpcPullAuditEventsClientTests
             invoker,
             NullLogger<GrpcPullAuditEventsClient>.Instance);
 
-        var result = await sut.PullAsync("site-a", BaseTime, batchSize: 256, CancellationToken.None);
+        var result = await sut.PullAsync("site-a", BaseTime, afterId: null, batchSize: 256, ct: CancellationToken.None);
 
         Assert.Empty(result.Events);
         Assert.False(result.MoreAvailable);
@@ -197,7 +197,7 @@ public class GrpcPullAuditEventsClientTests
             invoker,
             NullLogger<GrpcPullAuditEventsClient>.Instance);
 
-        var result = await sut.PullAsync("site-a", BaseTime, batchSize: 256, CancellationToken.None);
+        var result = await sut.PullAsync("site-a", BaseTime, afterId: null, batchSize: 256, ct: CancellationToken.None);
 
         Assert.Empty(result.Events);
         Assert.False(result.MoreAvailable);
@@ -221,7 +221,7 @@ public class GrpcPullAuditEventsClientTests
             NullLogger<GrpcPullAuditEventsClient>.Instance);
 
         // MUST NOT throw — must dial successfully.
-        var result = await sut.PullAsync("site-a", minUnspecified, batchSize: 256, CancellationToken.None);
+        var result = await sut.PullAsync("site-a", minUnspecified, afterId: null, batchSize: 256, ct: CancellationToken.None);
 
         Assert.Equal(1, invoker.CallCount);
         Assert.Equal("http://site-a:8083", invoker.Endpoint);
@@ -252,7 +252,7 @@ public class GrpcPullAuditEventsClientTests
             invoker,
             NullLogger<GrpcPullAuditEventsClient>.Instance);
 
-        var result = await sut.PullAsync("site-a", BaseTime, batchSize: 256, CancellationToken.None);
+        var result = await sut.PullAsync("site-a", BaseTime, afterId: null, batchSize: 256, ct: CancellationToken.None);
 
         Assert.Equal(new[] { "http://node-a:8083", "http://node-b:8083" }, invoker.Dialed);
         var evt = Assert.Single(result.Events);
@@ -271,7 +271,7 @@ public class GrpcPullAuditEventsClientTests
             invoker,
             NullLogger<GrpcPullAuditEventsClient>.Instance);
 
-        var result = await sut.PullAsync("site-a", BaseTime, batchSize: 256, CancellationToken.None);
+        var result = await sut.PullAsync("site-a", BaseTime, afterId: null, batchSize: 256, ct: CancellationToken.None);
 
         Assert.Empty(result.Events);
         Assert.Equal(new[] { "http://node-a:8083" }, invoker.Dialed);

tests/ZB.MOM.WW.ScadaBridge.AuditLog.Tests/Central/SiteAuditReconciliationActorTests.cs

@@ -194,7 +194,7 @@ public class SiteAuditReconciliationActorTests : TestKit, IClassFixture<MsSqlMig
     /// </summary>
     private sealed class ScriptedPullClient : IPullAuditEventsClient
     {
-        public List<(string SiteId, DateTime SinceUtc, int BatchSize)> Calls { get; } = new();
+        public List<(string SiteId, DateTime SinceUtc, string? AfterId, int BatchSize)> Calls { get; } = new();
         private readonly Dictionary<string, Queue<PullAuditEventsResponse>> _scripted = new();
         private readonly Dictionary<string, Exception> _throwOnSite = new();
 
@@ -211,9 +211,9 @@ public class SiteAuditReconciliationActorTests : TestKit, IClassFixture<MsSqlMig
         }
 
         public Task<PullAuditEventsResponse> PullAsync(
-            string siteId, DateTime sinceUtc, int batchSize, CancellationToken ct)
+            string siteId, DateTime sinceUtc, string? afterId, int batchSize, CancellationToken ct)
         {
-            Calls.Add((siteId, sinceUtc, batchSize));
+            Calls.Add((siteId, sinceUtc, afterId, batchSize));
             if (_throwOnSite.TryGetValue(siteId, out var ex))
             {
                 throw ex;
@@ -425,6 +425,104 @@ public class SiteAuditReconciliationActorTests : TestKit, IClassFixture<MsSqlMig
 
         Assert.Equal(DateTime.MinValue, client.Calls[0].SinceUtc);
         Assert.Equal(t3, client.Calls[1].SinceUtc);
+
+        // The composite half travels too (arch-review phase-2 residual #5): the first pull has
+        // no cursor at all, the second carries the id of the row at t3 so the site can retire
+        // it exactly instead of leaving the boundary instant permanently servable.
+        Assert.Null(client.Calls[0].AfterId);
+        Assert.Equal(e3.EventId.ToString(), client.Calls[1].AfterId);
+    }
+
+    [Fact]
+    public void Cursor_AdvancesOnTheIdTiebreak_WhenEveryRowSharesOneInstant()
+    {
+        // The case a bare timestamp cursor could never drain: a burst all stamped at the same
+        // instant. The timestamp cannot move, so only the id half can — and it must, or the
+        // next pull re-serves the identical window forever.
+        var sites = new StaticEnumerator(new SiteEntry("siteA", "http://siteA:8083"));
+        var t = new DateTime(2026, 5, 20, 10, 0, 0, DateTimeKind.Utc);
+
+        // Deterministic ids so "the greatest ordinal" is a fact, not a coin flip.
+        var low = NewEvent("siteA", t, Guid.Parse("11111111-1111-1111-1111-111111111111"));
+        var high = NewEvent("siteA", t, Guid.Parse("99999999-9999-9999-9999-999999999999"));
+        var mid = NewEvent("siteA", t, Guid.Parse("55555555-5555-5555-5555-555555555555"));
+
+        var client = new ScriptedPullClient().Script("siteA",
+            new PullAuditEventsResponse(new[] { low, high, mid }, MoreAvailable: true));
+        var repo = new RecordingRepo();
+
+        CreateActor(sites, client, repo, FastTickOptions());
+
+        AwaitAssert(() => Assert.True(client.Calls.Count >= 2,
+            $"need at least 2 pulls, got {client.Calls.Count}"),
+            duration: TimeSpan.FromSeconds(5),
+            interval: TimeSpan.FromMilliseconds(50));
+
+        Assert.Equal(t, client.Calls[1].SinceUtc);
+        Assert.Equal(high.EventId.ToString(), client.Calls[1].AfterId);
+    }
+
+    [Fact]
+    public void BothCursorHalves_AreHeldBack_WhileARowIsStillBeingRetried()
+    {
+        // A held-back cursor must hold BOTH halves: advancing the id while pinning the
+        // timestamp would skip past the very rows being retried.
+        var sites = new StaticEnumerator(new SiteEntry("siteA", "http://siteA:8083"));
+        var t = new DateTime(2026, 5, 20, 10, 0, 0, DateTimeKind.Utc);
+        var evt = NewEvent("siteA", t);
+
+        var client = new ScriptedPullClient().Script("siteA",
+            new PullAuditEventsResponse(new[] { evt }, MoreAvailable: false));
+        var repo = new AlwaysThrowingRepo();
+
+        CreateActor(sites, client, repo, FastTickOptions());
+
+        AwaitAssert(() => Assert.True(client.Calls.Count >= 2,
+            $"need at least 2 pulls, got {client.Calls.Count}"),
+            duration: TimeSpan.FromSeconds(5),
+            interval: TimeSpan.FromMilliseconds(50));
+
+        Assert.Equal(DateTime.MinValue, client.Calls[1].SinceUtc);
+        Assert.Null(client.Calls[1].AfterId);
+    }
+
+    /// <summary>Repository whose every insert throws, so the retry hold-back path is taken.</summary>
+    private sealed class AlwaysThrowingRepo : IAuditLogRepository
+    {
+        public Task InsertIfNotExistsAsync(AuditEvent evt, CancellationToken ct = default) =>
+            throw new InvalidOperationException("central insert failed");
+
+        public Task<IReadOnlyList<AuditEvent>> QueryAsync(
+            AuditLogQueryFilter filter, AuditLogPaging paging, CancellationToken ct = default)
+            => throw new NotSupportedException();
+
+        public Task<long> SwitchOutPartitionAsync(
+            DateTime monthBoundary, TimeSpan? commandTimeout = null, CancellationToken ct = default)
+            => throw new NotSupportedException();
+
+        public Task<long> PurgeChannelOlderThanAsync(
+            string channel, DateTime threshold, int batchSize, TimeSpan? commandTimeout = null,
+            CancellationToken ct = default)
+            => throw new NotSupportedException();
+
+        public Task<long> BackfillSourceNodeAsync(
+            string sentinel, DateTime before, int batchSize, CancellationToken ct = default)
+            => throw new NotSupportedException();
+
+        public Task<IReadOnlyList<DateTime>> GetPartitionBoundariesOlderThanAsync(
+            DateTime threshold, CancellationToken ct = default)
+            => throw new NotSupportedException();
+
+        public Task<ZB.MOM.WW.ScadaBridge.Commons.Types.AuditLogKpiSnapshot> GetKpiSnapshotAsync(
+            TimeSpan window, DateTime? nowUtc = null, CancellationToken ct = default)
+            => throw new NotSupportedException();
+
+        public Task<IReadOnlyList<ExecutionTreeNode>> GetExecutionTreeAsync(
+            Guid executionId, CancellationToken ct = default)
+            => throw new NotSupportedException();
+
+        public Task<IReadOnlyList<string>> GetDistinctSourceNodesAsync(CancellationToken ct = default)
+            => throw new NotSupportedException();
     }
 
     // ---------------------------------------------------------------------

tests/ZB.MOM.WW.ScadaBridge.AuditLog.Tests/Integration/OutageReconciliationTests.cs

@@ -85,7 +85,7 @@ public class OutageReconciliationTests : TestKit, IClassFixture<MsSqlMigrationFi
         }
 
         public async Task<PullAuditEventsResponse> PullAsync(
-            string siteId, DateTime sinceUtc, int batchSize, CancellationToken ct)
+            string siteId, DateTime sinceUtc, string? afterId, int batchSize, CancellationToken ct)
         {
             CallCount++;
 
@@ -94,16 +94,16 @@ public class OutageReconciliationTests : TestKit, IClassFixture<MsSqlMigrationFi
             // is retired FIRST; the rows this call serves are NOT retired, because
             // nothing yet proves central consumed them. A fault between here and
             // central's commit therefore re-serves them on the next tick instead of
-            // losing them. The actor sends no after_id, so the cursor is a bare
+            // losing them. The actor now sends the composite (timestamp, id) cursor,
-            // timestamp under the inclusive >= read contract and only rows strictly
+            // so retirement is exact: the rows AT the cursor instant are proven
-            // older than it are provably received.
+            // received too, where a bare timestamp could only prove strictly-older ones.
             if (sinceUtc > DateTime.MinValue)
             {
-                await _siteQueue.MarkReconciledUpToAsync(sinceUtc, null, ct).ConfigureAwait(false);
+                await _siteQueue.MarkReconciledUpToAsync(sinceUtc, afterId, ct).ConfigureAwait(false);
             }
 
             var rows = await _siteQueue
-                .ReadPendingSinceAsync(sinceUtc, batchSize, afterId: null, ct)
+                .ReadPendingSinceAsync(sinceUtc, batchSize, afterId, ct)
                 .ConfigureAwait(false);
 
             // MoreAvailable is true iff the read filled the batch — the actor

tests/ZB.MOM.WW.ScadaBridge.CentralUI.Tests/Monitoring/AlarmSummaryRenderTests.cs

@@ -331,6 +331,14 @@ public class AlarmSummaryRenderTests : BunitContext
 
         public bool IsLive(int siteId) => _live;
 
+        /// <summary>
+        /// The aggregator's own fan-out result. Settable so a test can prove the page renders
+        /// the not-reporting list sourced from the live cache while it is serving the site.
+        /// </summary>
+        public IReadOnlyList<string> NotReporting { get; set; } = Array.Empty<string>();
+
+        public IReadOnlyList<string> GetNotReportingInstances(int siteId) => NotReporting;
+
         public void PushAlarms(IReadOnlyList<AlarmStateChanged> alarms)
         {
             _current = alarms;

tests/ZB.MOM.WW.ScadaBridge.CentralUI.Tests/Monitoring/AlarmSummaryVirtualizeTests.cs

@@ -105,6 +105,8 @@ public class AlarmSummaryVirtualizeTests : BunitContext
 
         public bool IsLive(int siteId) => false;
 
+        public IReadOnlyList<string> GetNotReportingInstances(int siteId) => Array.Empty<string>();
+
         private sealed class NoOp : IDisposable
         {
             public void Dispose() { }

tests/ZB.MOM.WW.ScadaBridge.CentralUI.Tests/Services/SharedAlarmSummaryServiceTests.cs

@@ -56,13 +56,13 @@ public class SharedAlarmSummaryServiceTests : IDisposable
         _provider = services.BuildServiceProvider();
     }
 
-    private SharedAlarmSummaryService CreateSut(TimeSpan liveCacheTtl) =>
+    private SharedAlarmSummaryService CreateSut() =>
-        new(_provider.GetRequiredService<IServiceScopeFactory>(), _liveCache, liveCacheTtl, () => _now);
+        new(_provider.GetRequiredService<IServiceScopeFactory>(), _liveCache, () => _now);
 
     [Fact]
     public async Task ConcurrentCircuits_ShareOneFanOut()
     {
-        var sut = CreateSut(TimeSpan.FromSeconds(60));
+        var sut = CreateSut();
 
         var results = await Task.WhenAll(Enumerable.Range(0, 8).Select(_ => sut.GetSiteAlarmsAsync(SiteId)));
 
@@ -73,7 +73,7 @@ public class SharedAlarmSummaryServiceTests : IDisposable
     [Fact]
     public async Task ColdLiveCache_RefreshesWithinThePageTick()
     {
-        var sut = CreateSut(TimeSpan.FromSeconds(60));
+        var sut = CreateSut();
         _liveCache.Live = false;
 
         await sut.GetSiteAlarmsAsync(SiteId);
@@ -86,22 +86,51 @@ public class SharedAlarmSummaryServiceTests : IDisposable
     }
 
     [Fact]
-    public async Task LiveCacheServing_WidensTheWindowToTheReconcileInterval()
+    public async Task LiveCacheServing_SkipsTheFanOutEntirely()
     {
-        var sut = CreateSut(TimeSpan.FromSeconds(60));
+        // The aggregator's seed/reconcile already ran this exact fan-out and publishes both
+        // halves of the answer, so the façade must not run a second one — not now, not after
+        // any elapsed window (arch-review phase-2 residual #4).
+        var sut = CreateSut();
+        _liveCache.Live = true;
+        _liveCache.NotReporting = new[] { "inst-silent" };
+        _liveCache.Current = new[]
+        {
+            new AlarmStateChanged("inst-a", "A-alarm", AlarmState.Active, 500, T0),
+        };
+
+        var first = await sut.GetSiteAlarmsAsync(SiteId);
+        _now = T0.AddSeconds(61);
+        var second = await sut.GetSiteAlarmsAsync(SiteId);
+
+        await _instanceRepo.DidNotReceive().GetInstancesBySiteIdAsync(SiteId, Arg.Any<CancellationToken>());
+        await _snapshotClient.DidNotReceive().GetSnapshotAsync(
+            Arg.Any<string>(), Arg.Any<string>(), Arg.Any<CancellationToken>());
+
+        // Both halves come from the cache: the rows so a liveness flip mid-call lands on
+        // last-known state instead of a blank grid, and the not-reporting names the page shows.
+        foreach (var result in new[] { first, second })
+        {
+            Assert.Equal("A-alarm", Assert.Single(result.Alarms).Alarm.AlarmName);
+            Assert.Equal("inst-silent", Assert.Single(result.NotReportingInstances));
+        }
+    }
+
+    [Fact]
+    public async Task LiveCacheGoingCold_FallsBackToTheFanOut()
+    {
+        var sut = CreateSut();
         _liveCache.Live = true;
 
         await sut.GetSiteAlarmsAsync(SiteId);
-        _now = T0.AddSeconds(30);
+        await _instanceRepo.DidNotReceive().GetInstancesBySiteIdAsync(SiteId, Arg.Any<CancellationToken>());
-        await sut.GetSiteAlarmsAsync(SiteId);
+
+        // Aggregator died / stream degraded → the poll is the page's rebuild path again.
+        _liveCache.Live = false;
+        var result = await sut.GetSiteAlarmsAsync(SiteId);
 
-        // Live deltas own the rows; only the not-reporting list still comes from the
-        // fan-out, so a 30s-old answer is fine and costs no second fan-out.
         await _instanceRepo.Received(1).GetInstancesBySiteIdAsync(SiteId, Arg.Any<CancellationToken>());
-
+        Assert.Single(result.Alarms);
-        _now = T0.AddSeconds(61);
-        await sut.GetSiteAlarmsAsync(SiteId);
-        await _instanceRepo.Received(2).GetInstancesBySiteIdAsync(SiteId, Arg.Any<CancellationToken>());
     }
 
     [Fact]
@@ -113,7 +142,7 @@ public class SharedAlarmSummaryServiceTests : IDisposable
         _instanceRepo.GetInstancesBySiteIdAsync(otherSite, Arg.Any<CancellationToken>())
             .Returns(new List<Instance>());
 
-        var sut = CreateSut(TimeSpan.FromSeconds(60));
+        var sut = CreateSut();
 
         await sut.GetSiteAlarmsAsync(SiteId);
         await sut.GetSiteAlarmsAsync(otherSite);
@@ -125,7 +154,7 @@ public class SharedAlarmSummaryServiceTests : IDisposable
     [Fact]
     public void PureMethods_MatchTheDirectImplementation()
     {
-        var sut = CreateSut(TimeSpan.FromSeconds(60));
+        var sut = CreateSut();
         var direct = new AlarmSummaryService(_instanceRepo, _siteRepo, _snapshotClient);
         var alarms = new List<AlarmStateChanged>
         {
@@ -149,18 +178,24 @@ public class SharedAlarmSummaryServiceTests : IDisposable
 
     public void Dispose() => _provider.Dispose();
 
-    /// <summary>Liveness-only stub — the façade consults nothing else on the live cache.</summary>
+    /// <summary>
+    /// Read-side stub: liveness, the published alarm snapshot, and the aggregator's
+    /// not-reporting set — the three things the façade reads while the cache is serving a site.
+    /// </summary>
     private sealed class FakeLiveCache : ISiteAlarmLiveCache
     {
         public bool Live { get; set; }
+        public IReadOnlyList<AlarmStateChanged> Current { get; set; } = Array.Empty<AlarmStateChanged>();
+        public IReadOnlyList<string> NotReporting { get; set; } = Array.Empty<string>();
 
         public IDisposable Subscribe(int siteId, Action onChanged) => new NoOp();
 
-        public IReadOnlyList<AlarmStateChanged> GetCurrentAlarms(int siteId) =>
+        public IReadOnlyList<AlarmStateChanged> GetCurrentAlarms(int siteId) => Current;
-            Array.Empty<AlarmStateChanged>();
 
         public bool IsLive(int siteId) => Live;
 
+        public IReadOnlyList<string> GetNotReportingInstances(int siteId) => NotReporting;
+
         private sealed class NoOp : IDisposable
         {
             public void Dispose() { }

tests/ZB.MOM.WW.ScadaBridge.Communication.Tests/CentralCommunicationActorAuditTests.cs

@@ -1,151 +0,0 @@
-using Akka.Actor;
-using Akka.TestKit;
-using Akka.TestKit.Xunit2;
-using Microsoft.Extensions.DependencyInjection;
-using NSubstitute;
-using ZB.MOM.WW.Audit;
-using ZB.MOM.WW.ScadaBridge.Commons.Entities.Audit;
-using ZB.MOM.WW.ScadaBridge.Commons.Interfaces.Repositories;
-using ZB.MOM.WW.ScadaBridge.Commons.Messages.Audit;
-using ZB.MOM.WW.ScadaBridge.Commons.Types;
-using ZB.MOM.WW.ScadaBridge.Commons.Types.Audit;
-using ZB.MOM.WW.ScadaBridge.Commons.Types.Enums;
-using ZB.MOM.WW.ScadaBridge.Communication.Actors;
-
-namespace ZB.MOM.WW.ScadaBridge.Communication.Tests;
-
-/// <summary>
-/// Tests for the Audit Log (#23) site→central ingest routing on
-/// <see cref="CentralCommunicationActor"/>. A site delivers
-/// <see cref="IngestAuditEventsCommand"/> / <see cref="IngestCachedTelemetryCommand"/>
-/// to the actor, which forwards to the registered
-/// <c>AuditLogIngestActor</c> proxy and routes the reply back to the site.
-/// Mirrors the NotificationSubmit / RegisterNotificationOutbox pattern.
-/// </summary>
-public class CentralCommunicationActorAuditTests : TestKit
-{
-    public CentralCommunicationActorAuditTests() : base(@"akka.loglevel = DEBUG") { }
-
-    private IActorRef CreateActor(TimeSpan? auditIngestAskTimeout = null)
-    {
-        var mockRepo = Substitute.For<ISiteRepository>();
-        mockRepo.GetAllSitesAsync(Arg.Any<CancellationToken>())
-            .Returns(new List<Commons.Entities.Sites.Site>());
-
-        var services = new ServiceCollection();
-        services.AddScoped(_ => mockRepo);
-        var sp = services.BuildServiceProvider();
-
-        var transport = Substitute.For<ISiteCommandTransport>();
-        return Sys.ActorOf(Props.Create(() =>
-            new CentralCommunicationActor(sp, transport, auditIngestAskTimeout)));
-    }
-
-    // C3 (Task 2.5): canonical ZB.MOM.WW.Audit.AuditEvent via the shared factory.
-    private static AuditEvent SampleAuditEvent() =>
-        ScadaBridgeAuditEventFactory.Create(
-            channel: AuditChannel.ApiOutbound,
-            kind: AuditKind.ApiCall,
-            status: AuditStatus.Delivered);
-
-    private static SiteCall SampleSiteCall() => new()
-    {
-        TrackedOperationId = TrackedOperationId.New(),
-        Channel = "OutboundApi",
-        Target = "ExternalSystemA",
-        SourceSite = "site1",
-        Status = "Delivered",
-        RetryCount = 0,
-        CreatedAtUtc = DateTime.UtcNow,
-        UpdatedAtUtc = DateTime.UtcNow,
-        IngestedAtUtc = DateTime.UtcNow,
-    };
-
-    [Fact]
-    public void IngestAuditEventsCommand_WithRegisteredProxy_ForwardsAndRoutesReplyToSender()
-    {
-        var actor = CreateActor();
-        var auditProbe = CreateTestProbe();
-        actor.Tell(new RegisterAuditIngest(auditProbe.Ref));
-
-        var evt = SampleAuditEvent();
-        var cmd = new IngestAuditEventsCommand(new[] { evt });
-        actor.Tell(cmd);
-
-        // The audit-ingest proxy receives the command, with the original site
-        // sender preserved (Forward semantics).
-        auditProbe.ExpectMsg(cmd);
-
-        // When the proxy replies, the actor routes it back to the original sender.
-        var reply = new IngestAuditEventsReply(new[] { evt.EventId });
-        auditProbe.Reply(reply);
-
-        var received = ExpectMsg<IngestAuditEventsReply>();
-        Assert.Equal(new[] { evt.EventId }, received.AcceptedEventIds);
-    }
-
-    [Fact]
-    public void IngestAuditEventsCommand_WithNoProxyRegistered_RepliesEmptyAcceptedEventIds()
-    {
-        var actor = CreateActor();
-
-        actor.Tell(new IngestAuditEventsCommand(new[] { SampleAuditEvent() }));
-
-        var reply = ExpectMsg<IngestAuditEventsReply>();
-        Assert.Empty(reply.AcceptedEventIds);
-    }
-
-    [Fact]
-    public void IngestAuditEventsCommand_WhenProxyNeverReplies_PipesStatusFailureToSender()
-    {
-        // A short test-only Ask timeout (constructor seam) keeps the test fast —
-        // production uses the 30 s default.
-        var actor = CreateActor(auditIngestAskTimeout: TimeSpan.FromMilliseconds(200));
-        var auditProbe = CreateTestProbe();
-        actor.Tell(new RegisterAuditIngest(auditProbe.Ref));
-
-        var cmd = new IngestAuditEventsCommand(new[] { SampleAuditEvent() });
-        actor.Tell(cmd);
-
-        // The proxy receives the command but deliberately never replies.
-        auditProbe.ExpectMsg(cmd);
-
-        // The Ask times out; PipeTo forwards the faulted task as a Status.Failure
-        // to the original sender. This is the real transient signal the site's
-        // own Ask faults on — it is NOT swallowed into an empty ack.
-        var failure = ExpectMsg<Status.Failure>();
-        Assert.IsType<AskTimeoutException>(failure.Cause);
-    }
-
-    [Fact]
-    public void IngestCachedTelemetryCommand_WithRegisteredProxy_ForwardsAndRoutesReplyToSender()
-    {
-        var actor = CreateActor();
-        var auditProbe = CreateTestProbe();
-        actor.Tell(new RegisterAuditIngest(auditProbe.Ref));
-
-        var entry = new CachedTelemetryEntry(SampleAuditEvent(), SampleSiteCall());
-        var cmd = new IngestCachedTelemetryCommand(new[] { entry });
-        actor.Tell(cmd);
-
-        auditProbe.ExpectMsg(cmd);
-
-        var reply = new IngestCachedTelemetryReply(new[] { entry.Audit.EventId });
-        auditProbe.Reply(reply);
-
-        var received = ExpectMsg<IngestCachedTelemetryReply>();
-        Assert.Equal(new[] { entry.Audit.EventId }, received.AcceptedEventIds);
-    }
-
-    [Fact]
-    public void IngestCachedTelemetryCommand_WithNoProxyRegistered_RepliesEmptyAcceptedEventIds()
-    {
-        var actor = CreateActor();
-
-        var entry = new CachedTelemetryEntry(SampleAuditEvent(), SampleSiteCall());
-        actor.Tell(new IngestCachedTelemetryCommand(new[] { entry }));
-
-        var reply = ExpectMsg<IngestCachedTelemetryReply>();
-        Assert.Empty(reply.AcceptedEventIds);
-    }
-}

tests/ZB.MOM.WW.ScadaBridge.Communication.Tests/CentralCommunicationActorClientLifecycleTests.cs

@@ -37,7 +37,7 @@ public class CentralCommunicationActorClientLifecycleTests : TestKit
 
         var transport = Substitute.For<ISiteCommandTransport>();
         var actor = Sys.ActorOf(Props.Create(() => new CentralCommunicationActor(
-            provider, transport, (TimeSpan?)null)));
+            provider, transport)));
 
         // Trigger the refresh (also fires at PreStart, but drive it explicitly so
         // the assertion is deterministic). The load runs on a detached task and

tests/ZB.MOM.WW.ScadaBridge.Communication.Tests/CentralCommunicationActorReconcileTests.cs

@@ -63,7 +63,7 @@ public class CentralCommunicationActorReconcileTests : TestKit
         var sp = services.BuildServiceProvider();
 
         var transport = Substitute.For<ISiteCommandTransport>();
-        var actor = Sys.ActorOf(Props.Create(() => new CentralCommunicationActor(sp, transport, (TimeSpan?)null)));
+        var actor = Sys.ActorOf(Props.Create(() => new CentralCommunicationActor(sp, transport)));
 
         // Node B is missing inst-B entirely → it should come back as a gap item.
         actor.Tell(new ReconcileSiteRequest(

tests/ZB.MOM.WW.ScadaBridge.Communication.Tests/CentralCommunicationActorTests.cs

@@ -40,7 +40,7 @@ public class CentralCommunicationActorTests : TestKit
         var sp = services.BuildServiceProvider();
 
         var transport = Substitute.For<ISiteCommandTransport>();
-        var actor = Sys.ActorOf(Props.Create(() => new CentralCommunicationActor(sp, transport, (TimeSpan?)null)));
+        var actor = Sys.ActorOf(Props.Create(() => new CentralCommunicationActor(sp, transport)));
         return (actor, mockRepo);
     }
 
@@ -60,7 +60,7 @@ public class CentralCommunicationActorTests : TestKit
 
         var transport = Substitute.For<ISiteCommandTransport>();
         var centralActor = Sys.ActorOf(
-            Props.Create(() => new CentralCommunicationActor(sp, transport, (TimeSpan?)null)));
+            Props.Create(() => new CentralCommunicationActor(sp, transport)));
 
         var timestamp = DateTimeOffset.UtcNow;
         centralActor.Tell(new HeartbeatMessage("site1", "host1", true, timestamp));
@@ -87,7 +87,7 @@ public class CentralCommunicationActorTests : TestKit
 
         var transport = Substitute.For<ISiteCommandTransport>();
         var centralActor = Sys.ActorOf(
-            Props.Create(() => new CentralCommunicationActor(sp, transport, (TimeSpan?)null)));
+            Props.Create(() => new CentralCommunicationActor(sp, transport)));
 
         var ts = DateTimeOffset.UtcNow;
         centralActor.Tell(new SiteHeartbeatReplica(new HeartbeatMessage("site-1", "host-a", true, ts)));
@@ -116,7 +116,7 @@ public class CentralCommunicationActorTests : TestKit
         // The fix logs a Warning carrying the InvalidOperationException as the cause.
         EventFilter.Warning(contains: "Failed to load site addresses from the database").ExpectOne(() =>
         {
-            Sys.ActorOf(Props.Create(() => new CentralCommunicationActor(sp, transport, (TimeSpan?)null)));
+            Sys.ActorOf(Props.Create(() => new CentralCommunicationActor(sp, transport)));
         });
     }
 

tests/ZB.MOM.WW.ScadaBridge.Communication.Tests/CentralCommunicationActorTransportTests.cs

@@ -40,7 +40,7 @@ public class CentralCommunicationActorTransportTests : TestKit
         var sp = services.BuildServiceProvider();
 
         var transport = Substitute.For<ISiteCommandTransport>();
-        var actor = Sys.ActorOf(Props.Create(() => new CentralCommunicationActor(sp, transport, (TimeSpan?)null)));
+        var actor = Sys.ActorOf(Props.Create(() => new CentralCommunicationActor(sp, transport)));
         return (actor, transport, repo);
     }
 

tests/ZB.MOM.WW.ScadaBridge.Communication.Tests/CommunicationOptionsValidatorTests.cs

@@ -29,6 +29,59 @@ public class CommunicationOptionsValidatorTests
         Assert.Contains("DeploymentTimeout", result.FailureMessage);
     }
 
+    // ── Audit-ingest timeout ladder (arch-review phase-2 residual #2) ────────────
+
+    [Fact]
+    public void TheAuditIngestTimeoutLadder_IsStrictlyMonotonic_EndToEnd()
+    {
+        // 35 (site forward Ask) > 30 (gRPC deadline AND central's Ask of the ingest singleton)
+        // > 20 (actor budget) > 15 (SQL command). Ties are the bug this closes: the site Ask
+        // used to reuse NotificationForwardTimeout (30 s), so a slow-but-succeeding central
+        // write could be acked to a caller that had already given up and re-sent the batch.
+        var options = new CommunicationOptions();
+
+        Assert.Equal(TimeSpan.FromSeconds(35), options.AuditForwardTimeout);
+        Assert.True(options.AuditForwardTimeout > ZB.MOM.WW.ScadaBridge.Communication.Grpc.SiteStreamGrpcServer.AuditIngestAskTimeout);
+        Assert.Equal(TimeSpan.FromSeconds(30), ZB.MOM.WW.ScadaBridge.Communication.Grpc.SiteStreamGrpcServer.AuditIngestAskTimeout);
+    }
+
+    [Fact]
+    public void AuditForwardTimeout_EqualToTheAskTimeout_IsRejected()
+    {
+        // Equality is precisely the pre-fix state, so the validator must refuse it, not just
+        // refuse something smaller.
+        var result = Validate(new CommunicationOptions
+        {
+            AuditForwardTimeout = ZB.MOM.WW.ScadaBridge.Communication.Grpc.SiteStreamGrpcServer.AuditIngestAskTimeout,
+        });
+
+        Assert.True(result.Failed);
+        Assert.Contains("AuditForwardTimeout", result.FailureMessage);
+    }
+
+    [Fact]
+    public void AuditForwardTimeout_ShorterThanTheAskTimeout_IsRejected()
+    {
+        var result = Validate(new CommunicationOptions
+        {
+            AuditForwardTimeout = TimeSpan.FromSeconds(5),
+        });
+
+        Assert.True(result.Failed);
+        Assert.Contains("AuditForwardTimeout", result.FailureMessage);
+    }
+
+    [Fact]
+    public void AuditForwardTimeout_IsStillConfigurableUpwards()
+    {
+        var result = Validate(new CommunicationOptions
+        {
+            AuditForwardTimeout = TimeSpan.FromMinutes(2),
+        });
+
+        Assert.True(result.Succeeded, result.FailureMessage);
+    }
+
     [Fact]
     public void NonPositiveGrpcMaxConcurrentStreams_IsRejected()
     {

tests/ZB.MOM.WW.ScadaBridge.Communication.Tests/Grpc/CentralControlGrpcServiceAuditIngestTests.cs

@@ -0,0 +1,194 @@
+using Akka.Actor;
+using Akka.TestKit.Xunit2;
+using Grpc.Core;
+using Microsoft.Extensions.Logging.Abstractions;
+using Microsoft.Extensions.Options;
+using NSubstitute;
+using ZB.MOM.WW.Audit;
+using ZB.MOM.WW.ScadaBridge.Commons.Entities.Audit;
+using ZB.MOM.WW.ScadaBridge.Commons.Interfaces.Repositories;
+using ZB.MOM.WW.ScadaBridge.Commons.Messages.Audit;
+using ZB.MOM.WW.ScadaBridge.Commons.Types;
+using ZB.MOM.WW.ScadaBridge.Commons.Types.Audit;
+using ZB.MOM.WW.ScadaBridge.Commons.Types.Enums;
+using ZB.MOM.WW.ScadaBridge.Communication.Actors;
+using ZB.MOM.WW.ScadaBridge.Communication.Grpc;
+using Microsoft.Extensions.DependencyInjection;
+
+namespace ZB.MOM.WW.ScadaBridge.Communication.Tests.Grpc;
+
+/// <summary>
+/// Audit Log (#23) site→central ingest routing on <see cref="CentralControlGrpcService"/>.
+/// <para>
+/// The service Asks the <c>audit-log-ingest</c> singleton proxy DIRECTLY. It used to relay
+/// through <see cref="CentralCommunicationActor"/>, which re-Asked the same proxy with the same
+/// 30 s <see cref="SiteStreamGrpcServer.AuditIngestAskTimeout"/> — a second hop whose inner Ask
+/// expired at the same instant as the outer one, so it could only add latency. These tests pin
+/// the direct dispatch, the wiring-race reply, and the fact that the relay is gone.
+/// </para>
+/// </summary>
+public class CentralControlGrpcServiceAuditIngestTests : TestKit
+{
+    private static ServerCallContext NewContext(CancellationToken ct = default)
+    {
+        var context = Substitute.For<ServerCallContext>();
+        context.CancellationToken.Returns(ct);
+        return context;
+    }
+
+    private static CentralControlGrpcService CreateService() => new(
+        NullLogger<CentralControlGrpcService>.Instance,
+        Options.Create(new CommunicationOptions()));
+
+    [Fact]
+    public async Task IngestAuditEvents_AsksTheIngestProxy_AndNeverTheCommunicationActor()
+    {
+        var ingest = CreateTestProbe();
+        var control = CreateTestProbe();
+        var service = CreateService();
+        service.SetReady(control.Ref);
+        service.SetAuditIngestActor(ingest.Ref);
+
+        var evt = SampleAuditEvent();
+        var batch = new AuditEventBatch();
+        batch.Events.Add(AuditEventDtoMapper.ToDto(evt));
+
+        var call = service.IngestAuditEvents(batch, NewContext());
+
+        var received = ingest.ExpectMsg<IngestAuditEventsCommand>();
+        Assert.Equal(evt.EventId, Assert.Single(received.Events).EventId);
+        ingest.Reply(new IngestAuditEventsReply(new[] { evt.EventId }));
+
+        var ack = await call;
+        Assert.Equal(evt.EventId.ToString(), Assert.Single(ack.AcceptedEventIds));
+
+        // The old relay hop is gone: the control-plane actor sees nothing at all.
+        control.ExpectNoMsg(TimeSpan.FromMilliseconds(200));
+    }
+
+    [Fact]
+    public async Task IngestCachedTelemetry_AsksTheIngestProxy_AndNeverTheCommunicationActor()
+    {
+        var ingest = CreateTestProbe();
+        var control = CreateTestProbe();
+        var service = CreateService();
+        service.SetReady(control.Ref);
+        service.SetAuditIngestActor(ingest.Ref);
+
+        var evt = SampleAuditEvent();
+        var batch = new CachedTelemetryBatch();
+        batch.Packets.Add(new CachedTelemetryPacket
+        {
+            AuditEvent = AuditEventDtoMapper.ToDto(evt),
+            Operational = SiteCallDtoMapper.ToDto(SampleSiteCall()),
+        });
+
+        var call = service.IngestCachedTelemetry(batch, NewContext());
+
+        var received = ingest.ExpectMsg<IngestCachedTelemetryCommand>();
+        Assert.Single(received.Entries);
+        ingest.Reply(new IngestCachedTelemetryReply(new[] { evt.EventId }));
+
+        var ack = await call;
+        Assert.Equal(evt.EventId.ToString(), Assert.Single(ack.AcceptedEventIds));
+        control.ExpectNoMsg(TimeSpan.FromMilliseconds(200));
+    }
+
+    [Fact]
+    public async Task IngestAuditEvents_BeforeTheIngestProxyIsWired_ReturnsAnEmptyAck()
+    {
+        // The singleton starts moments after SetReady, so this window is real. An empty ack
+        // (NOT a fault, NOT Unavailable) leaves the site's rows Pending for the next drain —
+        // byte-for-byte what the removed relay replied when its proxy was still null.
+        var service = CreateService();
+        service.SetReady(CreateTestProbe().Ref);
+
+        var batch = new AuditEventBatch();
+        batch.Events.Add(AuditEventDtoMapper.ToDto(SampleAuditEvent()));
+
+        var ack = await service.IngestAuditEvents(batch, NewContext());
+
+        Assert.Empty(ack.AcceptedEventIds);
+    }
+
+    [Fact]
+    public async Task IngestCachedTelemetry_BeforeTheIngestProxyIsWired_ReturnsAnEmptyAck()
+    {
+        var service = CreateService();
+        service.SetReady(CreateTestProbe().Ref);
+
+        var batch = new CachedTelemetryBatch();
+        batch.Packets.Add(new CachedTelemetryPacket
+        {
+            AuditEvent = AuditEventDtoMapper.ToDto(SampleAuditEvent()),
+            Operational = SiteCallDtoMapper.ToDto(SampleSiteCall()),
+        });
+
+        var ack = await service.IngestCachedTelemetry(batch, NewContext());
+
+        Assert.Empty(ack.AcceptedEventIds);
+    }
+
+    [Fact]
+    public void SetAuditIngestActor_IsIndependentOfSetReady()
+    {
+        var service = CreateService();
+        Assert.False(service.IsAuditIngestBound);
+
+        service.SetAuditIngestActor(CreateTestProbe().Ref);
+
+        Assert.True(service.IsAuditIngestBound);
+        Assert.False(service.IsReady);
+    }
+
+    [Fact]
+    public void CentralCommunicationActor_NoLongerRelaysIngestCommands()
+    {
+        // Regression pin for the removed hop: the actor has no ingest receive at all, so an
+        // ingest command reaching it is an unhandled message with no reply — not a silent
+        // second path that could drift from the direct one.
+        var actor = CreateCentralCommunicationActor();
+
+        actor.Tell(new IngestAuditEventsCommand(new[] { SampleAuditEvent() }), TestActor);
+        actor.Tell(
+            new IngestCachedTelemetryCommand(
+                new[] { new CachedTelemetryEntry(SampleAuditEvent(), SampleSiteCall()) }),
+            TestActor);
+
+        ExpectNoMsg(TimeSpan.FromMilliseconds(300));
+    }
+
+    private IActorRef CreateCentralCommunicationActor()
+    {
+        var siteRepo = Substitute.For<ISiteRepository>();
+        siteRepo.GetAllSitesAsync(Arg.Any<CancellationToken>())
+            .Returns(new List<Commons.Entities.Sites.Site>());
+
+        var services = new ServiceCollection();
+        services.AddScoped(_ => siteRepo);
+        var sp = services.BuildServiceProvider();
+
+        var transport = Substitute.For<ISiteCommandTransport>();
+        return Sys.ActorOf(Props.Create(() => new CentralCommunicationActor(sp, transport)));
+    }
+
+    private static AuditEvent SampleAuditEvent() =>
+        ScadaBridgeAuditEventFactory.Create(
+            channel: AuditChannel.ApiOutbound,
+            kind: AuditKind.ApiCall,
+            status: AuditStatus.Delivered,
+            sourceSiteId: "site-a");
+
+    private static SiteCall SampleSiteCall() => new()
+    {
+        TrackedOperationId = TrackedOperationId.New(),
+        Channel = "OutboundApi",
+        Target = "ExternalSystemA",
+        SourceSite = "site-a",
+        Status = "Delivered",
+        RetryCount = 0,
+        CreatedAtUtc = DateTime.UtcNow,
+        UpdatedAtUtc = DateTime.UtcNow,
+        IngestedAtUtc = DateTime.UtcNow,
+    };
+}

tests/ZB.MOM.WW.ScadaBridge.Communication.Tests/HealthReportAckTests.cs

@@ -55,7 +55,7 @@ public class HealthReportAckTests : TestKit
         var sp = services.BuildServiceProvider();
 
         var transport = Substitute.For<ISiteCommandTransport>();
-        var actor = Sys.ActorOf(Props.Create(() => new CentralCommunicationActor(sp, transport, (TimeSpan?)null)));
+        var actor = Sys.ActorOf(Props.Create(() => new CentralCommunicationActor(sp, transport)));
 
         actor.Tell(SampleReport(seq: 3));
         var ack = ExpectMsg<SiteHealthReportAck>();

tests/ZB.MOM.WW.ScadaBridge.Communication.Tests/SiteAlarmLiveCacheServiceTests.cs

@@ -81,7 +81,7 @@ public class SiteAlarmLiveCacheServiceTests : TestKit
     }
 
     private SiteAlarmLiveCacheService CreateService(TimeSpan linger, out CountingFactory factory,
-        int maxSubscribersPerSite = 200)
+        int maxSubscribersPerSite = 200, IReadOnlyList<Instance>? enabledInstances = null)
     {
         // Site with gRPC addresses, and NO enabled instances → the seed fan-out returns
         // empty immediately (so IsLive flips true fast without any snapshot Asks).
@@ -97,7 +97,7 @@ public class SiteAlarmLiveCacheServiceTests : TestKit
 
         var instanceRepo = Substitute.For<ITemplateEngineRepository>();
         instanceRepo.GetInstancesBySiteIdAsync(SiteId, Arg.Any<CancellationToken>())
-            .Returns(new List<Instance>());
+            .Returns((IReadOnlyList<Instance>)(enabledInstances ?? new List<Instance>()));
 
         var services = new ServiceCollection();
         services.AddScoped(_ => siteRepo);
@@ -121,6 +121,45 @@ public class SiteAlarmLiveCacheServiceTests : TestKit
         return service;
     }
 
+    [Fact]
+    public void Seed_FanOut_Publishes_The_Instances_That_Failed_To_Answer()
+    {
+        // Arch-review phase-2 residual #4: the seed/reconcile fan-out already knows which
+        // Enabled instances failed to answer and used to discard it, forcing the Alarm Summary
+        // page to run a SECOND identical fan-out purely to rebuild that list. It is now
+        // published alongside the snapshot.
+        var instances = new List<Instance>
+        {
+            new("inst-b") { Id = 2, SiteId = SiteId, State = InstanceState.Enabled },
+            new("inst-a") { Id = 1, SiteId = SiteId, State = InstanceState.Enabled },
+            // Disabled instances are never fanned out, so they can never be "not reporting".
+            new("inst-off") { Id = 3, SiteId = SiteId, State = InstanceState.Disabled },
+        };
+
+        // The CommunicationService has no site actor wired, so every snapshot Ask faults —
+        // which is exactly the "instance did not answer" case.
+        var service = CreateService(TimeSpan.FromMilliseconds(200), out _, enabledInstances: instances);
+
+        using var sub = service.Subscribe(SiteId, () => { });
+        AwaitCondition(() => service.IsLive(SiteId), TimeSpan.FromSeconds(5));
+
+        AwaitCondition(
+            () => service.GetNotReportingInstances(SiteId).Count == 2,
+            TimeSpan.FromSeconds(5));
+
+        // Ordered by name (ordinal-ignore-case), matching AlarmSummaryService's poll output so
+        // the page renders identically whichever source supplied the list.
+        Assert.Equal(new[] { "inst-a", "inst-b" }, service.GetNotReportingInstances(SiteId));
+    }
+
+    [Fact]
+    public void NotReporting_Is_Empty_For_An_Unknown_Site()
+    {
+        var service = CreateService(TimeSpan.FromMilliseconds(200), out _);
+
+        Assert.Empty(service.GetNotReportingInstances(SiteId));
+    }
+
     [Fact]
     public void First_Subscriber_Starts_One_Aggregator_Shared_By_Multiple_Viewers()
     {

tests/ZB.MOM.WW.ScadaBridge.Communication.Tests/SyntheticHeartbeatTests.cs

@@ -0,0 +1,201 @@
+using System.Buffers.Binary;
+using Akka.Actor;
+using Akka.TestKit.Xunit2;
+using Google.Protobuf.WellKnownTypes;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Logging.Abstractions;
+using NSubstitute;
+using ZB.MOM.WW.ScadaBridge.Commons.Entities.Sites;
+using ZB.MOM.WW.ScadaBridge.Commons.Interfaces.Repositories;
+using ZB.MOM.WW.ScadaBridge.Commons.Interfaces.Services;
+using ZB.MOM.WW.ScadaBridge.Commons.Messages.Health;
+using ZB.MOM.WW.ScadaBridge.Communication.Actors;
+using ZB.MOM.WW.ScadaBridge.Communication.Grpc;
+using ZB.MOM.WW.ScadaBridge.HealthMonitoring;
+
+namespace ZB.MOM.WW.ScadaBridge.Communication.Tests;
+
+/// <summary>
+/// Arch-review phase-2 residual #3: <see cref="CentralChannelProvider"/>'s failback probe reuses
+/// the <c>Heartbeat</c> RPC to ask "does the preferred central endpoint answer again?". It is
+/// emitted by a site's TRANSPORT layer, not by any node's heartbeat timer, so central must not
+/// count it as liveness — otherwise a site whose real heartbeats had stopped keeps looking alive
+/// on the health dashboard for as long as its transport keeps probing.
+/// <para>
+/// The contract is the explicit additive <c>Synthetic</c> flag (<c>HeartbeatDto.synthetic</c>,
+/// proto field 5), NOT the <c>failback-probe</c> hostname, which is a log label only.
+/// </para>
+/// </summary>
+public class SyntheticHeartbeatTests : TestKit
+{
+    private static readonly DateTimeOffset T0 =
+        new(2026, 8, 14, 9, 0, 0, TimeSpan.Zero);
+
+    // ── The consumer: central skips liveness bookkeeping for a synthetic heartbeat ───
+
+    [Fact]
+    public void SyntheticHeartbeat_DoesNotMarkTheHealthAggregator()
+    {
+        var (actor, aggregator) = CreateCentralActor();
+
+        actor.Tell(new HeartbeatMessage("site-1", CentralChannelProvider.SyntheticProbeHostname,
+            IsActive: false, Timestamp: T0, Synthetic: true));
+
+        // Give the actor a real chance to (wrongly) mark before asserting the negative.
+        ExpectNoMsg(TimeSpan.FromMilliseconds(300));
+        aggregator.DidNotReceiveWithAnyArgs().MarkHeartbeat(default!, default);
+    }
+
+    [Fact]
+    public void RealHeartbeat_StillMarksTheHealthAggregator()
+    {
+        // The guard must be keyed on the flag alone — an ordinary heartbeat is unaffected,
+        // including one from a node that predates the field (proto3 defaults it to false).
+        var (actor, aggregator) = CreateCentralActor();
+
+        actor.Tell(new HeartbeatMessage("site-1", "node-a", IsActive: true, Timestamp: T0));
+
+        AwaitAssert(() => aggregator.Received(1).MarkHeartbeat("site-1", T0));
+    }
+
+    [Fact]
+    public void SyntheticHeartbeatReplica_IsAlsoSkipped()
+    {
+        // Belt-and-braces on the last hop before the aggregator: a peer central node that
+        // predates the flag could still replicate one.
+        var (actor, aggregator) = CreateCentralActor();
+
+        actor.Tell(new SiteHeartbeatReplica(new HeartbeatMessage(
+            "site-1", CentralChannelProvider.SyntheticProbeHostname,
+            IsActive: false, Timestamp: T0, Synthetic: true)));
+
+        ExpectNoMsg(TimeSpan.FromMilliseconds(300));
+        aggregator.DidNotReceiveWithAnyArgs().MarkHeartbeat(default!, default);
+    }
+
+    // ── The wire contract: the flag survives the round trip ─────────────────────────
+
+    [Theory]
+    [InlineData(true)]
+    [InlineData(false)]
+    public void TheSyntheticFlag_RoundTripsThroughTheDto(bool synthetic)
+    {
+        var msg = new HeartbeatMessage("site-1", "node-a", IsActive: true, Timestamp: T0,
+            Synthetic: synthetic);
+
+        var back = CentralControlDtoMapper.FromDto(CentralControlDtoMapper.ToDto(msg));
+
+        Assert.Equal(synthetic, back.Synthetic);
+        Assert.Equal(msg with { Synthetic = synthetic }, back);
+    }
+
+    [Fact]
+    public void ADtoFromAnOlderSite_DefaultsToNotSynthetic()
+    {
+        // proto3 default: a peer that never sets field 5 sends a REAL heartbeat, as before.
+        var dto = new HeartbeatDto
+        {
+            SiteId = "site-1",
+            NodeHostname = "node-a",
+            IsActive = true,
+            Timestamp = Timestamp.FromDateTimeOffset(T0),
+        };
+
+        Assert.False(CentralControlDtoMapper.FromDto(dto).Synthetic);
+    }
+
+    // ── The producer: the failback probe marks itself synthetic ─────────────────────
+
+    [Fact]
+    public async Task TheFailbackProbe_MarksItsHeartbeatSynthetic()
+    {
+        // Two endpoints so a flip is possible; the capture handler answers nothing useful, so
+        // the probe faults and re-arms — we only care about the request it put on the wire.
+        var capture = new HeartbeatCapturingHandler();
+        using var provider = new CentralChannelProvider(
+            new[] { "http://central-a:8083", "http://central-b:8083" },
+            new FixedPskProvider("k"),
+            "site-1",
+            new CommunicationOptions(),
+            NullLogger.Instance,
+            handlerFactory: _ => capture,
+            probeDeadline: TimeSpan.FromSeconds(2),
+            backoffBase: TimeSpan.FromMilliseconds(20),
+            backoffCap: TimeSpan.FromMilliseconds(50));
+
+        // Off the preferred endpoint → the background failback probe arms.
+        provider.ReportUnavailable(0);
+
+        var probe = await capture.WaitForHeartbeatAsync(TimeSpan.FromSeconds(10));
+
+        Assert.True(probe.Synthetic);
+        Assert.Equal("site-1", probe.SiteId);
+        // The hostname is a human-readable label that rides ALONGSIDE the flag; central keys
+        // its skip on the flag, never on this string.
+        Assert.Equal(CentralChannelProvider.SyntheticProbeHostname, probe.NodeHostname);
+        Assert.False(probe.IsActive);
+    }
+
+    private (IActorRef Actor, ICentralHealthAggregator Aggregator) CreateCentralActor()
+    {
+        var siteRepo = Substitute.For<ISiteRepository>();
+        siteRepo.GetAllSitesAsync(Arg.Any<CancellationToken>()).Returns(new List<Site>());
+
+        var aggregator = Substitute.For<ICentralHealthAggregator>();
+
+        var services = new ServiceCollection();
+        services.AddScoped(_ => siteRepo);
+        services.AddSingleton(aggregator);
+        var sp = services.BuildServiceProvider();
+
+        var actor = Sys.ActorOf(Props.Create(() =>
+            new CentralCommunicationActor(sp, Substitute.For<ISiteCommandTransport>())));
+        return (actor, aggregator);
+    }
+
+    private sealed class FixedPskProvider(string key) : ISitePskProvider
+    {
+        public ValueTask<string> GetAsync(string siteId, CancellationToken ct) => new(key);
+        public void Invalidate(string siteId) { }
+    }
+
+    /// <summary>
+    /// Captures the first <c>Heartbeat</c> request body and decodes the length-prefixed gRPC
+    /// frame back into a <see cref="HeartbeatDto"/>. The response is deliberately a bare 500 so
+    /// the probe treats the endpoint as still down; the provider swallows that and re-arms.
+    /// </summary>
+    private sealed class HeartbeatCapturingHandler : HttpMessageHandler
+    {
+        private readonly TaskCompletionSource<HeartbeatDto> _captured =
+            new(TaskCreationOptions.RunContinuationsAsynchronously);
+
+        public async Task<HeartbeatDto> WaitForHeartbeatAsync(TimeSpan timeout)
+        {
+            var completed = await Task.WhenAny(_captured.Task, Task.Delay(timeout));
+            Assert.Same(_captured.Task, completed);
+            return await _captured.Task;
+        }
+
+        protected override async Task<HttpResponseMessage> SendAsync(
+            HttpRequestMessage request, CancellationToken cancellationToken)
+        {
+            if (request.Content is not null &&
+                request.RequestUri?.AbsolutePath.EndsWith("/Heartbeat", StringComparison.Ordinal) == true)
+            {
+                var body = await request.Content.ReadAsByteArrayAsync(cancellationToken);
+                // gRPC frame: 1 compression byte + 4-byte big-endian length + payload.
+                if (body.Length >= 5)
+                {
+                    var length = BinaryPrimitives.ReadInt32BigEndian(body.AsSpan(1, 4));
+                    _captured.TrySetResult(
+                        HeartbeatDto.Parser.ParseFrom(body.AsSpan(5, length).ToArray()));
+                }
+            }
+
+            return new HttpResponseMessage(System.Net.HttpStatusCode.InternalServerError)
+            {
+                Version = request.Version,
+            };
+        }
+    }
+}

tests/ZB.MOM.WW.ScadaBridge.Host.Tests/CentralControlEndToEndTests.cs

@@ -62,6 +62,10 @@ public class CentralControlEndToEndTests : IAsyncLifetime
             NullLogger<CentralControlGrpcService>.Instance,
             Options.Create(new CommunicationOptions()));
         _service.SetReady(stub);
+        // The two ingest RPCs Ask the audit-ingest singleton DIRECTLY (no CentralCommunicationActor
+        // relay any more), so the service needs its own proxy handed over. Reusing the same stub
+        // keeps one actor answering both shapes.
+        _service.SetAuditIngestActor(stub);
 
         var pskProvider = new MapPskProvider(new Dictionary<string, string>
         {
@@ -174,7 +178,7 @@ public class CentralControlEndToEndTests : IAsyncLifetime
     // ---- One RPC per shape: unary (above) + the ingest bridge ----
 
     [Fact]
-    public async Task IngestAuditEvents_DecodesTheBatch_AsksTheActor_EncodesTheAck()
+    public async Task IngestAuditEvents_DecodesTheBatch_AsksTheIngestProxy_EncodesTheAck()
     {
         var client = Client(SiteAKey, SiteA);
 
@@ -185,12 +189,13 @@ public class CentralControlEndToEndTests : IAsyncLifetime
         var ack = await client.IngestAuditEventsAsync(batch);
 
         // The stub actor accepts one deterministic id per non-empty batch; its presence proves
-        // the full DTO→command→Ask→reply→ack bridge ran, gated call and all.
+        // the full DTO→command→Ask→reply→ack bridge ran straight to the ingest proxy, gated
+        // call and all.
         Assert.Contains(AcceptedId.ToString(), ack.AcceptedEventIds);
     }
 
     [Fact]
-    public async Task IngestAuditEvents_EmptyBatch_ShortCircuits_WithoutAskingTheActor()
+    public async Task IngestAuditEvents_EmptyBatch_ShortCircuits_WithoutAskingTheIngestProxy()
     {
         // Even the empty-batch fast path is behind the gate — it still needs a valid key.
         var client = Client(SiteAKey, SiteA);
@@ -238,9 +243,9 @@ public class CentralControlEndToEndTests : IAsyncLifetime
     }
 
     /// <summary>
-    /// Minimal stand-in for <c>CentralCommunicationActor</c>: answers the two RPC shapes this
+    /// Minimal stand-in for both actors the service Asks — <c>CentralCommunicationActor</c> for
-    /// test exercises. Replies straight to the Ask's temp sender, exactly as the real actor's
+    /// the unary control RPCs and the <c>audit-log-ingest</c> singleton for the ingest ones.
-    /// Forward/PipeTo paths do.
+    /// Replies straight to the Ask's temp sender, exactly as the real actors do.
     /// </summary>
     private sealed class StubCentralActor : ReceiveActor
     {

tests/ZB.MOM.WW.ScadaBridge.IntegrationTests/AuditLog/SiteAuditPushFlowTests.cs

@@ -59,12 +59,24 @@ public class SiteAuditPushFlowTests : TestKit
     private sealed class BridgeCentralTransport : ICentralTransport
     {
         private readonly IActorRef _central;
-        public BridgeCentralTransport(IActorRef central) => _central = central;
+        private readonly IActorRef _auditIngest;
+
+        /// <param name="central">Stands in for the central control plane's non-audit RPCs.</param>
+        /// <param name="auditIngest">
+        /// The central audit-ingest singleton. The two ingest RPCs go straight here, mirroring
+        /// <c>CentralControlGrpcService</c>, which Asks the proxy directly rather than relaying
+        /// through <c>CentralCommunicationActor</c>.
+        /// </param>
+        public BridgeCentralTransport(IActorRef central, IActorRef auditIngest)
+        {
+            _central = central;
+            _auditIngest = auditIngest;
+        }
 
         public void SubmitNotification(NotificationSubmit message, IActorRef replyTo) => _central.Tell(message, replyTo);
         public void QueryNotificationStatus(NotificationStatusQuery message, IActorRef replyTo) => _central.Tell(message, replyTo);
-        public void IngestAuditEvents(IngestAuditEventsCommand message, IActorRef replyTo) => _central.Tell(message, replyTo);
+        public void IngestAuditEvents(IngestAuditEventsCommand message, IActorRef replyTo) => _auditIngest.Tell(message, replyTo);
-        public void IngestCachedTelemetry(IngestCachedTelemetryCommand message, IActorRef replyTo) => _central.Tell(message, replyTo);
+        public void IngestCachedTelemetry(IngestCachedTelemetryCommand message, IActorRef replyTo) => _auditIngest.Tell(message, replyTo);
         public void ReconcileSite(ReconcileSiteRequest message, IActorRef replyTo) => _central.Tell(message, replyTo);
         public void ReportSiteHealth(SiteHealthReport message, IActorRef replyTo) => _central.Tell(message, replyTo);
         public void SendHeartbeat(HeartbeatMessage message, IActorRef self) => _central.Tell(message, self);
@@ -144,9 +156,11 @@ public class SiteAuditPushFlowTests : TestKit
                 centralRepo,
                 NullLogger<ZB.MOM.WW.ScadaBridge.AuditLog.Central.AuditLogIngestActor>.Instance)));
 
-        // Real CentralCommunicationActor. Its periodic site-address refresh
+        // Real CentralCommunicationActor. It is NOT on the audit path any more (the
-        // resolves an ISiteRepository from this provider; an empty result keeps
+        // central-hosted CentralControlGrpcService Asks the ingest singleton directly), but the
-        // the refresh a clean no-op and never touches the audit-ingest path.
+        // bridge transport still routes notifications/health/reconcile through it, so it is
+        // constructed exactly as production does. Its periodic site-address refresh resolves an
+        // ISiteRepository from this provider; an empty result keeps the refresh a clean no-op.
         var siteRepo = Substitute.For<ISiteRepository>();
         siteRepo.GetAllSitesAsync().Returns(Array.Empty<Site>());
         var centralServices = new ServiceCollection();
@@ -155,9 +169,7 @@ public class SiteAuditPushFlowTests : TestKit
 
         var centralCommActor = Sys.ActorOf(Props.Create(() => new CentralCommunicationActor(
             centralProvider,
-            Substitute.For<ISiteCommandTransport>(),
+            Substitute.For<ISiteCommandTransport>())));
-            TimeSpan.FromSeconds(5))));
-        centralCommActor.Tell(new RegisterAuditIngest(ingestActor));
 
         // ── Site side ─────────────────────────────────────────────────────
         // Real SqliteAuditWriter on a file-backed SQLite db (the site hot-path
@@ -177,7 +189,7 @@ public class SiteAuditPushFlowTests : TestKit
             CreateTestProbe().Ref, // deployment-manager proxy is unused here
             null,
             null,
-            new BridgeCentralTransport(centralCommActor))));
+            new BridgeCentralTransport(centralCommActor, ingestActor))));
 
         // The production site audit push client — the unit under integration.
         var auditClient = new SiteCommunicationAuditClient(