merge(r2): PLAN-R2-02 Communication & S&F (Critical N1)

docs/requirements/Component-Communication.md

@@ -75,8 +75,11 @@ Delivered 2026-07-10 (`docs/plans/2026-07-10-aggregated-live-alarm-stream-plan.m
 - **Central live cache** (`ISiteAlarmLiveCache`, singleton `SiteAlarmLiveCacheService`): a DI singleton on the active central node. For each site with ≥1 active viewer it runs ONE shared, **reference-counted** per-site aggregator (`SiteAlarmAggregatorActor`); the first `Subscribe(siteId, onChanged)` starts it, the last subscriber leaving stops it after a short **linger** to avoid re-seed thrash. `GetCurrentAlarms(siteId)` returns the current immutable snapshot; `IsLive(siteId)` reports whether the aggregator has seeded and published at least once.
 - **Seed-then-stream** (copied from `DebugStreamBridgeActor` ordering): open the `SubscribeSite` stream first (buffer live deltas), run the snapshot fan-out once via the existing `DebugViewSnapshot` path (bounded by `LiveAlarmCacheSeedConcurrency`), flush the buffer with **dedup by `(InstanceUniqueName, AlarmName, SourceReference)`**, then live pass-through. Placeholders are seeded from the snapshot and never expected on the live stream.
 - **Failover & drift**: NodeA↔NodeB reconnect (client via `SiteStreamGrpcClient.SubscribeSiteAsync`, factory endpoint failover) re-seeds rather than serving stale; a periodic **reconcile snapshot** (default 60s) corrects any missed enable/disable so the cache can't drift indefinitely. `[PERM]` (`docs/plans/2026-05-29-native-alarms-design.md`): the cache is **purely in-memory** — no EF entity/table/migration, no persisted central alarm store — so a new active node simply re-seeds from scratch.
-- **Options** (`Communication` section, `CommunicationOptions`; eagerly validated by `CommunicationOptionsValidator` / `ValidateOnStart`): `LiveAlarmCacheLinger` (default 30s), `LiveAlarmCacheReconcileInterval` (default 60s), `LiveAlarmCacheSeedConcurrency` (default 8), `LiveAlarmCacheMaxSubscribersPerSite` (default 200).
+- **Options** (`Communication` section, `CommunicationOptions`; eagerly validated by `CommunicationOptionsValidator` / `ValidateOnStart`): `LiveAlarmCacheLinger` (default 30s), `LiveAlarmCacheReconcileInterval` (default 60s), `LiveAlarmCacheSeedConcurrency` (default 8), `LiveAlarmCacheMaxSubscribersPerSite` (default 200), `LiveAlarmCachePublishCoalesce` (default 250ms; `0` = publish per delta — legacy — batches an alarm storm into one snapshot copy + one viewer fan-out per window; arch review 02 round 2, N6).
 - **Telemetry** (`ScadaBridgeTelemetry` meter): observable gauge `scadabridge.site.alarm_cache.aggregators.active` (running per-site aggregators) and counter `scadabridge.site.alarm_cache.reconnects` (site-wide stream reconnects — a NodeA↔NodeB flip or reconcile-driven reopen; a sustained climb signals a flapping site link).
+- **Accepted limitations (arch review 02 round 2, N8):**
+  - **Standby-node aggregators**: the live cache is per-node and `SetActorSystem` is wired on every central node, so browsing the standby node directly (diagnostic ports, e.g. 9002) starts a second, fully-functional read-only aggregator + `SubscribeSite` stream there. Accepted — not gated: the aggregator is read-only, bounded (one stream/site, viewer-capped), and torn down by the viewer linger; gating `SetActorSystem` behind the active check would break the diagnostic-browse path and buy nothing. The `[PERM]` "lives only on the active central node" claim is corrected to "per-node; in routine operation only the active node hosts viewers".
+  - **Site deleted while an Alarm Summary viewer is open**: the viewer's aggregator reconciles to an empty snapshot (the deleted site's instances vanish from the fan-out) until its viewers leave, then the linger stop reaps it; the site's cached gRPC channels are disposed at deletion via `SiteStreamGrpcClientFactory.RemoveSiteAsync` (see `ManagementActor.HandleDeleteSite`, arch review 02 round 2, N8).
 
 #### Site-Side gRPC Streaming Components
 

docs/requirements/Component-StoreAndForward.md

@@ -80,7 +80,7 @@ There is **no maximum buffer size**. Messages accumulate in the buffer until del
 - The standby node applies the same operations to its own local SQLite database but is **passive**: it never runs the delivery sweep. The retry sweep is **gated to the active node** (the oldest Up member / singleton host, re-evaluated every sweep tick), so only one node delivers at a time. The standby applies replicated operations purely to keep its copy warm for a future failover.
 - On failover, the new active node has a near-complete copy of the buffer. In rare cases, the most recent operations may not have been replicated (e.g., a message added or removed just before failover). This can result in a few **duplicate deliveries** (message delivered but its `Remove` not yet replicated) or a few **missed retries** (message added but not replicated). Duplicate deliveries are therefore confined to the **failover window** — an in-flight delivery whose `Remove` had not yet replicated — and never occur in steady-state operation (the standby's gate keeps it from delivering the same rows). Both are acceptable trade-offs for the latency benefit.
 - On failover, the new active node's gate flips to active within one sweep interval and it resumes delivery from its local copy.
-- **Peer-join anti-entropy resync.** Asynchronous, no-ack replication keeps the standby warm in steady state, but a standby that was **down for an extended period** (a crash, a long maintenance window) misses every operation replicated while it was gone and would otherwise diverge from the active node's buffer forever. To close that gap, whenever a node **(re)tracks its peer**, a **standby** requests a full-buffer snapshot (`RequestSfBufferResync`); the **active** node answers with up to `MaxResyncRows` (10 000) of its oldest rows (`SfBufferSnapshot`), and the standby **replaces its entire local buffer** with that snapshot (`ReplaceAllAsync`, one transaction). Only the active node answers; only a standby applies (each side checks the repo-standard leader+Up active-node predicate, safe-by-default to standby). Because replicated applies are **upserts** (see the replication apply path), any Add/Remove/Park that lands after the resync merges cleanly onto the resynced state — no primary-key conflict, no lost delta. If the buffer exceeds the 10 000-row cap the snapshot is flagged `Truncated` and the standby logs a Warning; the residual divergence beyond the cap drains naturally as the active node delivers.
+- **Peer-join anti-entropy resync (chunked, ack-confirmed).** Asynchronous, no-ack replication keeps the standby warm in steady state, but a standby that was **down for an extended period** (a crash, a long maintenance window) misses every operation replicated while it was gone and would otherwise diverge from the active node's buffer forever. To close that gap, whenever a node **(re)tracks its peer**, a **standby** requests a full-buffer snapshot (`RequestSfBufferResync`); the **active** node loads up to `MaxResyncRows` (10 000) of its oldest rows and answers with a **sequence of byte-budgeted chunks** (`SfBufferSnapshotChunk`), each carrying a shared `ResyncId`, a 1-based `Sequence`, and the `TotalChunks` count. Chunking is mandatory because the monolithic snapshot exceeds Akka remoting's **default 128 000-byte frame** for any realistic backlog and `BuildHocon` sets no override — a single oversized message is silently undeliverable (review 02 round 2, **N2 High**). Rows accumulate into a chunk until the estimated payload budget (`MaxResyncChunkBytes` = 64 000, ≈50% frame headroom) or the row cap (`MaxResyncChunkRows` = 200) is hit; a single row whose payload alone exceeds the budget ships solo with a Warning. The standby **assembles all chunks of one `ResyncId`** (a new `ResyncId` discards any stale partial assembly; a partial that never completes is dropped after `resyncAssemblyTimeout`, default 30 s, and counted a replication failure), then **replaces its entire local buffer** with the assembled snapshot (`ReplaceAllAsync`, one transaction) and returns a delivery confirmation (`SfBufferResyncAck`). The active node arms an ack window (`resyncAckTimeout`, default 60 s): an acknowledged resync increments `scadabridge.store_and_forward.resync.completed`; an unacknowledged one (lost chunks / dead peer) logs a Warning and increments `scadabridge.store_and_forward.resync.ack_missing` — closing N2's silent-loss mode (previously nothing counted a lost snapshot and nothing retried until the next peer-track). Only the active node answers; only a standby applies, and both sides re-check at apply time (a mid-flight active-flip aborts the wipe) — each side checks the repo-standard **oldest-Up member** active-node predicate (singleton-host semantics via the shared `ActiveNodeEvaluator`, the **same predicate as the S&F delivery gate**; review 02 round 2, **N1 Critical** — using cluster *leadership* here let a rolling restart of the lower-address node make the delivering node wipe its own live buffer). Because replicated applies are **upserts** (see the replication apply path), any Add/Remove/Park that lands after the resync merges cleanly onto the resynced state — no primary-key conflict, no lost delta; the one accepted exception is the rare **N5** orphan-row race (a replicated `Remove` ordered before the snapshot chunks can re-add the removed row, re-delivered once and self-corrected at the next resync — inherent to no-ack replication). If the buffer exceeds the 10 000-row cap the snapshot is flagged `Truncated` and the standby logs a Warning; the residual divergence beyond the cap drains naturally as the active node delivers. The legacy monolithic `SfBufferSnapshot` message + standby handler are **retained** for rolling-upgrade compatibility (an old active node's monolithic snapshot is still applied by a new standby).
 
 ### Operation Tracking Table (lives in Site Runtime, not here)
 

src/ZB.MOM.WW.ScadaBridge.Commons/Observability/ScadaBridgeTelemetry.cs

@@ -47,6 +47,14 @@ public static class ScadaBridgeTelemetry
         Meter.CreateCounter<long>("scadabridge.store_and_forward.replication.failures", unit: "1",
             description: "S&F buffer replication operations that failed to dispatch/deliver to the peer node");
 
+    private static readonly Counter<long> _sfResyncCompleted =
+        Meter.CreateCounter<long>("scadabridge.store_and_forward.resync.completed", unit: "1",
+            description: "S&F anti-entropy resyncs the standby acknowledged as applied");
+
+    private static readonly Counter<long> _sfResyncAckMissing =
+        Meter.CreateCounter<long>("scadabridge.store_and_forward.resync.ack_missing", unit: "1",
+            description: "S&F resyncs answered by the active node but never acknowledged by the standby within the ack window (lost chunks / dead peer)");
+
     /// <summary>
     /// Incremented each time a per-site live-alarm aggregator re-establishes its site-wide
     /// gRPC stream — a NodeA↔NodeB failover flip or a reconcile-driven reopen after the
@@ -107,6 +115,12 @@ public static class ScadaBridgeTelemetry
     /// <summary>Records one failed S&amp;F replication dispatch.</summary>
     public static void RecordReplicationFailure() => _replicationFailures.Add(1);
 
+    /// <summary>Records one acknowledged (applied) S&amp;F buffer resync.</summary>
+    public static void RecordSfResyncCompleted() => _sfResyncCompleted.Add(1);
+
+    /// <summary>Records one resync whose ack window expired.</summary>
+    public static void RecordSfResyncAckMissing() => _sfResyncAckMissing.Add(1);
+
     /// <summary>Records that a site connection opened (increments the up-count gauge).</summary>
     public static void SiteConnectionOpened() => Interlocked.Increment(ref _siteConnectionsUp);
 

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

@@ -52,21 +52,26 @@ public sealed class SiteAlarmAggregatorActor : ReceiveActor, IWithTimers
     private readonly string _grpcNodeAAddress;
     private readonly string _grpcNodeBAddress;
     private readonly TimeSpan _reconcileInterval;
+    private readonly TimeSpan _publishCoalesce;
 
     private const int MaxRetries = 3;
     private const string ReconnectTimerKey = "alarm-grpc-reconnect";
     private const string StabilityTimerKey = "alarm-grpc-stability";
     private const string ReconcileTimerKey = "alarm-reconcile";
+    private const string PublishTimerKey = "alarm-publish-coalesce";
 
-    /// <summary>Delay between gRPC reconnection attempts. Settable for tests.</summary>
-    internal static TimeSpan ReconnectDelay { get; set; } = TimeSpan.FromSeconds(5);
+    /// <summary>True while a coalesced publish is armed (dirty deltas awaiting one tick). Actor-thread only.</summary>
+    private bool _publishPending;
+
+    /// <summary>Delay between gRPC reconnection attempts (ctor-injected; production default 5s).</summary>
+    private readonly TimeSpan _reconnectDelay;
 
     /// <summary>
     /// How long a freshly-opened gRPC stream must stay up before its retry budget is
-    /// considered recovered (mirrors <see cref="DebugStreamBridgeActor.StabilityWindow"/>).
-    /// Settable for tests.
+    /// considered recovered (mirrors <see cref="DebugStreamBridgeActor.StabilityWindow"/>);
+    /// ctor-injected (production default 60s).
     /// </summary>
-    internal static TimeSpan StabilityWindow { get; set; } = TimeSpan.FromSeconds(60);
+    private readonly TimeSpan _stabilityWindow;
 
     private int _retryCount;
     private bool _useNodeA = true;
@@ -94,6 +99,21 @@ public sealed class SiteAlarmAggregatorActor : ReceiveActor, IWithTimers
     /// <summary>Ordered buffer of live deltas that arrived while a fan-out was in flight. Actor-thread only.</summary>
     private readonly List<AlarmStateChanged> _buffer = new();
 
+    /// <summary>
+    /// A failover re-seed was requested while a fan-out was already in flight; it must run
+    /// right after the in-flight one completes rather than being silently dropped (N7.1) —
+    /// the in-flight snapshot's read-time predates the stream death. Actor-thread only.
+    /// </summary>
+    private bool _reseedQueued;
+
+    /// <summary>
+    /// Monotonic stream generation stamped on each opened gRPC stream and echoed back on its
+    /// error callback: a late error raced out of a previous (cancelled) stream carries a stale
+    /// generation and is ignored so it never burns retry budget or double-flips (N7.2).
+    /// Actor-thread only.
+    /// </summary>
+    private int _streamGeneration;
+
     private const int BufferWarnThreshold = 10_000;
     private bool _bufferWarned;
 
@@ -118,6 +138,15 @@ public sealed class SiteAlarmAggregatorActor : ReceiveActor, IWithTimers
     /// <param name="grpcNodeAAddress">gRPC address of the site's node A.</param>
     /// <param name="grpcNodeBAddress">gRPC address of the site's node B.</param>
     /// <param name="reconcileInterval">Periodic reconcile snapshot cadence.</param>
+    /// <param name="publishCoalesce">
+    /// Publish-coalescing window for live deltas: a positive value batches a delta storm
+    /// into one publish per window (review 02 round 2, N6); <see cref="TimeSpan.Zero"/>
+    /// restores per-delta publishing (legacy). Seed/reconcile publishes stay immediate.
+    /// </param>
+    /// <param name="reconnectDelay">Delay between gRPC reconnection attempts (production 5s).</param>
+    /// <param name="stabilityWindow">
+    /// How long a fresh gRPC stream must stay up before its retry budget recovers (production 60s).
+    /// </param>
     public SiteAlarmAggregatorActor(
         string siteIdentifier,
         string correlationId,
@@ -126,7 +155,10 @@ public sealed class SiteAlarmAggregatorActor : ReceiveActor, IWithTimers
         SiteStreamGrpcClientFactory grpcFactory,
         string grpcNodeAAddress,
         string grpcNodeBAddress,
-        TimeSpan reconcileInterval)
+        TimeSpan reconcileInterval,
+        TimeSpan publishCoalesce,
+        TimeSpan reconnectDelay,
+        TimeSpan stabilityWindow)
     {
         _siteIdentifier = siteIdentifier;
         _correlationId = correlationId;
@@ -136,6 +168,9 @@ public sealed class SiteAlarmAggregatorActor : ReceiveActor, IWithTimers
         _grpcNodeAAddress = grpcNodeAAddress;
         _grpcNodeBAddress = grpcNodeBAddress;
         _reconcileInterval = reconcileInterval;
+        _publishCoalesce = publishCoalesce;
+        _reconnectDelay = reconnectDelay;
+        _stabilityWindow = stabilityWindow;
 
         // Live delta from the site-wide alarm stream (marshalled in via Self.Tell).
         // A received delta must NOT reset the retry budget (a flapping stream that
@@ -152,6 +187,13 @@ public sealed class SiteAlarmAggregatorActor : ReceiveActor, IWithTimers
         // Periodic reconcile tick (and the re-seed kicked after a reconnect).
         Receive<RunReconcile>(_ => OnReconcileTick());
 
+        // Coalesced-publish tick: one publish for a batch of dirtying deltas (N6).
+        Receive<PublishCoalesced>(_ =>
+        {
+            _publishPending = false;
+            if (!_stopped) Publish();
+        });
+
         // Stream stayed up for StabilityWindow — recover the retry budget.
         Receive<GrpcAlarmStreamStable>(_ =>
         {
@@ -163,6 +205,15 @@ public sealed class SiteAlarmAggregatorActor : ReceiveActor, IWithTimers
         // gRPC stream error — flip node + reconnect + re-seed.
         Receive<GrpcAlarmStreamError>(msg =>
         {
+            // Ignore a late error raced out of a previous (cancelled) stream — the
+            // RpcException(Cancelled) filter at SiteStreamGrpcClient.cs covers the normal
+            // path, but a genuine socket fault can beat the cancel (N7.2).
+            if (msg.Generation != _streamGeneration)
+            {
+                _log.Debug("Ignoring stale gRPC error from stream generation {0} (current {1})",
+                    msg.Generation, _streamGeneration);
+                return;
+            }
             _log.Warning("Site-alarm gRPC stream error for {0}: {1}", _siteIdentifier, msg.Exception.Message);
             HandleGrpcError();
         });
@@ -247,7 +298,11 @@ public sealed class SiteAlarmAggregatorActor : ReceiveActor, IWithTimers
         if (_stopped) return;
         if (_fanoutInFlight)
         {
-            // Already seeding/reconciling — don't stack a second fan-out.
+            // A failover re-seed requested mid-fan-out must run right after the in-flight
+            // one — its snapshot read-time predates the stream death, so skipping it would
+            // serve stale up to the next 60s reconcile (N7.1). An initial-seed collision
+            // never queues (there is only ever one).
+            if (!isInitial) _reseedQueued = true;
             return;
         }
 
@@ -300,7 +355,18 @@ public sealed class SiteAlarmAggregatorActor : ReceiveActor, IWithTimers
         _log.Debug("Site-alarm {0} {1} complete: {2} alarm row(s)",
             _siteIdentifier, msg.IsInitial ? "seed" : "reconcile", _cache.Count);
 
+        // The fresh snapshot already carries the buffered deltas; drop any armed coalesce
+        // tick so we publish once, immediately.
+        Timers.Cancel(PublishTimerKey);
+        _publishPending = false;
         Publish();
+
+        // A failover re-seed requested while this fan-out was in flight runs now (N7.1).
+        if (_reseedQueued)
+        {
+            _reseedQueued = false;
+            StartFanout(isInitial: false);
+        }
     }
 
     private void OnSeedFailed(SeedFailed msg)
@@ -319,7 +385,18 @@ public sealed class SiteAlarmAggregatorActor : ReceiveActor, IWithTimers
         // Only publish if we already had a seed (so IsLive doesn't flip true on a
         // failed initial seed — the page keeps its poll fallback until we truly seed).
         if (_seeded)
+        {
+            Timers.Cancel(PublishTimerKey);
+            _publishPending = false;
             Publish();
+        }
+
+        // A failover re-seed requested while this fan-out was in flight runs now (N7.1).
+        if (_reseedQueued)
+        {
+            _reseedQueued = false;
+            StartFanout(isInitial: false);
+        }
     }
 
     /// <summary>
@@ -362,9 +439,23 @@ public sealed class SiteAlarmAggregatorActor : ReceiveActor, IWithTimers
             return;
         }
 
-        // Pass-through: apply and publish only if the cache actually changed.
+        // Pass-through: apply and (coalesced) publish only if the cache actually changed.
         if (ApplyDelta(delta, requireStrictlyNewer: false))
-            Publish();
+            SchedulePublish();
+    }
+
+    /// <summary>
+    /// Coalesced publish: with a positive window, the first dirtying delta arms a
+    /// single-shot timer and further deltas ride the same tick — one snapshot copy and
+    /// one viewer fan-out per window instead of per transition (N6). Zero = legacy
+    /// immediate publish. Last write wins, so batching never changes final state.
+    /// </summary>
+    private void SchedulePublish()
+    {
+        if (_publishCoalesce <= TimeSpan.Zero) { Publish(); return; }
+        if (_publishPending) return;
+        _publishPending = true;
+        Timers.StartSingleTimer(PublishTimerKey, new PublishCoalesced(), _publishCoalesce);
     }
 
     /// <summary>
@@ -416,8 +507,9 @@ public sealed class SiteAlarmAggregatorActor : ReceiveActor, IWithTimers
         _grpcCts?.Dispose();
         _grpcCts = new CancellationTokenSource();
 
-        Timers.StartSingleTimer(StabilityTimerKey, new GrpcAlarmStreamStable(), StabilityWindow);
+        Timers.StartSingleTimer(StabilityTimerKey, new GrpcAlarmStreamStable(), _stabilityWindow);
 
+        var generation = ++_streamGeneration;
         var client = _grpcFactory.GetOrCreate(_siteIdentifier, endpoint);
         var self = Self;
         var ct = _grpcCts.Token;
@@ -427,7 +519,7 @@ public sealed class SiteAlarmAggregatorActor : ReceiveActor, IWithTimers
             await client.SubscribeSiteAsync(
                 _correlationId,
                 alarm => self.Tell(alarm),
-                ex => self.Tell(new GrpcAlarmStreamError(ex)),
+                ex => self.Tell(new GrpcAlarmStreamError(ex, generation)),
                 ct);
         }, ct);
     }
@@ -474,7 +566,7 @@ public sealed class SiteAlarmAggregatorActor : ReceiveActor, IWithTimers
         if (_retryCount == 1)
             Self.Tell(new ReconnectAlarmStream());
         else
-            Timers.StartSingleTimer(ReconnectTimerKey, new ReconnectAlarmStream(), ReconnectDelay);
+            Timers.StartSingleTimer(ReconnectTimerKey, new ReconnectAlarmStream(), _reconnectDelay);
     }
 
     private void CleanupGrpc()
@@ -520,8 +612,12 @@ internal sealed record SeedFailed(Exception Exception, bool IsInitial);
 /// <summary>Internal: periodic reconcile tick (and the re-seed kicked after a reconnect).</summary>
 internal sealed record RunReconcile;
 
-/// <summary>Internal: site-alarm gRPC stream error occurred.</summary>
-internal sealed record GrpcAlarmStreamError(Exception Exception);
+/// <summary>Internal: coalesced-publish tick — flush the dirty cache to viewers once (N6).</summary>
+internal sealed record PublishCoalesced;
+
+/// <summary>Internal: site-alarm gRPC stream error occurred, stamped with the stream
+/// generation it came from so a late error from a cancelled stream can be ignored (N7.2).</summary>
+internal sealed record GrpcAlarmStreamError(Exception Exception, int Generation);
 
 /// <summary>Internal: reconnect the site-alarm gRPC stream (flip node).</summary>
 internal sealed record ReconnectAlarmStream;

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

@@ -67,10 +67,10 @@ public class SiteCommunicationActor : ReceiveActor, IWithTimers
     /// <param name="deploymentManagerProxy">Local reference to the Deployment Manager singleton proxy.</param>
     /// <param name="isActiveCheck">
     /// Optional override returning <c>true</c> when this node
-    /// is the active member of the site cluster. <c>null</c> uses the real
-    /// Akka <see cref="Cluster"/> leader check (the default for production
-    /// wiring); tests pass a stub so they do not need to load Akka.Cluster
-    /// into the <c>TestKit</c> ActorSystem.
+    /// is the active member of the site cluster. <c>null</c> uses the shared oldest-Up
+    /// evaluator (production wiring passes the Host's singleton-host delegate); tests
+    /// pass a stub so they do not need to load Akka.Cluster into the <c>TestKit</c>
+    /// ActorSystem.
     /// </param>
     public SiteCommunicationActor(
         string siteId,
@@ -506,24 +506,16 @@ public class SiteCommunicationActor : ReceiveActor, IWithTimers
     }
 
     /// <summary>
-    /// Default active-node check used when no override is
-    /// supplied. Mirrors <c>ActiveNodeGate</c> in the Host (and
-    /// <c>ActiveNodeHealthCheck</c>): the node is the active member of the
-    /// site cluster when it is the current cluster leader AND its own
-    /// <see cref="MemberStatus"/> is <see cref="MemberStatus.Up"/>. Any other
-    /// state (still joining, leaving, no leader yet) reports standby —
-    /// safe-by-default, matching the standby case.
+    /// Default active-node check used when no override is supplied: oldest-Up member
+    /// semantics via the shared <see cref="ClusterState.ActiveNodeEvaluator"/> — the
+    /// same predicate as the S&F delivery gate and the replication resync authority
+    /// (review 02 round 2, N1). Unscoped by role: a site cluster's members all carry
+    /// the site role, so role scoping is a no-op here; production wiring passes the
+    /// Host's role-scoped IClusterNodeProvider delegate anyway. Any other state
+    /// (still joining, leaving) reports standby — safe-by-default.
     /// </summary>
-    private bool DefaultIsActiveCheck()
-    {
-        var cluster = Cluster.Get(Context.System);
-        var self = cluster.SelfMember;
-        if (self.Status != MemberStatus.Up)
-            return false;
-
-        var leader = cluster.State.Leader;
-        return leader != null && leader == self.Address;
-    }
+    private bool DefaultIsActiveCheck() =>
+        ClusterState.ActiveNodeEvaluator.SelfIsOldestUp(Cluster.Get(Context.System));
 
     // ── Internal messages ──
 

src/ZB.MOM.WW.ScadaBridge.Communication/ClusterState/ActiveNodeEvaluator.cs

@@ -0,0 +1,39 @@
+using Akka.Cluster;
+
+namespace ZB.MOM.WW.ScadaBridge.Communication.ClusterState;
+
+/// <summary>
+/// THE single definition of "active node" (review 01 [High]; review 02 round 2 [Critical] N1):
+/// a node is active when it is the OLDEST Up member (optionally within a role scope) — i.e.
+/// the member the ClusterSingletonManager places singletons on. Cluster LEADERSHIP (lowest
+/// address) is an Akka-internal concept that diverges from singleton placement permanently
+/// once the original first node restarts and rejoins; every product-level active/standby
+/// decision must use this evaluator, never <c>cluster.State.Leader</c>.
+/// <para>
+/// Lives in Communication (not Host) so BOTH <c>SiteCommunicationActor</c> and
+/// <c>SiteReplicationActor</c> can default to it — Host cannot be referenced from either.
+/// The Host's <c>ClusterActivityEvaluator.SelfIsOldest</c> delegates here, so the S&amp;F
+/// delivery gate (<c>IClusterNodeProvider.SelfIsPrimary</c>), the resync authority checks,
+/// and the heartbeat IsActive stamp all share one implementation.
+/// </para>
+/// </summary>
+public static class ActiveNodeEvaluator
+{
+    /// <summary>True when self is Up and no other Up member (in the role scope) is older.</summary>
+    /// <param name="cluster">The Akka cluster to evaluate.</param>
+    /// <param name="role">Optional role scope; when set, only members with this role are considered.</param>
+    /// <returns><c>true</c> when self is Up and the oldest Up member in the role scope.</returns>
+    public static bool SelfIsOldestUp(Cluster cluster, string? role = null)
+    {
+        var self = cluster.SelfMember;
+        if (self.Status != MemberStatus.Up)
+            return false;
+        if (role != null && !self.HasRole(role))
+            return false;
+
+        return cluster.State.Members
+            .Where(m => m.Status == MemberStatus.Up)
+            .Where(m => role == null || m.HasRole(role))
+            .All(m => m.UniqueAddress.Equals(self.UniqueAddress) || self.IsOlderThan(m));
+    }
+}

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

@@ -122,4 +122,13 @@ public class CommunicationOptions
     /// count; this just bounds the subscriber list. Default 200.
     /// </summary>
     public int LiveAlarmCacheMaxSubscribersPerSite { get; set; } = 200;
+
+    /// <summary>
+    /// Publish-coalescing window for live alarm deltas: an applied delta marks the cache
+    /// dirty and one publish (fresh snapshot + per-viewer onChanged fan-out) fires after
+    /// this window, batching an alarm storm into ~4 publishes/second instead of one per
+    /// transition (review 02 round 2, N6). Zero = publish per delta (legacy). Seed and
+    /// reconcile publishes are always immediate. Default 250 ms.
+    /// </summary>
+    public TimeSpan LiveAlarmCachePublishCoalesce { get; set; } = TimeSpan.FromMilliseconds(250);
 }

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

@@ -85,5 +85,10 @@ public sealed class CommunicationOptionsValidator : OptionsValidatorBase<Communi
         // Per-site viewer cap must admit at least one viewer, else the page could never go live.
         builder.RequireThat(options.LiveAlarmCacheMaxSubscribersPerSite >= 1,
             $"Communication:LiveAlarmCacheMaxSubscribersPerSite must be at least 1 (was {options.LiveAlarmCacheMaxSubscribersPerSite}).");
+
+        // Publish-coalescing window drives a single-shot timer; TimeSpan.Zero is valid
+        // (publish per delta — legacy), only a negative value is invalid.
+        builder.RequireThat(options.LiveAlarmCachePublishCoalesce >= TimeSpan.Zero,
+            $"Communication:LiveAlarmCachePublishCoalesce must be zero or a positive duration (was {options.LiveAlarmCachePublishCoalesce}).");
     }
 }

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

@@ -11,9 +11,12 @@ namespace ZB.MOM.WW.ScadaBridge.Communication;
 /// reflects alarm transitions in near-real-time instead of re-polling every 15s.
 /// <para>
 /// <b>Hard constraint (locked <c>[PERM]</c>):</b> there is NO persisted central alarm
-/// store. This cache is purely in-memory and lives only on the active central node —
-/// no EF entity/table/migration backs it. On a NodeA↔NodeB failover the new active
-/// node re-seeds from scratch.
+/// store. This cache is purely in-memory, per-node — no EF entity/table/migration backs
+/// it. In routine operation only the active node (behind Traefik) hosts viewers, so only
+/// it runs aggregators; browsing the standby node directly (diagnostic ports) starts an
+/// independent read-only aggregator there — accepted (arch review 02 round 2, N8):
+/// bounded (one stream/site, viewer-capped), read-only, and torn down by the viewer
+/// linger. On a NodeA↔NodeB failover the new active node re-seeds from scratch.
 /// </para>
 /// <para>
 /// <b>Reference-counted lifecycle:</b> the first <see cref="Subscribe"/> for a site

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

@@ -233,7 +233,10 @@ public sealed class SiteAlarmLiveCacheService : ISiteAlarmLiveCache
                     _grpcFactory,
                     grpcA,
                     grpcB,
-                    _options.LiveAlarmCacheReconcileInterval));
+                    _options.LiveAlarmCacheReconcileInterval,
+                    _options.LiveAlarmCachePublishCoalesce,
+                    TimeSpan.FromSeconds(5),    // reconnect delay — former ReconnectDelay static default
+                    TimeSpan.FromSeconds(60))); // stability window — former StabilityWindow static default
 
                 entry.Actor = system.ActorOf(props, $"site-alarm-aggregator-{entry.SiteId}-{Guid.NewGuid():N}");
                 entry.StartRetryTimer?.Dispose();
@@ -291,7 +294,9 @@ public sealed class SiteAlarmLiveCacheService : ISiteAlarmLiveCache
             return null;
         }
 
-        if (string.IsNullOrWhiteSpace(site.GrpcNodeAAddress) || string.IsNullOrWhiteSpace(site.GrpcNodeBAddress))
+        var nodeA = site.GrpcNodeAAddress;
+        var nodeB = site.GrpcNodeBAddress;
+        if (string.IsNullOrWhiteSpace(nodeA) && string.IsNullOrWhiteSpace(nodeB))
         {
             _logger.LogWarning(
                 "Live alarm cache: site {SiteId} ({SiteIdentifier}) has no gRPC node addresses; " +
@@ -299,7 +304,13 @@ public sealed class SiteAlarmLiveCacheService : ISiteAlarmLiveCache
             return null;
         }
 
-        return (site.SiteIdentifier, site.GrpcNodeAAddress!, site.GrpcNodeBAddress!);
+        // Single-endpoint site (review 02 round 2, N9): reuse the sole configured
+        // endpoint for both slots — the aggregator's NodeA↔NodeB failover flip
+        // degenerates to a reconnect against the same node, which is exactly the
+        // right behavior for a one-node site.
+        var grpcA = !string.IsNullOrWhiteSpace(nodeA) ? nodeA! : nodeB!;
+        var grpcB = !string.IsNullOrWhiteSpace(nodeB) ? nodeB! : nodeA!;
+        return (site.SiteIdentifier, grpcA, grpcB);
     }
 
     // ── Snapshot fan-out (the seed / reconcile) ─────────────────────────────────

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

@@ -772,10 +772,20 @@ akka {{
         var replicationLogger = _serviceProvider.GetRequiredService<ILoggerFactory>()
             .CreateLogger<SiteReplicationActor>();
 
+        // ONE active-node predicate instance governs the S&F delivery gate, the resync
+        // authority checks (SiteReplicationActor), and the heartbeat IsActive stamp
+        // (SiteCommunicationActor, wired below) — review 02 round 2, N1. Null in
+        // non-clustered test hosts: the actors fall back to the shared oldest-Up
+        // evaluator, never to a leader check.
+        var clusterNodeProvider = _serviceProvider.GetService<ZB.MOM.WW.ScadaBridge.HealthMonitoring.IClusterNodeProvider>();
+        Func<bool>? activeNodeCheck = clusterNodeProvider != null
+            ? () => clusterNodeProvider.SelfIsPrimary
+            : null;
+
         var replicationActor = _actorSystem!.ActorOf(
             Props.Create(() => new SiteReplicationActor(
                 storage, sfStorage, replicationService, siteRole, replicationLogger,
-                deploymentConfigFetcher, null, siteRuntimeOptionsValue, null)),
+                deploymentConfigFetcher, activeNodeCheck, siteRuntimeOptionsValue, null)),
             "site-replication");
 
         // Wire S&F replication handler to forward operations via the replication actor
@@ -823,7 +833,8 @@ akka {{
             Props.Create(() => new SiteCommunicationActor(
                 _nodeOptions.SiteId!,
                 _communicationOptions,
-                dmProxy)),
+                dmProxy,
+                activeNodeCheck)),
             "site-communication");
 
         // Register local handlers with SiteCommunicationActor
@@ -872,10 +883,11 @@ akka {{
             // tick so failover resumes delivery within one RetryTimerInterval.
             // IClusterNodeProvider.SelfIsPrimary is the canonical "this node is the
             // oldest Up member (singleton host)" check from the cluster-infrastructure
-            // fix plan — the shared helper this seam was designed to accept. In a
-            // non-clustered test host the provider is unregistered, so the gate stays
-            // unset and the sweep is ungated (legacy behaviour, preserved).
-            var clusterNodeProvider = _serviceProvider.GetService<ZB.MOM.WW.ScadaBridge.HealthMonitoring.IClusterNodeProvider>();
+            // fix plan — the shared helper this seam was designed to accept. The provider
+            // is resolved once above (the same instance gating the resync authority and
+            // heartbeat IsActive stamp — N1). In a non-clustered test host the provider is
+            // unregistered, so the gate stays unset and the sweep is ungated (legacy
+            // behaviour, preserved).
             if (clusterNodeProvider != null)
             {
                 storeAndForwardService.SetDeliveryGate(() => clusterNodeProvider.SelfIsPrimary);

src/ZB.MOM.WW.ScadaBridge.Host/Health/ClusterActivityEvaluator.cs

@@ -13,23 +13,15 @@ namespace ZB.MOM.WW.ScadaBridge.Host.Health;
 /// </summary>
 public static class ClusterActivityEvaluator
 {
-    /// <summary>True when self is Up and no other Up member (in the role scope) is older.</summary>
+    /// <summary>True when self is Up and no other Up member (in the role scope) is older.
+    /// Delegates to the shared <see cref="Communication.ClusterState.ActiveNodeEvaluator"/> —
+    /// one implementation for the delivery gate, the resync authority checks, and the
+    /// heartbeat IsActive stamp (review 02 round 2, N1).</summary>
     /// <param name="cluster">The Akka cluster to evaluate.</param>
     /// <param name="role">Optional role scope; when set, only members with this role are considered.</param>
     /// <returns><c>true</c> when self is Up and the oldest Up member in the role scope.</returns>
-    public static bool SelfIsOldest(Cluster cluster, string? role = null)
-    {
-        var self = cluster.SelfMember;
-        if (self.Status != MemberStatus.Up)
-            return false;
-        if (role != null && !self.HasRole(role))
-            return false;
-
-        return cluster.State.Members
-            .Where(m => m.Status == MemberStatus.Up)
-            .Where(m => role == null || m.HasRole(role))
-            .All(m => m.UniqueAddress.Equals(self.UniqueAddress) || self.IsOlderThan(m));
-    }
+    public static bool SelfIsOldest(Cluster cluster, string? role = null) =>
+        Communication.ClusterState.ActiveNodeEvaluator.SelfIsOldestUp(cluster, role);
 
     /// <summary>The oldest Up member in the role scope, or null while none is Up. Used for Primary/Standby labelling.</summary>
     /// <param name="cluster">The Akka cluster to evaluate.</param>

src/ZB.MOM.WW.ScadaBridge.ManagementService/ManagementActor.cs

@@ -1583,6 +1583,18 @@ public class ManagementActor : ReceiveActor
         await repo.SaveChangesAsync();
         var commService = sp.GetService<CommunicationService>();
         commService?.RefreshSiteAddresses();
+
+        // Dispose the deleted site's cached gRPC channels — RemoveSiteAsync is the
+        // factory's designed site-deletion disposal path and previously had no
+        // production caller (arch review 02 round 2, N8): a deleted site's channels
+        // (both node endpoints) otherwise persist until process restart. Null-safe:
+        // test/composition roots without the factory skip it. Any live-alarm
+        // aggregator for the site reconciles to an empty snapshot and is reaped by
+        // the viewer linger (documented acceptance — see Component-Communication.md).
+        var grpcFactory = sp.GetService<ZB.MOM.WW.ScadaBridge.Communication.Grpc.SiteStreamGrpcClientFactory>();
+        if (grpcFactory is not null && site is not null)
+            await grpcFactory.RemoveSiteAsync(site.SiteIdentifier);
+
         await AuditAsync(sp, user, "Delete", "Site", cmd.SiteId.ToString(), site?.Name ?? cmd.SiteId.ToString(), null);
         return true;
     }

src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Actors/SiteReplicationActor.cs

@@ -18,7 +18,7 @@ namespace ZB.MOM.WW.ScadaBridge.SiteRuntime.Actors;
 /// Inbound: receives replicated operations from peer and applies to local SQLite.
 /// Uses fire-and-forget (Tell) — no ack wait per design.
 /// </summary>
-public class SiteReplicationActor : ReceiveActor
+public class SiteReplicationActor : ReceiveActor, IWithTimers
 {
     private readonly SiteStorageService _storage;
     private readonly StoreAndForwardStorage _sfStorage;
@@ -32,6 +32,30 @@ public class SiteReplicationActor : ReceiveActor
     private readonly TimeSpan _configFetchRetryDelay;
     private Address? _peerAddress;
 
+    /// <summary>Akka timer scheduler injected by the framework via <see cref="IWithTimers"/>.</summary>
+    public ITimerScheduler Timers { get; set; } = null!;
+
+    // ── Chunked-resync assembly (standby side; actor-thread only) ──
+    private string? _assemblingResyncId;
+    private int _assemblingTotalChunks;
+    private bool _assemblingTruncated;
+    private readonly Dictionary<int, List<StoreAndForwardMessage>> _assemblingChunks = new();
+    private const string ResyncAssemblyTimerKey = "sf-resync-assembly-timeout";
+
+    /// <summary>How long a partial chunk assembly may wait for its missing chunks before
+    /// being discarded (a lost chunk = lost resync; the next peer-track retries). Ctor
+    /// test seam; production default 30 s.</summary>
+    private readonly TimeSpan _resyncAssemblyTimeout;
+
+    // ── Resync delivery confirmation (active side; actor-thread only) ──
+    private string? _pendingAckResyncId;
+    private const string ResyncAckTimerKey = "sf-resync-ack-timeout";
+
+    /// <summary>How long the active node waits for the standby's <see cref="SfBufferResyncAck"/>
+    /// before warning + counting the resync as unacknowledged (lost chunks / dead peer). Ctor
+    /// test seam; production default 60 s.</summary>
+    private readonly TimeSpan _resyncAckTimeout;
+
     /// <summary>
     /// Maximum rows an active node returns in a single anti-entropy resync snapshot.
     /// A standby whose buffer exceeded this (Truncated snapshot) resyncs the oldest
@@ -39,6 +63,48 @@ public class SiteReplicationActor : ReceiveActor
     /// </summary>
     private const int MaxResyncRows = 10_000;
 
+    /// <summary>
+    /// Estimated per-chunk payload budget for a resync snapshot. Akka remoting's default
+    /// <c>maximum-frame-size</c> is 128 000 bytes and <c>BuildHocon</c> sets no override,
+    /// so the monolithic <see cref="SfBufferSnapshot"/> is silently undeliverable for any
+    /// realistic backlog (review 02 round 2, N2). 64 000 bytes leaves ≈50% headroom for
+    /// the JSON envelope, CLR type manifests, and the non-payload columns.
+    /// </summary>
+    internal const int MaxResyncChunkBytes = 64_000;
+
+    /// <summary>Row cap per resync chunk (bounds a chunk even when every row is tiny).</summary>
+    internal const int MaxResyncChunkRows = 200;
+
+    /// <summary>
+    /// Splits a resync snapshot into chunks that fit Akka remoting's default
+    /// 128 000-byte frame (review 02 round 2, N2): rows accumulate until the estimated
+    /// payload budget or the row cap is hit. Estimation is payload-dominated
+    /// (payload_json length + 512 bytes fixed overhead per row); a single row whose
+    /// payload exceeds the budget ships alone (Warning at the call site). Order is
+    /// preserved (oldest-first, matching GetAllMessagesAsync).
+    /// </summary>
+    internal static List<List<StoreAndForwardMessage>> ChunkForRemoting(
+        IReadOnlyList<StoreAndForwardMessage> rows, int maxChunkBytes, int maxChunkRows)
+    {
+        var chunks = new List<List<StoreAndForwardMessage>>();
+        var current = new List<StoreAndForwardMessage>();
+        var currentBytes = 0;
+        foreach (var row in rows)
+        {
+            var estimate = (row.PayloadJson?.Length ?? 0) + 512;
+            if (current.Count > 0 && (currentBytes + estimate > maxChunkBytes || current.Count >= maxChunkRows))
+            {
+                chunks.Add(current);
+                current = new List<StoreAndForwardMessage>();
+                currentBytes = 0;
+            }
+            current.Add(row);
+            currentBytes += estimate;
+        }
+        if (current.Count > 0) chunks.Add(current);
+        return chunks;
+    }
+
     /// <summary>
     /// Initializes a new <see cref="SiteReplicationActor"/> and registers Akka message handlers.
     /// </summary>
@@ -54,10 +120,11 @@ public class SiteReplicationActor : ReceiveActor
     /// config never crosses the intra-site Akka hop. Null on nodes/tests without a fetcher.
     /// </param>
     /// <param name="isActiveOverride">
-    /// Test seam for the active-node check that gates the buffer-resync roles (a
-    /// standby requests a resync, the active node answers). Null (production) uses
-    /// the repo-standard leader+Up check via <see cref="Cluster"/> — swap point for
-    /// plan 01's shared active-node helper.
+    /// Active-node check that gates the buffer-resync roles (a standby requests a
+    /// resync, the active node answers). Production wiring passes the Host's
+    /// <c>IClusterNodeProvider.SelfIsPrimary</c> delegate (the same instance gating the
+    /// S&amp;F delivery sweep); null falls back to the shared oldest-Up evaluator
+    /// (<see cref="Communication.ClusterState.ActiveNodeEvaluator"/>).
     /// </param>
     /// <param name="options">Site runtime options, including the config-fetch retry count; production defaults apply when null.</param>
     /// <param name="configFetchRetryDelay">Delay between config-fetch retry attempts; defaults to 2 seconds when null.</param>
@@ -70,7 +137,9 @@ public class SiteReplicationActor : ReceiveActor
         IDeploymentConfigFetcher? configFetcher = null,
         Func<bool>? isActiveOverride = null,
         SiteRuntimeOptions? options = null,
-        TimeSpan? configFetchRetryDelay = null)
+        TimeSpan? configFetchRetryDelay = null,
+        TimeSpan? resyncAssemblyTimeout = null,
+        TimeSpan? resyncAckTimeout = null)
     {
         _storage = storage;
         _sfStorage = sfStorage;
@@ -80,6 +149,8 @@ public class SiteReplicationActor : ReceiveActor
         _logger = logger;
         _cluster = Cluster.Get(Context.System);
         _isActive = isActiveOverride ?? DefaultIsActive;
+        _resyncAssemblyTimeout = resyncAssemblyTimeout ?? TimeSpan.FromSeconds(30);
+        _resyncAckTimeout = resyncAckTimeout ?? TimeSpan.FromSeconds(60);
         // UA2: bound the standby's replicated-config fetch retries. At least one
         // attempt always runs; the fixed inter-attempt delay is a test seam
         // (production default 2 s).
@@ -110,7 +181,30 @@ public class SiteReplicationActor : ReceiveActor
 
         // Anti-entropy — full S&F buffer resync on peer (re)join
         Receive<RequestSfBufferResync>(HandleRequestSfBufferResync);
-        Receive<SfBufferSnapshot>(HandleSfBufferSnapshot);
+        Receive<SfResyncSnapshotLoaded>(HandleSfResyncSnapshotLoaded);
+        Receive<SfBufferSnapshotChunk>(HandleSfBufferSnapshotChunk);
+        Receive<ResyncAssemblyTimedOut>(HandleResyncAssemblyTimedOut);
+        Receive<SfBufferResyncAck>(msg =>
+        {
+            if (msg.ResyncId == _pendingAckResyncId)
+            {
+                _pendingAckResyncId = null;
+                Timers.Cancel(ResyncAckTimerKey);
+            }
+            ScadaBridgeTelemetry.RecordSfResyncCompleted();
+            _logger.LogInformation("S&F resync {ResyncId} acknowledged by standby: {Rows} row(s) applied",
+                msg.ResyncId, msg.RowCount);
+        });
+        Receive<ResyncAckTimedOut>(msg =>
+        {
+            if (msg.ResyncId != _pendingAckResyncId) return;
+            _pendingAckResyncId = null;
+            ScadaBridgeTelemetry.RecordSfResyncAckMissing();
+            _logger.LogWarning(
+                "S&F resync {ResyncId} was never acknowledged within {Window} — snapshot chunks may have been lost (frame drop / dead peer); the next peer-track retries",
+                msg.ResyncId, _resyncAckTimeout);
+        });
+        Receive<SfBufferSnapshot>(HandleSfBufferSnapshot); // legacy monolithic handler — retained for rolling compat
     }
 
     /// <inheritdoc />
@@ -181,21 +275,17 @@ public class SiteReplicationActor : ReceiveActor
     }
 
     /// <summary>
-    /// Repo-standard active-node check: this node is active when it is the current
-    /// cluster leader AND its own <see cref="MemberStatus"/> is
-    /// <see cref="MemberStatus.Up"/>. Mirrors <c>SiteCommunicationActor.DefaultIsActiveCheck</c>
-    /// (swap point for plan 01's shared helper). Any other state reports standby —
-    /// safe-by-default.
+    /// Repo-standard active-node check: this node is active when it is the OLDEST Up
+    /// member carrying the site role — the same oldest-Up semantics as the S&F delivery
+    /// gate (IClusterNodeProvider.SelfIsPrimary → ClusterActivityEvaluator → shared
+    /// ActiveNodeEvaluator). NEVER the cluster leader: leadership is lowest-address and
+    /// diverges from singleton/delivery placement permanently after the lower-address
+    /// node restarts — the divergence that made the delivering node wipe its own live
+    /// buffer via a wrong-direction resync (review 02 round 2, N1 Critical). Any other
+    /// state reports standby — safe-by-default.
     /// </summary>
-    private bool DefaultIsActive()
-    {
-        var self = _cluster.SelfMember;
-        if (self.Status != MemberStatus.Up)
-            return false;
-
-        var leader = _cluster.State.Leader;
-        return leader != null && leader == self.Address;
-    }
+    private bool DefaultIsActive() =>
+        Communication.ClusterState.ActiveNodeEvaluator.SelfIsOldestUp(_cluster, _siteRole);
 
     /// <summary>
     /// Evaluates the active-node check, treating a throwing check as standby
@@ -391,9 +481,10 @@ public class SiteReplicationActor : ReceiveActor
 
     /// <summary>
     /// Active-node side of the anti-entropy resync: answers a standby's
-    /// <see cref="RequestSfBufferResync"/> with a full-buffer <see cref="SfBufferSnapshot"/>
-    /// (up to <see cref="MaxResyncRows"/> oldest rows). A non-active node ignores the
-    /// request — only the authoritative node may answer.
+    /// <see cref="RequestSfBufferResync"/> with a sequence of byte-budgeted
+    /// <see cref="SfBufferSnapshotChunk"/>s (up to <see cref="MaxResyncRows"/> oldest rows).
+    /// A non-active node ignores the request — only the authoritative node may answer.
+    /// The snapshot is piped back to Self so chunking + ack bookkeeping stays actor-safe.
     /// </summary>
     private void HandleRequestSfBufferResync(RequestSfBufferResync msg)
     {
@@ -405,9 +496,37 @@ public class SiteReplicationActor : ReceiveActor
 
         var replyTo = Sender;
         _sfStorage.GetAllMessagesAsync(MaxResyncRows).PipeTo(
-            replyTo,
             Self,
-            success: result => new SfBufferSnapshot(result.Messages, result.Truncated));
+            failure: ex => new Status.Failure(ex),
+            success: result => new SfResyncSnapshotLoaded(replyTo, result.Messages, result.Truncated));
+    }
+
+    /// <summary>
+    /// Active-node continuation: the resync snapshot finished loading; chunk it to fit the
+    /// remoting frame and send the sequenced chunks to the requester (all sharing one
+    /// resyncId). Task 7 arms the ack-timeout here.
+    /// </summary>
+    private void HandleSfResyncSnapshotLoaded(SfResyncSnapshotLoaded msg)
+    {
+        var chunks = ChunkForRemoting(msg.Messages, MaxResyncChunkBytes, MaxResyncChunkRows);
+        if (chunks.Count == 0) chunks.Add(new List<StoreAndForwardMessage>()); // empty buffer still resyncs (clears the standby)
+        var resyncId = Guid.NewGuid().ToString("N");
+        for (var i = 0; i < chunks.Count; i++)
+        {
+            if (chunks[i].Count == 1 && (chunks[i][0].PayloadJson?.Length ?? 0) + 512 > MaxResyncChunkBytes)
+                _logger.LogWarning(
+                    "Resync row {Id} alone exceeds the chunk budget ({Bytes}B payload); sending solo — it may exceed the remoting frame",
+                    chunks[i][0].Id, chunks[i][0].PayloadJson?.Length ?? 0);
+            msg.ReplyTo.Tell(new SfBufferSnapshotChunk(resyncId, i + 1, chunks.Count, chunks[i], msg.Truncated), Self);
+        }
+        _logger.LogInformation(
+            "Answered S&F resync request with {Rows} row(s) in {Chunks} chunk(s), resyncId={ResyncId}",
+            msg.Messages.Count, chunks.Count, resyncId);
+        // Arm the ack window: absence of an SfBufferResyncAck within it surfaces as a
+        // Warning + counter (the silent-loss mode N2 flagged). Single-outstanding-resync
+        // bookkeeping: a new request supersedes by overwriting the id and restarting the timer.
+        _pendingAckResyncId = resyncId;
+        Timers.StartSingleTimer(ResyncAckTimerKey, new ResyncAckTimedOut(resyncId), _resyncAckTimeout);
     }
 
     /// <summary>
@@ -435,13 +554,120 @@ public class SiteReplicationActor : ReceiveActor
         _logger.LogInformation(
             "Applying S&F buffer resync snapshot ({Count} rows), replacing local buffer", msg.Messages.Count);
 
-        _sfStorage.ReplaceAllAsync(msg.Messages)
+        Task.Run(async () =>
+            {
+                // Belt-and-braces (N1): re-check at apply time. ReplaceAllAsync discards
+                // every in-flight row (StoreAndForwardStorage.cs "Never call on an active
+                // node"); a flip between message receipt and this point must abort.
+                if (SafeIsActive())
+                {
+                    _logger.LogWarning(
+                        "Discarding S&F buffer resync snapshot: this node became active before apply");
+                    return;
+                }
+                await _sfStorage.ReplaceAllAsync(msg.Messages);
+            })
             .ContinueWith(t =>
             {
                 if (t.IsFaulted)
                     _logger.LogError(t.Exception, "Failed to apply S&F buffer resync snapshot");
             });
     }
+
+    /// <summary>
+    /// Standby-node side of the chunked anti-entropy resync: accumulates the chunks of one
+    /// <c>ResyncId</c>, and once all have arrived, assembles them in sequence order and
+    /// replaces the local buffer atomically, then acks. A new <c>ResyncId</c> discards any
+    /// stale partial assembly (review 02 round 2, N2). An active node ignores chunks.
+    /// </summary>
+    private void HandleSfBufferSnapshotChunk(SfBufferSnapshotChunk msg)
+    {
+        if (SafeIsActive())
+        {
+            _logger.LogDebug("Ignoring S&F resync chunk — this node is active");
+            return;
+        }
+
+        if (_assemblingResyncId != msg.ResyncId)
+        {
+            if (_assemblingResyncId != null)
+                _logger.LogWarning(
+                    "Discarding partial S&F resync assembly {Old} ({Have}/{Want} chunks): a new resync {New} superseded it",
+                    _assemblingResyncId, _assemblingChunks.Count, _assemblingTotalChunks, msg.ResyncId);
+            _assemblingResyncId = msg.ResyncId;
+            _assemblingTotalChunks = msg.TotalChunks;
+            _assemblingTruncated = msg.Truncated;
+            _assemblingChunks.Clear();
+        }
+
+        _assemblingChunks[msg.Sequence] = msg.Messages;
+        Timers.StartSingleTimer(ResyncAssemblyTimerKey, new ResyncAssemblyTimedOut(msg.ResyncId), _resyncAssemblyTimeout);
+
+        if (_assemblingChunks.Count < _assemblingTotalChunks)
+            return;
+
+        // Complete: assemble in sequence order and apply atomically.
+        var assembled = Enumerable.Range(1, _assemblingTotalChunks)
+            .SelectMany(seq => _assemblingChunks[seq])
+            .ToList();
+        var resyncId = _assemblingResyncId!;
+        var truncated = _assemblingTruncated;
+        _assemblingResyncId = null;
+        _assemblingChunks.Clear();
+        Timers.Cancel(ResyncAssemblyTimerKey);
+
+        if (truncated)
+            _logger.LogWarning(
+                "S&F buffer resync snapshot truncated at {Cap} rows; divergence beyond the cap drains naturally",
+                MaxResyncRows);
+        _logger.LogInformation(
+            "Applying chunked S&F resync {ResyncId} ({Count} rows), replacing local buffer", resyncId, assembled.Count);
+
+        // KNOWN, ACCEPTED race (review 02 round 2, N5 — do NOT "fix" this into something
+        // worse): a replicated Remove sent after the active node read its snapshot but
+        // before the snapshot's chunks is ordered BEFORE them on the wire (same
+        // sender/receiver pair), so this apply can re-add the removed row → an orphan
+        // Pending row that a later failover re-delivers ONCE. Bounded, self-correcting at
+        // the next resync, and inherent to no-ack replication; a delivered-side dedup or
+        // op-sequencing scheme would cost far more than one rare duplicate.
+        var replyTo = Sender;
+        Task.Run(async () =>
+            {
+                if (SafeIsActive()) // belt-and-braces, mirrors the monolithic path (T3)
+                {
+                    _logger.LogWarning("Discarding chunked S&F resync {ResyncId}: node became active before apply", resyncId);
+                    return;
+                }
+                await _sfStorage.ReplaceAllAsync(assembled);
+                replyTo.Tell(new SfBufferResyncAck(resyncId, assembled.Count));
+            })
+            .ContinueWith(t =>
+            {
+                if (t.IsFaulted)
+                    _logger.LogError(t.Exception, "Failed to apply chunked S&F resync {ResyncId}", resyncId);
+            });
+    }
+
+    private void HandleResyncAssemblyTimedOut(ResyncAssemblyTimedOut msg)
+    {
+        if (_assemblingResyncId != msg.ResyncId) return; // superseded already
+        _logger.LogWarning(
+            "S&F resync assembly {ResyncId} timed out with {Have}/{Want} chunks — discarding partial (a lost chunk; the next peer-track retries)",
+            msg.ResyncId, _assemblingChunks.Count, _assemblingTotalChunks);
+        ScadaBridgeTelemetry.RecordReplicationFailure();
+        _assemblingResyncId = null;
+        _assemblingChunks.Clear();
+    }
+
+    /// <summary>Internal: the resync snapshot finished loading; chunk and send to the requester.</summary>
+    internal sealed record SfResyncSnapshotLoaded(
+        IActorRef ReplyTo, List<StoreAndForwardMessage> Messages, bool Truncated);
+
+    /// <summary>Internal: a partial chunk assembly for <paramref name="ResyncId"/> exceeded its window.</summary>
+    internal sealed record ResyncAssemblyTimedOut(string ResyncId);
+
+    /// <summary>Internal: the active node's ack window for <paramref name="ResyncId"/> expired.</summary>
+    internal sealed record ResyncAckTimedOut(string ResyncId);
 }
 
 /// <summary>
@@ -458,3 +684,24 @@ public sealed record RequestSfBufferResync;
 /// Akka remoting; the message list rides the default serializer.
 /// </summary>
 public sealed record SfBufferSnapshot(List<StoreAndForwardMessage> Messages, bool Truncated);
+
+/// <summary>
+/// Active→standby: one sequenced chunk of a full-buffer anti-entropy snapshot
+/// (review 02 round 2, N2 — the monolithic <see cref="SfBufferSnapshot"/> exceeds Akka
+/// remoting's default 128 000-byte frame for any realistic backlog). All chunks of one
+/// resync share <paramref name="ResyncId"/>; <paramref name="Sequence"/> is 1-based up to
+/// <paramref name="TotalChunks"/>. Additive message — the legacy monolithic snapshot
+/// handler is retained for rolling upgrades. Crosses intra-site Akka remoting (NOT
+/// ClusterClient — ClusterClientContractLockTests is intentionally not involved).
+/// </summary>
+public sealed record SfBufferSnapshotChunk(
+    string ResyncId, int Sequence, int TotalChunks,
+    List<StoreAndForwardMessage> Messages, bool Truncated);
+
+/// <summary>
+/// Standby→active: delivery confirmation — the standby assembled all chunks of
+/// <paramref name="ResyncId"/> and applied them atomically (<paramref name="RowCount"/>
+/// rows installed). Absence within the ack window is surfaced by the active node
+/// (Warning + counter) — the silent-loss mode N2 flagged.
+/// </summary>
+public sealed record SfBufferResyncAck(string ResyncId, int RowCount);

src/ZB.MOM.WW.ScadaBridge.StoreAndForward/StoreAndForwardOptionsValidator.cs

@@ -31,5 +31,13 @@ public sealed class StoreAndForwardOptionsValidator : OptionsValidatorBase<Store
         builder.RequireThat(options.DefaultMaxRetries >= 0,
             $"ScadaBridge:StoreAndForward:DefaultMaxRetries must be >= 0 " +
             $"(was {options.DefaultMaxRetries}).");
+
+        builder.RequireThat(options.SweepBatchLimit >= 0,
+            $"ScadaBridge:StoreAndForward:SweepBatchLimit must be >= 0 " +
+            $"(was {options.SweepBatchLimit}); it bounds due rows per retry sweep — 0 means unlimited (legacy).");
+
+        builder.RequireThat(options.SweepTargetParallelism >= 1,
+            $"ScadaBridge:StoreAndForward:SweepTargetParallelism must be >= 1 " +
+            $"(was {options.SweepTargetParallelism}); it caps concurrent (category,target) sweep lanes — 1 means serial.");
     }
 }

src/ZB.MOM.WW.ScadaBridge.StoreAndForward/StoreAndForwardService.cs

@@ -394,7 +394,7 @@ public class StoreAndForwardService
         });
 
         _retryTimer = new Timer(
-            _ => Volatile.Write(ref _sweepTask, RetryPendingMessagesAsync()),
+            _ => KickSweep(),
             null,
             _options.RetryTimerInterval,
             _options.RetryTimerInterval);
@@ -658,17 +658,37 @@ public class StoreAndForwardService
     /// <summary>
     /// Kicks a background retry sweep now (fire-and-forget). No-op before
     /// <see cref="StartAsync"/> (storage may be uninitialized and the timer, whose
-    /// presence gates this, is not yet set). Overlap-safe: the sweep's
-    /// <see cref="_retryInProgress"/> CAS makes a concurrent kick a cheap no-op.
+    /// presence gates this, is not yet set). Overlap-safe: publishes into
+    /// <see cref="_sweepTask"/> only when the CAS is won — see <see cref="KickSweep"/>.
     /// </summary>
     public void TriggerSweep()
     {
         if (_retryTimer == null) return;
-        Volatile.Write(ref _sweepTask, RetryPendingMessagesAsync());
+        KickSweep();
+    }
+
+    /// <summary>The current drain handle — test seam for the N3 clobber regression.</summary>
+    internal Task? CurrentSweepTaskForTest => Volatile.Read(ref _sweepTask);
+
+    /// <summary>
+    /// Starts a sweep IF none is in flight, publishing the new task into
+    /// <see cref="_sweepTask"/> only when this call wins the <see cref="_retryInProgress"/>
+    /// CAS. A kick that loses the CAS returns without touching <see cref="_sweepTask"/> —
+    /// pre-fix it overwrote the drain handle with an instantly-completed no-op, so
+    /// <see cref="StopAsync"/> proceeded with disposal under a still-running sweep
+    /// (review 02 round 2, N3) — defeating the drain exactly when a sweep outlives a tick.
+    /// </summary>
+    private void KickSweep()
+    {
+        if (Interlocked.CompareExchange(ref _retryInProgress, 1, 0) != 0)
+            return;
+        Volatile.Write(ref _sweepTask, RunSweepOwnedAsync());
     }
 
     /// <summary>
     /// Background retry sweep. Processes all pending messages that are due for retry.
+    /// Self-CASes for direct callers (existing tests); the entry CAS is skipped when
+    /// ownership is already held by <see cref="KickSweep"/>.
     /// </summary>
     /// <returns>A task representing the asynchronous retry sweep.</returns>
     internal async Task RetryPendingMessagesAsync()
@@ -676,7 +696,16 @@ public class StoreAndForwardService
         // Prevent overlapping retry sweeps
         if (Interlocked.CompareExchange(ref _retryInProgress, 1, 0) != 0)
             return;
+        await RunSweepOwnedAsync();
+    }
 
+    /// <summary>
+    /// The actual retry sweep body. The caller MUST already own the
+    /// <see cref="_retryInProgress"/> flag (won the CAS); this method releases it in its
+    /// finally. Never call directly without holding ownership.
+    /// </summary>
+    private async Task RunSweepOwnedAsync()
+    {
         try
         {
             var gate = _deliveryGate;

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

@@ -0,0 +1,57 @@
+using System.Net;
+using System.Net.Sockets;
+using Akka.Actor;
+using Akka.Configuration;
+using Xunit;
+using ZB.MOM.WW.ScadaBridge.Communication.ClusterState;
+
+namespace ZB.MOM.WW.ScadaBridge.Communication.Tests;
+
+public class ActiveNodeEvaluatorTests : IAsyncLifetime
+{
+    private ActorSystem? _system;
+
+    public async Task InitializeAsync()
+    {
+        var port = FreePort();
+        var config = ConfigurationFactory.ParseString($@"
+akka {{
+    actor.provider = cluster
+    remote.dot-netty.tcp {{ hostname = ""127.0.0.1"", port = {port} }}
+    cluster {{
+        seed-nodes = [""akka.tcp://ane-test@127.0.0.1:{port}""]
+        roles = [""site-x""]
+        min-nr-of-members = 1
+    }}
+}}");
+        _system = ActorSystem.Create("ane-test", config);
+        var cluster = Akka.Cluster.Cluster.Get(_system);
+        var deadline = DateTime.UtcNow.AddSeconds(20);
+        while (cluster.SelfMember.Status != Akka.Cluster.MemberStatus.Up && DateTime.UtcNow < deadline)
+            await Task.Delay(100);
+    }
+
+    public async Task DisposeAsync() { if (_system != null) await _system.Terminate(); }
+
+    [Fact]
+    public void SelfIsOldestUp_SoleUpMember_ReturnsTrue()
+    {
+        var cluster = Akka.Cluster.Cluster.Get(_system!);
+        Assert.True(ActiveNodeEvaluator.SelfIsOldestUp(cluster));
+        Assert.True(ActiveNodeEvaluator.SelfIsOldestUp(cluster, "site-x"));
+    }
+
+    [Fact]
+    public void SelfIsOldestUp_RoleNotHeld_ReturnsFalse()
+    {
+        var cluster = Akka.Cluster.Cluster.Get(_system!);
+        Assert.False(ActiveNodeEvaluator.SelfIsOldestUp(cluster, "role-nonexistent"));
+    }
+
+    private static int FreePort()
+    {
+        using var l = new TcpListener(IPAddress.Loopback, 0);
+        l.Start();
+        return ((IPEndPoint)l.LocalEndpoint).Port;
+    }
+}

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

@@ -86,4 +86,22 @@ public class CommunicationOptionsValidatorTests
         Assert.True(result.Failed);
         Assert.Contains("LiveAlarmCacheMaxSubscribersPerSite", result.FailureMessage);
     }
+
+    // ── R2 T10: live-delta publish-coalescing window (N6) ────────────────────────
+
+    [Fact]
+    public void ZeroLiveAlarmCachePublishCoalesce_IsValid()
+    {
+        // Zero = publish per delta (legacy behavior).
+        var result = Validate(new CommunicationOptions { LiveAlarmCachePublishCoalesce = TimeSpan.Zero });
+        Assert.True(result.Succeeded, result.FailureMessage);
+    }
+
+    [Fact]
+    public void NegativeLiveAlarmCachePublishCoalesce_IsRejected()
+    {
+        var result = Validate(new CommunicationOptions { LiveAlarmCachePublishCoalesce = TimeSpan.FromMilliseconds(-1) });
+        Assert.True(result.Failed);
+        Assert.Contains("LiveAlarmCachePublishCoalesce", result.FailureMessage);
+    }
 }

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

@@ -24,10 +24,12 @@ public class SiteAlarmAggregatorActorTests : TestKit
 
     public SiteAlarmAggregatorActorTests() : base(@"akka.loglevel = WARNING")
     {
-        SiteAlarmAggregatorActor.ReconnectDelay = TimeSpan.FromMilliseconds(50);
-        SiteAlarmAggregatorActor.StabilityWindow = TimeSpan.FromSeconds(30);
     }
 
+    // Former process-global static test seams, now ctor-injected per actor (R2 T12).
+    private static readonly TimeSpan TestReconnectDelay = TimeSpan.FromMilliseconds(50);
+    private static readonly TimeSpan TestStabilityWindow = TimeSpan.FromSeconds(30);
+
     // ── Test doubles ────────────────────────────────────────────────────────────
 
     /// <summary>
@@ -137,7 +139,7 @@ public class SiteAlarmAggregatorActorTests : TestKit
     }
 
     private (IActorRef Actor, SeedStub Seed, PublishSink Sink, MockSiteAlarmStreamClientFactory Factory) CreateActor(
-        TimeSpan? reconcileInterval = null)
+        TimeSpan? reconcileInterval = null, TimeSpan? publishCoalesce = null)
     {
         var seed = new SeedStub();
         var sink = new PublishSink();
@@ -145,7 +147,10 @@ public class SiteAlarmAggregatorActorTests : TestKit
 
         var props = Props.Create(() => new SiteAlarmAggregatorActor(
             SiteId, "corr-1", seed.Seed, sink.Publish, factory, GrpcNodeA, GrpcNodeB,
-            reconcileInterval ?? TimeSpan.FromMinutes(10)));
+            reconcileInterval ?? TimeSpan.FromMinutes(10),
+            publishCoalesce ?? TimeSpan.Zero,
+            TestReconnectDelay,
+            TestStabilityWindow));
 
         var actor = Sys.ActorOf(props);
         return (actor, seed, sink, factory);
@@ -396,6 +401,89 @@ public class SiteAlarmAggregatorActorTests : TestKit
         AwaitCondition(() => TotalSubs() > before, TimeSpan.FromSeconds(3));
     }
 
+    // ── R2 T11: queued re-seed after reconnect + stream-generation stamp (N7.1/N7.2) ──
+
+    [Fact]
+    public void ReseedRequestedDuringInFlightFanout_IsQueued_NotDropped()
+    {
+        var (actor, seed, sink, factory) = CreateActor(publishCoalesce: TimeSpan.Zero);
+        AwaitCondition(() => seed.CallCount == 1, TimeSpan.FromSeconds(3));
+        seed.CompleteNext();                       // initial seed done (CallCount 1)
+        AwaitAssert(() => Assert.Equal(1, sink.Count));
+        AwaitCondition(() => factory.ClientFor(GrpcNodeA).Subs.Count == 1, TimeSpan.FromSeconds(3));
+
+        actor.Tell(new RunReconcile());            // reconcile fan-out now in flight (CallCount 2)
+        AwaitAssert(() => Assert.Equal(2, seed.CallCount));
+
+        // Stream error while the reconcile is in flight → the failover re-seed must not be
+        // silently skipped. Pre-fix: StartFanout no-ops and CallCount stays 2 until the next
+        // 60s reconcile tick.
+        factory.ClientFor(GrpcNodeA).Subs.Last().OnError(new Exception("stream fault"));
+
+        seed.CompleteNext();                       // finish the in-flight reconcile
+        AwaitAssert(() => Assert.Equal(3, seed.CallCount)); // queued re-seed ran immediately after
+    }
+
+    [Fact]
+    public void LateErrorFromAPreviousStreamGeneration_IsIgnored()
+    {
+        var (_, seed, _, factory) = CreateActor(publishCoalesce: TimeSpan.Zero);
+        AwaitCondition(() => seed.CallCount == 1, TimeSpan.FromSeconds(3));
+        seed.CompleteNext();
+        AwaitCondition(() => factory.ClientFor(GrpcNodeA).Subs.Count == 1, TimeSpan.FromSeconds(3));
+
+        int TotalSubs() => factory.ClientFor(GrpcNodeA).Subs.Count + factory.ClientFor(GrpcNodeB).Subs.Count;
+
+        var firstSub = factory.ClientFor(GrpcNodeA).Subs.Single();
+        firstSub.OnError(new Exception("real fault"));      // gen 1 error → flip, reconnect
+        AwaitCondition(() => factory.ClientFor(GrpcNodeB).Subs.Count == 1, TimeSpan.FromSeconds(5)); // gen 2 stream open
+        AwaitAssert(() => Assert.Equal(2, TotalSubs()));
+
+        firstSub.OnError(new Exception("late zombie fault")); // stale gen-1 error races in
+        // Pre-fix this burns retry budget and opens a THIRD stream; post-fix it is ignored.
+        Thread.Sleep(400);
+        Assert.Equal(2, TotalSubs());
+    }
+
+    // ── R2 T10: live-delta publish coalescing (N6) ──
+
+    [Fact]
+    public void DeltaBurst_IsCoalesced_IntoFewPublishes_ContainingEveryRow()
+    {
+        var (_, seed, sink, factory) = CreateActor(publishCoalesce: TimeSpan.FromMilliseconds(150));
+        AwaitCondition(() => seed.CallCount == 1, TimeSpan.FromSeconds(3));
+        seed.CompleteNext(); // empty initial seed
+        AwaitAssert(() => Assert.Equal(1, sink.Count)); // seed publish, immediate
+
+        AwaitCondition(() => factory.ClientFor(GrpcNodeA).Subs.Count == 1, TimeSpan.FromSeconds(3));
+        var sub = factory.ClientFor(GrpcNodeA).Subs.Single();
+        var now = DateTimeOffset.UtcNow;
+        for (var i = 0; i < 50; i++)
+            sub.OnAlarm(Alarm($"A{i}", "", 900, now)); // 50 distinct keys, one burst
+
+        AwaitAssert(() =>
+        {
+            Assert.Equal(50, sink.Latest!.Count);   // nothing lost
+            Assert.InRange(sink.Count, 2, 4);       // pre-fix: 51 publishes (1 seed + 1 per delta)
+        }, TimeSpan.FromSeconds(3));
+    }
+
+    [Fact]
+    public void ZeroCoalesce_PreservesLegacyPerDeltaPublish()
+    {
+        var (_, seed, sink, factory) = CreateActor(publishCoalesce: TimeSpan.Zero);
+        AwaitCondition(() => seed.CallCount == 1, TimeSpan.FromSeconds(3));
+        seed.CompleteNext();
+        AwaitAssert(() => Assert.Equal(1, sink.Count));
+
+        AwaitCondition(() => factory.ClientFor(GrpcNodeA).Subs.Count == 1, TimeSpan.FromSeconds(3));
+        var sub = factory.ClientFor(GrpcNodeA).Subs.Single();
+        var now = DateTimeOffset.UtcNow;
+        sub.OnAlarm(Alarm("A1", "", 900, now));
+        sub.OnAlarm(Alarm("A2", "", 900, now));
+        AwaitAssert(() => Assert.Equal(3, sink.Count)); // 1 seed + 1 per delta
+    }
+
     [Fact]
     public void Stop_Message_TearsDown_Grpc_And_Stops_Actor()
     {

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

@@ -258,4 +258,48 @@ public class SiteAlarmLiveCacheServiceTests : TestKit
         Assert.False(service.IsLive(999));
         Assert.Empty(service.GetCurrentAlarms(999));
     }
+
+    // ── R2 T15: single-endpoint sites can go live (N9) ──
+
+    [Fact]
+    public void SiteWithOnlyNodeAEndpoint_StartsAnAggregator_AgainstThatEndpoint()
+    {
+        var site = new Site("Three", "site-3")
+        {
+            Id = 3,
+            GrpcNodeAAddress = "http://only-node:8083",
+            GrpcNodeBAddress = null, // single-endpoint site — pre-fix: never goes live
+        };
+
+        var siteRepo = Substitute.For<ISiteRepository>();
+        siteRepo.GetSiteByIdAsync(3, Arg.Any<CancellationToken>()).Returns(site);
+
+        var instanceRepo = Substitute.For<ITemplateEngineRepository>();
+        instanceRepo.GetInstancesBySiteIdAsync(3, Arg.Any<CancellationToken>())
+            .Returns(new List<Instance>());
+
+        var services = new ServiceCollection();
+        services.AddScoped(_ => siteRepo);
+        services.AddScoped(_ => instanceRepo);
+        var provider = services.BuildServiceProvider();
+
+        var options = Options.Create(new CommunicationOptions
+        {
+            LiveAlarmCacheLinger = TimeSpan.FromSeconds(30),
+            LiveAlarmCacheReconcileInterval = TimeSpan.FromMinutes(10),
+        });
+        var comm = new CommunicationService(Options.Create(new CommunicationOptions()),
+            NullLogger<CommunicationService>.Instance);
+        var factory = new CountingFactory();
+
+        var service = new SiteAlarmLiveCacheService(
+            provider, comm, factory, options, NullLogger<SiteAlarmLiveCacheService>.Instance);
+        service.SetActorSystem(Sys);
+
+        using var sub = service.Subscribe(3, () => { });
+
+        // Pre-fix: ResolveSiteAsync required BOTH endpoints and bailed → never live, no client.
+        AwaitCondition(() => service.IsLive(3), TimeSpan.FromSeconds(5));
+        Assert.True(factory.GetOrCreateCount >= 1); // a client was created for the single endpoint
+    }
 }

tests/ZB.MOM.WW.ScadaBridge.IntegrationTests/Cluster/SfBufferResyncPredicateTests.cs

@@ -0,0 +1,94 @@
+using Akka.Actor;
+using Microsoft.Extensions.Logging.Abstractions;
+using ZB.MOM.WW.ScadaBridge.SiteRuntime.Actors;
+using ZB.MOM.WW.ScadaBridge.SiteRuntime.Persistence;
+using ZB.MOM.WW.ScadaBridge.StoreAndForward;
+using ZB.MOM.WW.ScadaBridge.Commons.Types.Enums;
+
+namespace ZB.MOM.WW.ScadaBridge.IntegrationTests.Cluster;
+
+/// <summary>
+/// N1 regression (review 02 round 2, Critical): the resync authority must use the same
+/// oldest-Up predicate as the S&F delivery gate. Divergence scenario = the delivering node
+/// is OLDEST but not LEADER (leader = lowest address), the exact state a rolling restart of
+/// the lower-address node produces. Pre-fix the delivering node requests a resync from the
+/// stale peer and ReplaceAllAsync wipes its live buffer.
+/// </summary>
+public class SfBufferResyncPredicateTests
+{
+    [Fact]
+    public async Task OldestButNotLeaderNode_KeepsItsBuffer_AndSeedsTheJoiner()
+    {
+        // Two explicit ports, deliberately assigned so the FIRST-started (oldest,
+        // delivering) node has the HIGHER address → the second node is cluster leader.
+        var p1 = TwoNodeClusterFixture.GetFreeTcpPort();
+        var p2 = TwoNodeClusterFixture.GetFreeTcpPort();
+        var (portHigh, portLow) = p1 > p2 ? (p1, p2) : (p2, p1);
+
+        await using var fixture = await TwoNodeClusterFixture.StartAsync(
+            role: "site-int", portA: portHigh, portB: portLow);
+
+        // Real S&F storage + replication actor per node, production default predicate
+        // (no isActiveOverride) — the exact wiring under test.
+        var (storageOldest, _) = await CreateReplicationActorAsync(fixture.NodeA, "oldest");
+        var (storageJoiner, _) = await CreateReplicationActorAsync(fixture.NodeB, "joiner");
+
+        // The delivering (oldest) node has a live buffered row the standby never saw.
+        await storageOldest.EnqueueAsync(NewMessage("live-row"));
+
+        // Trigger peer (re)tracking on both sides: each actor got InitialStateAsSnapshot
+        // in PreStart, but the enqueue raced it — re-deliver via a fresh MemberUp is not
+        // needed; OnPeerTracked already fired on join. The resync exchange is async:
+        // wait until the JOINER holds the row (proves the snapshot flowed oldest→joiner,
+        // the correct direction). Pre-fix this times out (the joiner, as leader, never
+        // requests) AND the oldest node's row is deleted by the stale wipe.
+        await AwaitAsync(async () => await storageJoiner.GetMessageByIdAsync("live-row") != null,
+            TimeSpan.FromSeconds(20),
+            "joiner never received the resync snapshot (resync ran in the wrong direction)");
+
+        // And the delivering node's buffer is untouched — the N1 wipe assertion.
+        Assert.NotNull(await storageOldest.GetMessageByIdAsync("live-row"));
+    }
+
+    private static async Task<(StoreAndForwardStorage Storage, IActorRef Actor)> CreateReplicationActorAsync(
+        ActorSystem node, string tag)
+    {
+        var sfDb = Path.Combine(Path.GetTempPath(), $"sf-resync-{tag}-{Guid.NewGuid():N}.db");
+        var siteDb = Path.Combine(Path.GetTempPath(), $"site-resync-{tag}-{Guid.NewGuid():N}.db");
+        var sfStorage = new StoreAndForwardStorage($"Data Source={sfDb}",
+            NullLogger<StoreAndForwardStorage>.Instance);
+        await sfStorage.InitializeAsync();
+        var siteStorage = new SiteStorageService($"Data Source={siteDb}",
+            NullLogger<SiteStorageService>.Instance);
+        var replicationService = new ReplicationService(
+            new StoreAndForwardOptions(), NullLogger<ReplicationService>.Instance);
+        // Name MUST be "site-replication" — SendToPeer targets /user/site-replication.
+        var actor = node.ActorOf(Props.Create(() => new SiteReplicationActor(
+            siteStorage, sfStorage, replicationService, "site-int",
+            NullLogger<SiteReplicationActor>.Instance, null, null, null, null)),
+            "site-replication");
+        return (sfStorage, actor);
+    }
+
+    private static StoreAndForwardMessage NewMessage(string id) => new()
+    {
+        Id = id,
+        Category = StoreAndForwardCategory.Notification,
+        Target = "central",
+        PayloadJson = "{}",
+        CreatedAt = DateTimeOffset.UtcNow,
+        Status = StoreAndForwardMessageStatus.Pending,
+        MaxRetries = 0,
+    };
+
+    private static async Task AwaitAsync(Func<Task<bool>> condition, TimeSpan timeout, string why)
+    {
+        var deadline = DateTime.UtcNow + timeout;
+        while (DateTime.UtcNow < deadline)
+        {
+            if (await condition()) return;
+            await Task.Delay(250);
+        }
+        throw new TimeoutException(why);
+    }
+}

tests/ZB.MOM.WW.ScadaBridge.IntegrationTests/Cluster/TwoNodeClusterFixture.cs

@@ -22,11 +22,12 @@ public sealed class TwoNodeClusterFixture : IAsyncDisposable
     public int PortB { get; private set; }
 
     public static async Task<TwoNodeClusterFixture> StartAsync(
-        string role = "Central", TimeSpan? stableAfter = null)
+        string role = "Central", TimeSpan? stableAfter = null,
+        int? portA = null, int? portB = null)
     {
         var f = new TwoNodeClusterFixture();
-        f.PortA = GetFreeTcpPort();
-        f.PortB = GetFreeTcpPort();
+        f.PortA = portA ?? GetFreeTcpPort();
+        f.PortB = portB ?? GetFreeTcpPort();
         f.NodeA = f.StartNode(f.PortA, role, stableAfter);
         await WaitForMembersUp(f.NodeA, 1, TimeSpan.FromSeconds(20));
         f.NodeB = f.StartNode(f.PortB, role, stableAfter);
@@ -98,7 +99,7 @@ public sealed class TwoNodeClusterFixture : IAsyncDisposable
         throw new TimeoutException($"Member {removed} was never removed from {cluster.SelfAddress}'s view — SBR did not down it.");
     }
 
-    private static int GetFreeTcpPort()
+    public static int GetFreeTcpPort()
     {
         var listener = new System.Net.Sockets.TcpListener(System.Net.IPAddress.Loopback, 0);
         listener.Start();

tests/ZB.MOM.WW.ScadaBridge.IntegrationTests/Grpc/SiteAlarmStreamEndToEndTests.cs

@@ -223,7 +223,8 @@ public class SiteAlarmStreamEndToEndTests : TestKit
         var sink = new PublishSink();
         var aggregator = Sys.ActorOf(Props.Create(() => new SiteAlarmAggregatorActor(
             "site-alpha", "e2e-parity", _ => Task.FromResult<IReadOnlyList<AlarmStateChanged>>(new[] { seedRow }),
-            sink.Publish, factory, "http://a:5100", "http://b:5100", TimeSpan.FromMinutes(10))));
+            sink.Publish, factory, "http://a:5100", "http://b:5100", TimeSpan.FromMinutes(10),
+            TimeSpan.Zero, TimeSpan.FromSeconds(5), TimeSpan.FromSeconds(60))));
 
         await WaitForConditionAsync(() => sink.Latest is { Count: 1 }); // seed applied
         aggregator.Tell(liveRow);

tests/ZB.MOM.WW.ScadaBridge.ManagementService.Tests/ManagementActorTests.cs

@@ -5,8 +5,10 @@ using Microsoft.Extensions.Logging.Abstractions;
 using Microsoft.Extensions.Options;
 using NSubstitute;
 using ZB.MOM.WW.ScadaBridge.Communication;
+using ZB.MOM.WW.ScadaBridge.Communication.Grpc;
 using NSubstitute.ExceptionExtensions;
 using ZB.MOM.WW.ScadaBridge.Commons.Entities.Instances;
+using ZB.MOM.WW.ScadaBridge.Commons.Entities.Sites;
 using ZB.MOM.WW.ScadaBridge.Commons.Entities.Schemas;
 using ZB.MOM.WW.ScadaBridge.Commons.Entities.SecuredWrites;
 using ZB.MOM.WW.ScadaBridge.Commons.Entities.Scripts;
@@ -3599,6 +3601,51 @@ public class ManagementActorTests : TestKit, IDisposable
         ExpectMsg<ManagementUnauthorized>(TimeSpan.FromSeconds(5));
     }
 
+    // ── R2 T13: site delete disposes the site's cached gRPC channels (N8) ──
+
+    [Fact]
+    public void DeleteSite_DisposesTheSitesCachedGrpcChannels()
+    {
+        var siteRepo = Substitute.For<ISiteRepository>();
+        siteRepo.GetSiteByIdAsync(7, Arg.Any<CancellationToken>())
+            .Returns(new Site("Seven", "site-7") { Id = 7 });
+        siteRepo.GetInstancesBySiteIdAsync(7, Arg.Any<CancellationToken>())
+            .Returns((IReadOnlyList<Instance>)new List<Instance>());
+        _services.AddScoped(_ => siteRepo);
+
+        var factory = new TrackingGrpcFactory();
+        var cached = (TrackingClient)factory.GetOrCreate("site-7", "http://node-a:8083");
+        _services.AddSingleton<SiteStreamGrpcClientFactory>(factory);
+
+        var actor = CreateActor();
+        actor.Tell(Envelope(new DeleteSiteCommand(7), "Administrator"));
+        ExpectMsg<ManagementSuccess>(TimeSpan.FromSeconds(5));
+
+        AwaitAssert(() =>
+        {
+            Assert.True(cached.Disposed);
+            Assert.Null(factory.TryGet("site-7", "http://node-a:8083"));
+        }, TimeSpan.FromSeconds(5));
+    }
+
+    private sealed class TrackingClient : SiteStreamGrpcClient
+    {
+        public TrackingClient(string endpoint) : base(endpoint) { }
+        public bool Disposed { get; private set; }
+        public override void Dispose() => Disposed = true;
+        public override ValueTask DisposeAsync()
+        {
+            Disposed = true;
+            return ValueTask.CompletedTask;
+        }
+    }
+
+    private sealed class TrackingGrpcFactory : SiteStreamGrpcClientFactory
+    {
+        public TrackingGrpcFactory() : base(NullLoggerFactory.Instance) { }
+        protected override SiteStreamGrpcClient CreateClient(string grpcEndpoint) => new TrackingClient(grpcEndpoint);
+    }
+
     /// <summary>
     /// Records remote-query relays for the actor-dispatch tests. The browse /
     /// search / verify / cert-trust methods on <see cref="CommunicationService"/>

tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/Actors/SiteReplicationActorTests.cs

@@ -1,4 +1,5 @@
 using System.Collections.Concurrent;
+using System.Diagnostics.Metrics;
 using Akka.Actor;
 using Akka.TestKit.Xunit2;
 using Microsoft.Extensions.Logging;
@@ -9,6 +10,7 @@ using ZB.MOM.WW.ScadaBridge.SiteRuntime.Deployment;
 using ZB.MOM.WW.ScadaBridge.SiteRuntime.Messages;
 using ZB.MOM.WW.ScadaBridge.SiteRuntime.Persistence;
 using ZB.MOM.WW.ScadaBridge.StoreAndForward;
+using ZB.MOM.WW.ScadaBridge.Commons.Observability;
 using ZB.MOM.WW.ScadaBridge.Commons.Types.Enums;
 
 namespace ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests.Actors;
@@ -248,8 +250,10 @@ akka {
     }
 
     [Fact]
-    public async Task ActiveNode_AnswersResyncRequest_WithFullBufferSnapshot()
+    public async Task ActiveNode_AnswersResyncRequest_WithChunkedSnapshot()
     {
+        // Post-R2-T5 the active node answers with byte-budgeted SfBufferSnapshotChunk(s)
+        // (a single small row rides one chunk) rather than the monolithic SfBufferSnapshot.
         await _sfStorage.EnqueueAsync(NewSfMessage("m1"));
         var probe = CreateTestProbe();
         var actor = ActorOf(Props.Create(() => new ResyncTestActor(
@@ -258,9 +262,11 @@ akka {
 
         actor.Tell(new RequestSfBufferResync(), TestActor);
 
-        var snapshot = ExpectMsg<SfBufferSnapshot>(TimeSpan.FromSeconds(3));
-        Assert.Single(snapshot.Messages);
-        Assert.False(snapshot.Truncated);
+        var chunk = ExpectMsg<SfBufferSnapshotChunk>(TimeSpan.FromSeconds(3));
+        Assert.Equal(1, chunk.TotalChunks);
+        Assert.Equal(1, chunk.Sequence);
+        Assert.Single(chunk.Messages);
+        Assert.False(chunk.Truncated);
     }
 
     [Fact]
@@ -281,6 +287,170 @@ akka {
         }, TimeSpan.FromSeconds(5));
     }
 
+    // ── R2 T5: chunked resync answer ──
+
+    [Fact]
+    public void ChunkForRemoting_SplitsByByteBudget_PreservingOrderAndSequence()
+    {
+        var rows = Enumerable.Range(0, 10)
+            .Select(i => NewMessage($"m{i}", payloadJson: new string('x', 20_000)))
+            .ToList();
+
+        var chunks = SiteReplicationActor.ChunkForRemoting(rows, maxChunkBytes: 64_000, maxChunkRows: 200);
+
+        Assert.True(chunks.Count > 1); // 10 × 20 KB cannot ride one 64 KB chunk
+        Assert.Equal(rows.Select(r => r.Id), chunks.SelectMany(c => c).Select(r => r.Id)); // order preserved
+        Assert.All(chunks, c => Assert.True(
+            c.Sum(r => r.PayloadJson.Length) <= 64_000 || c.Count == 1)); // budget honored (oversized row isolated)
+    }
+
+    [Fact]
+    public void ChunkForRemoting_RowCapHonored_AndSingleOversizedRowIsolated()
+    {
+        var many = Enumerable.Range(0, 500).Select(i => NewMessage($"s{i}", payloadJson: "{}")).ToList();
+        Assert.All(SiteReplicationActor.ChunkForRemoting(many, 64_000, 200), c => Assert.True(c.Count <= 200));
+
+        var oversized = new List<StoreAndForwardMessage>
+            { NewMessage("big", payloadJson: new string('y', 100_000)), NewMessage("small", payloadJson: "{}") };
+        var chunks = SiteReplicationActor.ChunkForRemoting(oversized, 64_000, 200);
+        Assert.Equal(2, chunks.Count); // the oversized row rides alone
+    }
+
+    [Fact]
+    public async Task ActiveNode_AnswersResyncRequest_WithSequencedChunks_SharingOneResyncId()
+    {
+        for (var i = 0; i < 3; i++)
+            await _sfStorage.EnqueueAsync(NewMessage($"c{i}", payloadJson: new string('z', 30_000)));
+        var actor = CreateResyncActor(isActive: () => true);
+
+        actor.Tell(new RequestSfBufferResync(), TestActor);
+
+        var first = ExpectMsg<SfBufferSnapshotChunk>(TimeSpan.FromSeconds(5));
+        var rest = Enumerable.Range(1, first.TotalChunks - 1)
+            .Select(_ => ExpectMsg<SfBufferSnapshotChunk>(TimeSpan.FromSeconds(5)))
+            .Prepend(first)
+            .ToList();
+
+        Assert.True(first.TotalChunks > 1);
+        Assert.All(rest, c => Assert.Equal(first.ResyncId, c.ResyncId));
+        Assert.Equal(Enumerable.Range(1, first.TotalChunks), rest.Select(c => c.Sequence));
+        Assert.Equal(3, rest.Sum(c => c.Messages.Count));
+    }
+
+    // ── R2 T6: standby chunk assembly + atomic apply + ack ──
+
+    [Fact]
+    public async Task StandbyNode_AssemblesChunks_AppliesOnce_AndAcks()
+    {
+        await _sfStorage.EnqueueAsync(NewMessage("stale"));
+        var actor = CreateResyncActor(isActive: () => false);
+        var resyncId = "r1";
+
+        actor.Tell(new SfBufferSnapshotChunk(resyncId, 1, 2,
+            new List<StoreAndForwardMessage> { NewMessage("f1") }, false), TestActor);
+        actor.Tell(new SfBufferSnapshotChunk(resyncId, 2, 2,
+            new List<StoreAndForwardMessage> { NewMessage("f2") }, false), TestActor);
+
+        var ack = ExpectMsg<SfBufferResyncAck>(TimeSpan.FromSeconds(5));
+        Assert.Equal(resyncId, ack.ResyncId);
+        Assert.Equal(2, ack.RowCount);
+        await AwaitAssertAsync(async () =>
+        {
+            Assert.Null(await _sfStorage.GetMessageByIdAsync("stale"));   // replaced wholesale
+            Assert.NotNull(await _sfStorage.GetMessageByIdAsync("f1"));
+            Assert.NotNull(await _sfStorage.GetMessageByIdAsync("f2"));
+        });
+    }
+
+    [Fact]
+    public async Task StandbyNode_NewResyncId_DiscardsStalePartialAssembly()
+    {
+        var actor = CreateResyncActor(isActive: () => false);
+        actor.Tell(new SfBufferSnapshotChunk("old", 1, 2,
+            new List<StoreAndForwardMessage> { NewMessage("orphan") }, false), TestActor);
+        actor.Tell(new SfBufferSnapshotChunk("new", 1, 1,
+            new List<StoreAndForwardMessage> { NewMessage("fresh") }, false), TestActor);
+
+        ExpectMsg<SfBufferResyncAck>(TimeSpan.FromSeconds(5)); // "new" completed
+        await AwaitAssertAsync(async () =>
+        {
+            Assert.NotNull(await _sfStorage.GetMessageByIdAsync("fresh"));
+            Assert.Null(await _sfStorage.GetMessageByIdAsync("orphan")); // stale partial never applied
+        });
+    }
+
+    [Fact]
+    public void ActiveNode_IgnoresChunks_NeverAcks()
+    {
+        var actor = CreateResyncActor(isActive: () => true);
+        actor.Tell(new SfBufferSnapshotChunk("r", 1, 1,
+            new List<StoreAndForwardMessage> { NewMessage("x") }, false), TestActor);
+        ExpectNoMsg(TimeSpan.FromMilliseconds(300));
+    }
+
+    // ── R2 T7: active-side resync ack confirmation + telemetry ──
+    //
+    // NOTE (deviation from plan): the actor logs via Microsoft ILogger (NullLogger in
+    // tests), NOT Akka's EventStream, so the plan's EventFilter.Warning assertions can
+    // never observe these warnings. We observe the two OTel counters via a MeterListener
+    // instead — the equivalent, and stronger, observable signal.
+
+    [Fact]
+    public async Task ActiveNode_ReceivingAck_CountsResyncCompleted()
+    {
+        long completed = 0;
+        using var listener = ListenCounter("scadabridge.store_and_forward.resync.completed",
+            m => Interlocked.Add(ref completed, m));
+
+        await _sfStorage.EnqueueAsync(NewMessage("m1"));
+        var actor = CreateResyncActor(isActive: () => true, ackTimeout: TimeSpan.FromSeconds(30));
+        actor.Tell(new RequestSfBufferResync(), TestActor);
+        var chunk = ExpectMsg<SfBufferSnapshotChunk>(TimeSpan.FromSeconds(5));
+
+        actor.Tell(new SfBufferResyncAck(chunk.ResyncId, 1), TestActor);
+
+        await AwaitAssertAsync(() =>
+        {
+            Assert.True(Interlocked.Read(ref completed) >= 1); // ack recorded the completion
+            return Task.CompletedTask;
+        }, TimeSpan.FromSeconds(5));
+    }
+
+    [Fact]
+    public async Task ActiveNode_MissingAck_WarnsAfterAckTimeout()
+    {
+        long ackMissing = 0;
+        using var listener = ListenCounter("scadabridge.store_and_forward.resync.ack_missing",
+            m => Interlocked.Add(ref ackMissing, m));
+
+        await _sfStorage.EnqueueAsync(NewMessage("m1"));
+        var actor = CreateResyncActor(isActive: () => true, ackTimeout: TimeSpan.FromMilliseconds(200));
+        actor.Tell(new RequestSfBufferResync(), TestActor);
+        ExpectMsg<SfBufferSnapshotChunk>(TimeSpan.FromSeconds(5));
+        // No ack is sent → the ack window expires and the resync is counted unacknowledged.
+
+        await AwaitAssertAsync(() =>
+        {
+            Assert.True(Interlocked.Read(ref ackMissing) >= 1);
+            return Task.CompletedTask;
+        }, TimeSpan.FromSeconds(5));
+    }
+
+    /// <summary>Attaches a <see cref="MeterListener"/> to a single ScadaBridge counter by name,
+    /// forwarding each recorded increment to <paramref name="onMeasurement"/>.</summary>
+    private static MeterListener ListenCounter(string instrumentName, Action<long> onMeasurement)
+    {
+        var listener = new MeterListener();
+        listener.InstrumentPublished = (inst, l) =>
+        {
+            if (inst.Meter.Name == ScadaBridgeTelemetry.MeterName && inst.Name == instrumentName)
+                l.EnableMeasurementEvents(inst);
+        };
+        listener.SetMeasurementEventCallback<long>((_, m, _, _) => onMeasurement(m));
+        listener.Start();
+        return listener;
+    }
+
     private static StoreAndForwardMessage NewSfMessage(string id) => new()
     {
         Id = id,
@@ -294,6 +464,31 @@ akka {
         Status = StoreAndForwardMessageStatus.Pending,
     };
 
+    /// <summary>
+    /// Builds a resync-test message with a settable payload (additive to
+    /// <see cref="NewSfMessage"/> — the chunker sizes on <c>PayloadJson</c> length).
+    /// </summary>
+    private static StoreAndForwardMessage NewMessage(string id, string payloadJson = "{}") => new()
+    {
+        Id = id,
+        Category = StoreAndForwardCategory.ExternalSystem,
+        Target = "t",
+        PayloadJson = payloadJson,
+        RetryCount = 0,
+        MaxRetries = 50,
+        RetryIntervalMs = 30000,
+        CreatedAt = DateTimeOffset.UtcNow,
+        Status = StoreAndForwardMessageStatus.Pending,
+    };
+
+    /// <summary>Constructs a <see cref="ResyncTestActor"/> with the given active-node check
+    /// (the resync chunk/ack tests Tell to and expect from <see cref="TestKit.TestActor"/>).
+    /// <paramref name="ackTimeout"/> is the active-side ack window seam (T7).</summary>
+    private IActorRef CreateResyncActor(Func<bool> isActive, TimeSpan? ackTimeout = null) =>
+        ActorOf(Props.Create(() => new ResyncTestActor(
+            _storage, _sfStorage, _replicationService, SiteRole,
+            NullLogger<SiteReplicationActor>.Instance, CreateTestProbe().Ref, isActive, ackTimeout)));
+
     /// <summary>Test message: drives <see cref="SiteReplicationActor.OnPeerTracked"/> directly,
     /// standing in for the MemberUp→TryTrackPeer path (a single-node TestKit cannot form a real peer).</summary>
     private sealed record TriggerPeerTracked;
@@ -309,9 +504,10 @@ akka {
         public ResyncTestActor(
             SiteStorageService storage, StoreAndForwardStorage sfStorage,
             ReplicationService replicationService, string siteRole,
-            ILogger<SiteReplicationActor> logger, IActorRef peerProbe, Func<bool> isActive)
+            ILogger<SiteReplicationActor> logger, IActorRef peerProbe, Func<bool> isActive,
+            TimeSpan? ackTimeout = null)
             : base(storage, sfStorage, replicationService, siteRole, logger,
-                   configFetcher: null, isActiveOverride: isActive)
+                   configFetcher: null, isActiveOverride: isActive, resyncAckTimeout: ackTimeout)
         {
             _peerProbe = peerProbe;
             Receive<TriggerPeerTracked>(_ => OnPeerTracked());

tests/ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests/ResyncWireSerializationPinTests.cs

@@ -0,0 +1,95 @@
+using Akka.TestKit.Xunit2;
+using ZB.MOM.WW.ScadaBridge.Commons.Types.Enums;
+using ZB.MOM.WW.ScadaBridge.SiteRuntime.Actors;
+using ZB.MOM.WW.ScadaBridge.StoreAndForward;
+
+namespace ZB.MOM.WW.ScadaBridge.SiteRuntime.Tests;
+
+/// <summary>
+/// Characterization pin for the chunked anti-entropy resync contract (review 02 round 2,
+/// N2). <see cref="SfBufferSnapshotChunk"/> (active→standby) and <see cref="SfBufferResyncAck"/>
+/// (standby→active) ride intra-site Akka remoting on the default reflective-JSON wire
+/// format. A rename/move, dropped setter, or non-default-constructible message would
+/// silently break resync across a rolling upgrade and only surface as a divergent buffer
+/// after a failover. These pin round-trip fidelity and type identity. (These messages are
+/// NOT ClusterClient traffic, so they are intentionally absent from ClusterClientContractLockTests.)
+/// </summary>
+public class ResyncWireSerializationPinTests : TestKit
+{
+    private T RoundTrip<T>(T message)
+    {
+        var serialization = Sys.Serialization;
+        var serializer = serialization.FindSerializerFor(message);
+        var bytes = serializer.ToBinary(message);
+        return (T)serialization.Deserialize(bytes, serializer.Identifier, message!.GetType());
+    }
+
+    private static StoreAndForwardMessage FullMessage() => new()
+    {
+        Id = Guid.NewGuid().ToString("N"),
+        Category = StoreAndForwardCategory.Notification,
+        Target = "Operators",
+        PayloadJson = "{\"notificationId\":\"abc\"}",
+        RetryCount = 4,
+        MaxRetries = 0,
+        RetryIntervalMs = 30000,
+        CreatedAt = DateTimeOffset.UtcNow,
+        LastAttemptAt = DateTimeOffset.UtcNow,
+        Status = StoreAndForwardMessageStatus.Parked,
+        LastError = "central rejected",
+        OriginInstanceName = "Plant.Pump3",
+        ExecutionId = Guid.NewGuid(),
+        SourceScript = "ScriptActor:MonitorSpeed",
+        ParentExecutionId = Guid.NewGuid(),
+    };
+
+    [Fact]
+    public void SfBufferSnapshotChunk_WithFullMessage_RoundTripsOnTheWire()
+    {
+        var message = FullMessage();
+        var original = new SfBufferSnapshotChunk(
+            "resync-1", 2, 5, new List<StoreAndForwardMessage> { message }, Truncated: true);
+
+        var back = RoundTrip(original);
+
+        Assert.Equal(original.ResyncId, back.ResyncId);
+        Assert.Equal(original.Sequence, back.Sequence);
+        Assert.Equal(original.TotalChunks, back.TotalChunks);
+        Assert.Equal(original.Truncated, back.Truncated);
+        var m = Assert.Single(back.Messages);
+        Assert.Equal(message.Id, m.Id);
+        Assert.Equal(message.Category, m.Category);
+        Assert.Equal(message.Target, m.Target);
+        Assert.Equal(message.PayloadJson, m.PayloadJson);
+        Assert.Equal(message.RetryCount, m.RetryCount);
+        Assert.Equal(message.MaxRetries, m.MaxRetries);
+        Assert.Equal(message.RetryIntervalMs, m.RetryIntervalMs);
+        Assert.Equal(message.CreatedAt, m.CreatedAt);
+        Assert.Equal(message.LastAttemptAt, m.LastAttemptAt);
+        Assert.Equal(message.Status, m.Status);
+        Assert.Equal(message.LastError, m.LastError);
+        Assert.Equal(message.OriginInstanceName, m.OriginInstanceName);
+        Assert.Equal(message.ExecutionId, m.ExecutionId);
+        Assert.Equal(message.SourceScript, m.SourceScript);
+        Assert.Equal(message.ParentExecutionId, m.ParentExecutionId);
+    }
+
+    [Fact]
+    public void SfBufferResyncAck_RoundTripsOnTheWire()
+    {
+        var original = new SfBufferResyncAck("resync-1", 42);
+
+        var back = RoundTrip(original);
+
+        Assert.Equal(original.ResyncId, back.ResyncId);
+        Assert.Equal(original.RowCount, back.RowCount);
+    }
+
+    // Type-identity pins: the reflective-JSON wire embeds CLR type manifests, so a
+    // rename/move of either type silently breaks resync across a rolling upgrade.
+    [Theory]
+    [InlineData(typeof(SfBufferSnapshotChunk), "ZB.MOM.WW.ScadaBridge.SiteRuntime.Actors.SfBufferSnapshotChunk")]
+    [InlineData(typeof(SfBufferResyncAck), "ZB.MOM.WW.ScadaBridge.SiteRuntime.Actors.SfBufferResyncAck")]
+    public void ResyncContract_TypeIdentity_IsPinned(Type type, string expectedFullName) =>
+        Assert.Equal(expectedFullName, type.FullName);
+}

tests/ZB.MOM.WW.ScadaBridge.StoreAndForward.Tests/StoreAndForwardOptionsValidatorTests.cs

@@ -47,4 +47,33 @@ public class StoreAndForwardOptionsValidatorTests
         Assert.True(result.Failed);
         Assert.Contains("SqliteDbPath", result.FailureMessage);
     }
+
+    // ── R2 T9: sweep-tuning eager validation (N4) ──
+
+    [Theory]
+    [InlineData(-1)]
+    [InlineData(-500)]
+    public void Validate_NegativeSweepBatchLimit_Fails(int limit)
+    {
+        var result = Validate(new StoreAndForwardOptions { SweepBatchLimit = limit });
+        Assert.True(result.Failed);
+        Assert.Contains("SweepBatchLimit", result.FailureMessage);
+    }
+
+    [Fact]
+    public void Validate_ZeroSweepBatchLimit_IsValidUnlimitedLegacy()
+    {
+        var result = Validate(new StoreAndForwardOptions { SweepBatchLimit = 0 });
+        Assert.True(result.Succeeded, result.FailureMessage);
+    }
+
+    [Theory]
+    [InlineData(0)]
+    [InlineData(-4)]
+    public void Validate_NonPositiveSweepTargetParallelism_Fails(int parallelism)
+    {
+        var result = Validate(new StoreAndForwardOptions { SweepTargetParallelism = parallelism });
+        Assert.True(result.Failed);
+        Assert.Contains("SweepTargetParallelism", result.FailureMessage);
+    }
 }

tests/ZB.MOM.WW.ScadaBridge.StoreAndForward.Tests/StoreAndForwardServiceTests.cs

@@ -821,4 +821,67 @@ public class StoreAndForwardServiceTests : IAsyncLifetime, IDisposable
         }
         finally { await service.StopAsync(); }
     }
+
+    // ── R2 T8: _sweepTask clobber (N3) ──
+
+    [Fact]
+    public async Task TriggerSweep_WhileSweepInFlight_DoesNotClobberTheDrainHandle()
+    {
+        var service = CreateService(retryTimerInterval: TimeSpan.FromHours(1));
+        var entered = new TaskCompletionSource(TaskCreationOptions.RunContinuationsAsynchronously);
+        var release = new TaskCompletionSource(TaskCreationOptions.RunContinuationsAsynchronously);
+        service.RegisterDeliveryHandler(StoreAndForwardCategory.ExternalSystem, async _ =>
+        {
+            entered.TrySetResult();
+            await release.Task;
+            return true;
+        });
+        await service.StartAsync();
+        try
+        {
+            await service.EnqueueAsync(StoreAndForwardCategory.ExternalSystem, "t", "{}",
+                attemptImmediateDelivery: false, retryInterval: TimeSpan.Zero);
+
+            service.TriggerSweep();                       // real sweep, blocked in the handler
+            await entered.Task.WaitAsync(TimeSpan.FromSeconds(5));
+            service.TriggerSweep();                       // redundant kick — pre-fix clobbers _sweepTask
+
+            var handle = service.CurrentSweepTaskForTest;
+            Assert.NotNull(handle);
+            Assert.False(handle!.IsCompleted);            // pre-fix: true (a completed no-op replaced the real sweep)
+        }
+        finally
+        {
+            release.TrySetResult();
+            await service.StopAsync();
+        }
+    }
+
+    [Fact]
+    public async Task StopAsync_WaitsForTheRealInFlightSweep_EvenAfterARedundantTrigger()
+    {
+        var service = CreateService(retryTimerInterval: TimeSpan.FromHours(1));
+        var entered = new TaskCompletionSource(TaskCreationOptions.RunContinuationsAsynchronously);
+        var release = new TaskCompletionSource(TaskCreationOptions.RunContinuationsAsynchronously);
+        service.RegisterDeliveryHandler(StoreAndForwardCategory.ExternalSystem, async _ =>
+        {
+            entered.TrySetResult();
+            await release.Task;
+            return true;
+        });
+        await service.StartAsync();
+        await service.EnqueueAsync(StoreAndForwardCategory.ExternalSystem, "t", "{}",
+            attemptImmediateDelivery: false, retryInterval: TimeSpan.Zero);
+
+        service.TriggerSweep();
+        await entered.Task.WaitAsync(TimeSpan.FromSeconds(5));
+        service.TriggerSweep(); // the clobbering kick
+
+        var stop = service.StopAsync();
+        await Task.Delay(300);
+        Assert.False(stop.IsCompleted); // pre-fix: StopAsync already returned (awaited the no-op)
+
+        release.TrySetResult();
+        await stop.WaitAsync(TimeSpan.FromSeconds(5)); // drains the real sweep promptly once released
+    }
 }