docs+log(siteeventlogging): explain the site_events purge oplog-backlog burst (R7)

The daily site_events retention purge (and the storage-cap trim) is CDC-captured
on a replication-enabled site node exactly like any other write — correct by
design, since LocalDb Phase 2 deliberately has no purge-exemption path — so the
backlog jumps by the deleted batch size at purge time. LocalDbOplogBacklog /
localdb_oplog_depth spike, drain, and an operator watching the gauge with no
context reads it as a replication fault.

Documentation + one log line, no behaviour change:

- topology-guide.md gains "Reading the replication backlog — the daily
  site_events purge burst": when it fires (PurgeInterval 24h, anchored to the
  active node's PROCESS START, not a wall-clock hour, so it moves after every
  failover), where it shows (replicated nodes only — not rig site-b/site-c),
  the healthy signature (LocalDbReplicationConnected stays true, backlog
  returns to ~0) and what a genuine fault looks like instead.
- Component-SiteEventLogging.md Storage records the same under retention/purge;
  Component-HealthMonitoring.md gains the two previously-undocumented
  LocalDbReplicationConnected / LocalDbOplogBacklog metric rows carrying the
  caveat, with cross-references both ways.
- EventLogPurgeService emits one Information line naming the row count and the
  expected transient backlog when a purge deleted rows on a replication-enabled
  node, so the spike is correlatable in the log. Replication-awareness comes in
  as a Host-supplied SiteEventLogReplicationCheck delegate, mirroring the
  existing SiteEventLogActiveNodeCheck seam: SiteLocalDbSetup.ReplicationIsConfigured
  goes internal so the PeerAddress-OR-ApiKey rule stays in one place and
  SiteEventLogging never learns to read LocalDb config. Unregistered ⇒ no note,
  matching the default that replication is opt-in and off.

Both delete paths carry the note (a cap trim is usually the larger burst); the
predicate is try/caught since a log-wording check must never break the purge.

Tests: 5 new EventLogPurgeServiceTests cases (replicated logs it, unreplicated
does not, zero-rows does not, cap purge logs it, throwing predicate still purges
and swallows) via a local capturing ILogger. SiteEventLogging 81/81 green,
Host 490/490 green, full solution build clean (0 warnings).

CLAUDE.md

@@ -83,7 +83,7 @@ spec for each is `docs/requirements/Component-<Name>.md`, and `README.md` carrie
   - `site_events.id` changed from autoincrement INTEGER to an application-minted **GUID**. Last-writer-wins keys on the primary key, so two nodes independently minting `id=1,2,3…` would destroy each other's events rather than merge them. The event-log read path uses a composite `(timestamp, id)` keyset cursor with an **opaque string** continuation token; `EventLogEntry.Id` and both `ContinuationToken`s are `string`/`string?` on the site↔central Akka DTOs.
   - `ScadaBridge:OperationTracking:ConnectionString`, `ScadaBridge:SiteEventLog:DatabasePath` and — as of Phase 2 — `ScadaBridge:StoreAndForward:SqliteDbPath` + `ScadaBridge:Database:SiteDbPath` are all **migration-only** — nothing reads them but `SiteLocalDbLegacyMigrator`, which copies a pre-Phase-1 file in once (deterministic `mig-{NodeName}-{legacyId}` event ids, `INSERT OR IGNORE`, runs AFTER `RegisterReplicated` so migrated rows replicate) and renames it `.migrated`. Delete the keys once a node has migrated.
   - This incidentally fixes a data-loss bug: both legacy databases defaulted to CWD-relative paths **outside** the mounted volume and were discarded on every container recreate.
-  - **Replication is default-OFF and opt-in** via `LocalDb:Replication:PeerAddress` + a matching `ApiKey` on both nodes. `LocalDbSyncAuthInterceptor` is **fail-closed**: no configured key means no sync stream is accepted at all, so a key typo does not degrade to unauthenticated replication — the pair simply stops converging. The sync endpoint shares the existing site gRPC h2c listener (8083); no new port. Rig posture: **site-a replicated, site-b/site-c deliberately not**, so both states are proven side-by-side. Status surfaces on the site health report as `LocalDbReplicationConnected` / `LocalDbOplogBacklog` (both nullable — null means "no data", NOT "disconnected with an empty backlog") and as `localdb_*` Prometheus series. Note `ZbTelemetryOptions.Meters` is an **allowlist** (`SiteServiceRegistration.ObservedMeters`); an unlisted meter exports nothing, silently.
+  - **Replication is default-OFF and opt-in** via `LocalDb:Replication:PeerAddress` + a matching `ApiKey` on both nodes. `LocalDbSyncAuthInterceptor` is **fail-closed**: no configured key means no sync stream is accepted at all, so a key typo does not degrade to unauthenticated replication — the pair simply stops converging. The sync endpoint shares the existing site gRPC h2c listener (8083); no new port. Rig posture: **site-a replicated, site-b/site-c deliberately not**, so both states are proven side-by-side. Status surfaces on the site health report as `LocalDbReplicationConnected` / `LocalDbOplogBacklog` (both nullable — null means "no data", NOT "disconnected with an empty backlog") and as `localdb_*` Prometheus series. **A transient backlog spike is expected daily and is not a fault:** the `site_events` retention purge (and the storage-cap trim) is CDC-captured like any other write — there is deliberately no purge-exemption path — so the backlog jumps by the deleted batch size and drains as the peer acks it; the purge logs an Information note saying so on a replication-enabled node (`SiteEventLogReplicationCheck`, supplied by the Host from `SiteLocalDbSetup.ReplicationIsConfigured` so SiteEventLogging never reads LocalDb config itself). Healthy = `LocalDbReplicationConnected` stays true and the backlog returns to ~0; see `docs/deployment/topology-guide.md` → *Reading the replication backlog*. Note `ZbTelemetryOptions.Meters` is an **allowlist** (`SiteServiceRegistration.ObservedMeters`); an unlisted meter exports nothing, silently.
   - Design: **scadaproj** `docs/plans/2026-07-19-scadabridge-localdb-design.md` (that doc lives in the umbrella repo, not here); Phase 1 plan + Phase 2 gate are here under `docs/plans/`. Phase 2 **deleted** `SiteReplicationActor`, its `ReplicationMessages`, StoreAndForward's `ReplicationService`, and `StoreAndForwardStorage.ReplaceAllAsync` — do not reintroduce them:
 
     - CDC replication does all three jobs now: config deploys reach the standby as ordinary row changes — **the standby makes no fetch at all** during a deploy (`SiteReconciliationActor`'s node-STARTUP fetch when central reports gaps is a different, surviving path) — and buffer mutations replicate via triggers on `sf_messages`. `ReplaceAllAsync` was a destructive delete-all-then-insert-all resync and is **unsafe to reintroduce**: a mass DELETE on a replicated table would be captured and shipped to the peer. LocalDb's snapshot resync merges per row under LWW and never deletes, which is also why the old N1 directional-authority guard is gone — there is no wipe left to gate.

docs/deployment/topology-guide.md

@@ -198,6 +198,50 @@ that drops a table its peer still replicates stops syncing with a schema-mismatc
 diverging silently. Turning it on again later re-baselines, which is what makes the ledger prune on
 deregistration safe.
 
+#### Reading the replication backlog — the daily `site_events` purge burst
+
+The site health report carries `LocalDbReplicationConnected` and `LocalDbOplogBacklog` (nullable —
+**null means "no reading", not "disconnected with an empty backlog"**), and the same numbers export
+as the `localdb_*` Prometheus series (`localdb_oplog_depth` is the backlog gauge). A healthy pair
+sits at a backlog of roughly zero, so a sudden spike naturally reads as a replication problem.
+
+**One expected spike is not a problem: the daily `site_events` retention purge.** `site_events` is
+one of the ten replicated tables, and its retention DELETE is captured by CDC exactly like an
+ordinary write — by design, since LocalDb Phase 2 there is deliberately no purge-exemption path
+(the same property that makes a mass DELETE dangerous, which is why `ReplaceAllAsync` was deleted
+rather than reinstated). One oplog row is therefore queued per deleted event, and the backlog jumps
+by the size of the day's expired batch.
+
+- **When.** Every `ScadaBridge:SiteEventLog:PurgeInterval` (default **24 h**), plus once at
+  startup. The timer is anchored to the **active node's process start**, not to a wall-clock hour,
+  so the burst lands at a different time of day after each failover or restart — do not expect it
+  at a fixed hour. The storage-cap trim (default 1 GB) can produce the same shape off-schedule, and
+  is usually the larger of the two.
+- **Where it shows.** Only on a node with replication configured — the rig's site-a. site-b/site-c
+  have no capture triggers at all and report no backlog for a purge.
+- **What healthy looks like.** `LocalDbReplicationConnected` stays **true** across the spike, and
+  the backlog drains back to ~0 as the peer acks the batch — within seconds to a couple of minutes
+  depending on batch size (delta messages are bounded by `LocalDb:Replication:MaxBatchBytes`,
+  default 2 MB, and secondarily by `MaxBatchSize`). No dead letters, no schema-mismatch errors.
+- **What is actually wrong.** `LocalDbReplicationConnected` **false** while the backlog climbs, a
+  backlog that keeps rising across successive readings rather than draining, or a backlog that
+  never returns near zero between bursts. Those point at the sync stream — an ApiKey mismatch
+  (fail-closed: the pair simply stops converging), an unreachable peer, or an asymmetric
+  registered-table set.
+
+**Correlating it in the log.** When a purge on a replication-enabled node actually deletes rows it
+logs an Information line next to the purge count:
+
+```
+Purged 41230 events older than 30 days
+Purged 41230 site_events rows on a replication-enabled node — a transient LocalDb oplog backlog
+is expected while the deletes replicate to the peer. It drains on its own;
+LocalDbReplicationConnected staying true with the backlog returning to ~0 is the healthy signature.
+```
+
+An unreplicated node logs only the first line. If a backlog spike has no such line near it in the
+active node's log, the purge is *not* the explanation and the spike is worth investigating.
+
 ### Site Pair Upgrades — stop and start BOTH nodes together
 
 **A rolling upgrade of a site pair, one node at a time, is no longer supported.** It worked while

docs/requirements/Component-HealthMonitoring.md

@@ -37,6 +37,8 @@ Site clusters (metric collection and reporting). Central cluster (aggregation an
 | `SiteAuditBacklog` | Audit Log (site) | Count of `Pending` rows in the site-local `AuditLog` plus oldest-pending-age plus on-disk bytes. A configurable threshold drives a Health dashboard warning on the affected site tile. |
 | `SiteAuditWriteFailures` | Audit Log (site) | Count of failed hot-path audit appends at the site since the last health report. |
 | `AuditRedactionFailure` | Audit Log (central) | Count of payload redactor errors (over-redacted payloads, safety-net hit) since the last interval. |
+| `LocalDbReplicationConnected` | Consolidated site LocalDb | Whether a peer sync session is currently running on this node. **Nullable — null means "no reading", not "disconnected".** Reported only by a node with replication configured. |
+| `LocalDbOplogBacklog` | Consolidated site LocalDb | Unacked oplog depth (Prometheus `localdb_oplog_depth`). **Nullable — null means "no reading", NOT "connected with an empty backlog"** (a failed poll rendered as 0 would report a pair that cannot read its own oplog as perfectly healthy). **Expected transient spikes:** the daily `site_events` retention purge and the storage-cap trim are CDC-captured like any other write, so the backlog jumps by the batch size at purge time and drains as the peer acks it — see `Component-SiteEventLogging.md` → Storage and `docs/deployment/topology-guide.md` → *Reading the replication backlog* for the healthy-versus-faulty signature. |
 
 ## Reporting Protocol
 
@@ -113,6 +115,7 @@ These tiles are **point-in-time** like the Notification Outbox and Site Call Aud
 - **Cluster Infrastructure (site)**: Provides node role status.
 - **Notification Outbox (central)**: Provides central-computed outbox KPIs — queue depth, stuck count, parked count — for the headline dashboard tiles.
 - **Site Call Audit (central)**: Provides central-computed cached-call KPIs — buffered count, parked count, failed/delivered (last interval), oldest pending age, stuck count — for the headline dashboard tiles.
+- **Site Event Logging (site)**: Provides `SiteEventLogWriteFailures`. Its daily retention purge and storage-cap trim are also the expected cause of transient `LocalDbOplogBacklog` spikes on a replicated node — see [Component-SiteEventLogging.md](Component-SiteEventLogging.md) → Storage.
 - **Audit Log (#23)**: Provides the site-reported `SiteAuditBacklog` / `SiteAuditWriteFailures` metrics (via the site health report) and the central-computed `AuditRedactionFailure` metric, plus the central audit-row rate feeding the **Audit** dashboard tile group (Audit volume, Audit error rate, Audit backlog).
 
 ## Interactions

docs/requirements/Component-SiteEventLogging.md

@@ -92,8 +92,27 @@ Each event entry contains:
   On a site pair running **without** replication configured (by deliberate choice, e.g. the
   rig's site-b/site-c), the log stays node-local and a failover does start it fresh — the
   documented trade of not configuring a peer, unchanged by this policy.
-- **Retention**: 30 days. A **daily background job** runs on the active node and deletes all events older than 30 days. Hard delete — no archival. (Today, retention/cap deletes on a replicated node are captured by CDC like any other write — see the Volume Policy section above for why this is a small residual cost now that per-run rows are off by default.)
+- **Retention**: 30 days. A **daily background job** runs on the active node and deletes all events older than 30 days. Hard delete — no archival. Deletes are sliced into bounded 1000-row batches per DELETE statement rather than one unbounded statement, so a large expired backlog does not hold the write lock (and every concurrent recorder flush) for the duration of the purge.
 - **Storage cap**: A configurable maximum database size (default: 1 GB) is enforced. If the storage cap is reached before the 30-day retention window, the oldest events are purged first. This prevents disk exhaustion from alarm storms, script failure loops, or connection flapping.
+- **Purge deletes are CDC-captured on a replicated site — the resulting backlog spike is expected.**
+  Retention and cap deletes are ordinary row changes on a replicated table; there is deliberately no
+  purge-exemption path (LocalDb Phase 2 — CDC does all three jobs, and a table-wide exemption is the
+  same mechanism that made `ReplaceAllAsync` unsafe). One oplog row is queued per deleted event, so
+  the site health report's `LocalDbOplogBacklog` (Prometheus `localdb_oplog_depth`) jumps by the size
+  of the batch at purge time and drains as the peer acks it. **Healthy signature:**
+  `LocalDbReplicationConnected` stays true across the spike and the backlog returns to ~0; a backlog
+  that keeps climbing, or climbs while `LocalDbReplicationConnected` is false, is a genuine
+  replication fault and not the purge. The purge logs an Information line naming the row count and
+  the expected transient backlog whenever it deletes rows on a replication-enabled node, so an
+  operator can correlate a spike with the purge that caused it — a spike with no such line nearby is
+  not the purge. Node-local wiring: the Host supplies the replication predicate
+  (`SiteLocalDbSetup.ReplicationIsConfigured` — the `PeerAddress`-OR-`ApiKey` rule) as
+  `SiteEventLogReplicationCheck`; this component never reads LocalDb configuration itself, and with
+  no predicate registered the note is suppressed. Operator detail —
+  including that the purge timer is anchored to the active node's process start, not a wall-clock
+  hour — is in `docs/deployment/topology-guide.md` → *Reading the replication backlog*. See also the
+  Volume Policy section above for why this residual cost is small now that per-run script rows are
+  off by default.
 
 ## Central Access
 
@@ -121,4 +140,4 @@ Each event entry contains:
 - **Communication Layer**: Receives remote queries from central and returns results.
 - **Central UI**: Site Event Log Viewer displays queried events.
 - **Store-and-Forward Engine**: Its notification path (the site→central forward of script-generated notifications) reports forward failures and long-buffered notifications as Notification-category events. Routine enqueue and forward-success events are deliberately not logged — central's authoritative `Notifications` table (owned by the Notification Outbox component) is the audit record of record; site-side logging covers only the in-transit blind spot when central is unreachable.
-- **Health Monitoring**: Script error rates and alarm evaluation error rates can be derived from event log data.
+- **Health Monitoring**: Script error rates and alarm evaluation error rates can be derived from event log data; this component also reports `SiteEventLogWriteFailures`, and its purge is the expected cause of the transient `LocalDbOplogBacklog` spikes described in [Component-HealthMonitoring.md](Component-HealthMonitoring.md) → Monitored Metrics.

src/ZB.MOM.WW.ScadaBridge.Host/SiteLocalDbSetup.cs

@@ -195,6 +195,14 @@ public static class SiteLocalDbSetup
     /// replicates.
     /// </para>
     /// <para>
+    /// <b><c>internal</c>, not private, so there is exactly one copy of this rule.</b>
+    /// <c>SiteServiceRegistration</c> reuses it to supply SiteEventLogging's
+    /// <c>SiteEventLogReplicationCheck</c> — the predicate that decides whether the daily
+    /// <c>site_events</c> purge adds its "expect a transient oplog backlog" operator note.
+    /// A second hand-rolled PeerAddress-OR-ApiKey test would be free to drift out of step
+    /// with the one that actually installs the triggers.
+    /// </para>
+    /// <para>
     /// <b>Both directions of a change to this predicate are now handled at boot</b> (LocalDb
     /// 0.2.0). Flipping it to false deregisters, so a file registered by an older build stops
     /// capturing on the next start instead of paying for triggers forever; flipping it to true
@@ -208,7 +216,7 @@ public static class SiteLocalDbSetup
     /// <c>docs/deployment/topology-guide.md</c>.
     /// </para>
     /// </remarks>
-    private static bool ReplicationIsConfigured(IConfiguration config)
+    internal static bool ReplicationIsConfigured(IConfiguration config)
     {
         var section = config.GetSection("LocalDb:Replication");
 

src/ZB.MOM.WW.ScadaBridge.Host/SiteServiceRegistration.cs

@@ -210,6 +210,18 @@ public static class SiteServiceRegistration
             return () => nodeProvider.SelfIsPrimary;
         });
 
+        // Wording-only companion to the gate above: when a purge on a replication-enabled node
+        // actually deletes rows, it appends an operator note explaining the LocalDbOplogBacklog
+        // spike that follows. site_events is a replicated table and a retention/cap DELETE is
+        // CDC-captured like any other write (LocalDb Phase 2 — no purge-exemption path), so the
+        // backlog genuinely rises at purge time and reads like a replication fault to anyone who
+        // does not know a purge just ran. Delegating to SiteLocalDbSetup.ReplicationIsConfigured
+        // keeps the PeerAddress-OR-ApiKey rule in one place — SiteEventLogging must not learn to
+        // read LocalDb configuration itself.
+        var replicationConfigured = SiteLocalDbSetup.ReplicationIsConfigured(config);
+        SiteEventLogReplicationCheck replicationCheck = () => replicationConfigured;
+        services.AddSingleton(replicationCheck);
+
         // Health checks — the shared ZB.MOM.WW.Health probes, mapped by MapZbHealth on the site's
         // HTTP/1.1 listener (Program.cs). Site nodes served no health endpoints before this; the
         // family overview dashboard probes every node the same way, so a site node has to answer

src/ZB.MOM.WW.ScadaBridge.SiteEventLogging/EventLogPurgeService.cs

@@ -20,6 +20,32 @@ namespace ZB.MOM.WW.ScadaBridge.SiteEventLogging;
 /// <returns><c>true</c> if this node is the active site member and should run the purge; <c>false</c> to skip.</returns>
 public delegate bool SiteEventLogActiveNodeCheck();
 
+/// <summary>
+/// Predicate the <see cref="EventLogPurgeService"/> consults when a purge actually deleted
+/// rows, to decide whether to add the "expect a transient replication backlog" operator note
+/// to its purge log line.
+///
+/// <para>
+/// <c>site_events</c> is one of LocalDb's replicated tables, and CDC captures a retention
+/// DELETE exactly like any other row change — by design, since Phase 2 there is deliberately
+/// no purge-exemption path. So on a replication-enabled node the purge briefly inflates the
+/// oplog (health field <c>LocalDbOplogBacklog</c>, Prometheus <c>localdb_oplog_depth</c>),
+/// which reads like a replication fault to an operator who does not know a purge just ran.
+/// The log line is the breadcrumb that ties the two together; it is a message-wording choice
+/// only and changes no purge behaviour.
+/// </para>
+///
+/// <para>
+/// Registration is the Host's responsibility — SiteEventLogging has no view of
+/// <c>LocalDb:Replication</c> and must not grow one; the Host already owns that predicate
+/// (<c>SiteLocalDbSetup.ReplicationIsConfigured</c>, the <c>PeerAddress</c>-OR-<c>ApiKey</c>
+/// rule) and passes it in. When no implementation is registered the note is suppressed, which
+/// matches the product default: replication is opt-in and off unless configured.
+/// </para>
+/// </summary>
+/// <returns><c>true</c> if this node has LocalDb replication configured; <c>false</c> otherwise.</returns>
+public delegate bool SiteEventLogReplicationCheck();
+
 /// <summary>
 /// Background service that periodically purges old events from the SQLite event log.
 /// Enforces both time-based retention (default 30 days) and storage cap (default 1GB).
@@ -37,6 +63,7 @@ public class EventLogPurgeService : BackgroundService
     private readonly SiteEventLogOptions _options;
     private readonly ILogger<EventLogPurgeService> _logger;
     private readonly SiteEventLogActiveNodeCheck _isActiveNode;
+    private readonly SiteEventLogReplicationCheck _isReplicationConfigured;
 
     /// <summary>Initializes a new instance of <see cref="EventLogPurgeService"/>.</summary>
     /// <param name="eventLogger">The concrete event logger providing lock-guarded database access.</param>
@@ -49,11 +76,18 @@ public class EventLogPurgeService : BackgroundService
     /// the Host on a site node — each tick early-exits on the standby so the
     /// daily purge runs only on the active node, matching the design.
     /// </param>
+    /// <param name="isReplicationConfigured">
+    /// Optional LocalDb-replication check. When <c>null</c> — non-clustered hosts,
+    /// unit tests — the purge log omits the replication-backlog operator note, matching
+    /// the product default that replication is opt-in and off unless configured.
+    /// See <see cref="SiteEventLogReplicationCheck"/>.
+    /// </param>
     public EventLogPurgeService(
         SiteEventLogger eventLogger,
         IOptions<SiteEventLogOptions> options,
         ILogger<EventLogPurgeService> logger,
-        SiteEventLogActiveNodeCheck? isActiveNode = null)
+        SiteEventLogActiveNodeCheck? isActiveNode = null,
+        SiteEventLogReplicationCheck? isReplicationConfigured = null)
     {
         // Depend on the concrete recorder directly: purge must funnel database access
         // through its lock-guarded WithConnection. Taking ISiteEventLogger and
@@ -62,6 +96,7 @@ public class EventLogPurgeService : BackgroundService
         _options = options.Value;
         _logger = logger;
         _isActiveNode = isActiveNode ?? (static () => true);
+        _isReplicationConfigured = isReplicationConfigured ?? (static () => false);
     }
 
     /// <inheritdoc />
@@ -161,9 +196,49 @@ public class EventLogPurgeService : BackgroundService
         if (totalDeleted > 0)
         {
             _logger.LogInformation("Purged {Count} events older than {Days} days", totalDeleted, _options.RetentionDays);
+            LogReplicationBacklogNote(totalDeleted);
         }
     }
 
+    /// <summary>
+    /// Emits the operator breadcrumb that explains the oplog backlog spike a purge produces on a
+    /// replication-enabled node. No-op when this node has no replication peer.
+    /// </summary>
+    /// <remarks>
+    /// <c>site_events</c> is a replicated table, and a retention/cap DELETE is captured by CDC
+    /// exactly like an ordinary write — there is deliberately no purge-exemption path (LocalDb
+    /// Phase 2: CDC does all three jobs). The daily purge therefore pushes one oplog row per
+    /// deleted event, and the site health report's <c>LocalDbOplogBacklog</c> (Prometheus
+    /// <c>localdb_oplog_depth</c>) spikes until the peer acks them. That is expected and drains
+    /// on its own; without this line an operator correlating the spike has nothing in the log to
+    /// tie it to. Information level because it is only interesting next to the purge line it
+    /// follows — the spike itself is not a fault.
+    /// </remarks>
+    /// <param name="deletedRows">Number of rows the purge just deleted; always &gt; 0 at the call sites.</param>
+    private void LogReplicationBacklogNote(int deletedRows)
+    {
+        bool replicated;
+        try
+        {
+            replicated = _isReplicationConfigured();
+        }
+        catch (Exception checkEx)
+        {
+            // A log-wording predicate must never break the purge loop.
+            _logger.LogDebug(checkEx, "Replication check threw while composing the purge log note; note suppressed");
+            return;
+        }
+
+        if (!replicated)
+            return;
+
+        _logger.LogInformation(
+            "Purged {Count} site_events rows on a replication-enabled node — a transient LocalDb oplog backlog " +
+            "is expected while the deletes replicate to the peer. It drains on its own; " +
+            "LocalDbReplicationConnected staying true with the backlog returning to ~0 is the healthy signature.",
+            deletedRows);
+    }
+
     private void PurgeByStorageCap()
     {
         var capBytes = (long)_options.MaxStorageMb * 1024 * 1024;
@@ -180,6 +255,8 @@ public class EventLogPurgeService : BackgroundService
         // The loop also stops if the on-disk size fails to decrease across an
         // iteration (e.g. if vacuum cannot reclaim space), so a cap that can never
         // be met does not silently empty the entire table.
+        var totalDeleted = 0;
+
         while (currentSizeBytes > capBytes)
         {
             var previousSizeBytes = currentSizeBytes;
@@ -214,6 +291,7 @@ public class EventLogPurgeService : BackgroundService
             if (deleted == 0)
                 break;
 
+            totalDeleted += deleted;
             currentSizeBytes = GetDatabaseSizeBytes();
 
             if (currentSizeBytes >= previousSizeBytes)
@@ -228,6 +306,13 @@ public class EventLogPurgeService : BackgroundService
                 break;
             }
         }
+
+        // Same CDC cost as the retention purge above — a cap-driven trim is usually the
+        // larger of the two, so it gets the same operator breadcrumb.
+        if (totalDeleted > 0)
+        {
+            LogReplicationBacklogNote(totalDeleted);
+        }
     }
 
     /// <summary>

src/ZB.MOM.WW.ScadaBridge.SiteEventLogging/ServiceCollectionExtensions.cs

@@ -30,11 +30,18 @@ public static class ServiceCollectionExtensions
         // it unregistered, and the purge defaults to "always run" (the
         // pre-fix behaviour). Building the service via a factory so the
         // optional delegate flows from DI rather than the constructor default.
+        //
+        // SiteEventLogReplicationCheck flows the same way and is likewise Host-supplied:
+        // it only selects the wording of the purge log line (whether to add the
+        // "expect a transient LocalDb oplog backlog" note), and this component has no
+        // view of LocalDb:Replication. Unregistered ⇒ no note, matching the default
+        // that replication is opt-in and off.
         services.AddHostedService(sp => new EventLogPurgeService(
             sp.GetRequiredService<SiteEventLogger>(),
             sp.GetRequiredService<IOptions<SiteEventLogOptions>>(),
             sp.GetRequiredService<ILogger<EventLogPurgeService>>(),
-            sp.GetService<SiteEventLogActiveNodeCheck>()));
+            sp.GetService<SiteEventLogActiveNodeCheck>(),
+            sp.GetService<SiteEventLogReplicationCheck>()));
         return services;
     }
 

tests/ZB.MOM.WW.ScadaBridge.SiteEventLogging.Tests/EventLogPurgeServiceTests.cs

@@ -1,3 +1,4 @@
+using Microsoft.Extensions.Logging;
 using Microsoft.Extensions.Logging.Abstractions;
 using Microsoft.Extensions.Options;
 
@@ -45,14 +46,17 @@ public class EventLogPurgeServiceTests : IDisposable
 
     private EventLogPurgeService CreatePurgeService(
         SiteEventLogOptions? optionsOverride = null,
-        SiteEventLogActiveNodeCheck? isActiveNode = null)
+        SiteEventLogActiveNodeCheck? isActiveNode = null,
+        SiteEventLogReplicationCheck? isReplicationConfigured = null,
+        ILogger<EventLogPurgeService>? logger = null)
     {
         var opts = optionsOverride ?? _options;
         return new EventLogPurgeService(
             _eventLogger,
             Options.Create(opts),
-            NullLogger<EventLogPurgeService>.Instance,
-            isActiveNode);
+            logger ?? NullLogger<EventLogPurgeService>.Instance,
+            isActiveNode,
+            isReplicationConfigured);
     }
 
     private void InsertEventWithTimestamp(DateTimeOffset timestamp)
@@ -455,4 +459,135 @@ public class EventLogPurgeServiceTests : IDisposable
 
         Assert.Equal(0, GetEventCount());
     }
+
+    // ── R7: replication-backlog operator note on the purge log line ──
+
+    /// <summary>
+    /// Fragment unique to the R7 note, matched against the rendered log message.
+    /// Deliberately not the whole sentence — the wording is operator prose and may be
+    /// reworded; what must not regress is that the note fires (or does not) per the
+    /// replication predicate.
+    /// </summary>
+    private const string BacklogNoteFragment = "transient LocalDb oplog backlog";
+
+    [Fact]
+    public void PurgeByRetention_OnReplicatedNode_LogsTheOplogBacklogNote()
+    {
+        // R7: site_events is a replicated table and the retention DELETE is CDC-captured
+        // like any other write — no purge-exemption path by design — so the purge inflates
+        // LocalDbOplogBacklog / localdb_oplog_depth until the peer acks. The Information
+        // line is the breadcrumb that lets an operator tie the spike to the purge instead
+        // of reading it as a replication fault.
+        InsertEventWithTimestamp(DateTimeOffset.UtcNow.AddDays(-31));
+
+        var logger = new CapturingLogger();
+        var purge = CreatePurgeService(isReplicationConfigured: () => true, logger: logger);
+        purge.RunPurge();
+
+        Assert.Equal(0, GetEventCount());
+        var note = Assert.Single(logger.Entries, e => e.Message.Contains(BacklogNoteFragment, StringComparison.Ordinal));
+        Assert.Equal(LogLevel.Information, note.Level);
+    }
+
+    [Fact]
+    public void PurgeByRetention_OnUnreplicatedNode_DoesNotLogTheNote()
+    {
+        // The other half of the gate: site-b/site-c on the rig run with no replication
+        // configured, have no CDC triggers at all, and therefore no backlog to explain.
+        // Emitting the note there would be noise pointing at a metric that reads null.
+        InsertEventWithTimestamp(DateTimeOffset.UtcNow.AddDays(-31));
+
+        var logger = new CapturingLogger();
+        var purge = CreatePurgeService(isReplicationConfigured: () => false, logger: logger);
+        purge.RunPurge();
+
+        Assert.Equal(0, GetEventCount());
+        Assert.DoesNotContain(logger.Entries, e => e.Message.Contains(BacklogNoteFragment, StringComparison.Ordinal));
+    }
+
+    [Fact]
+    public void PurgeByRetention_WithNoRowsDeleted_DoesNotLogTheNote()
+    {
+        // The note is a companion to a purge line, not a tick heartbeat: a daily tick that
+        // deleted nothing produces no oplog rows and so must stay silent, otherwise the
+        // breadcrumb loses all correlating value.
+        InsertEventWithTimestamp(DateTimeOffset.UtcNow);
+
+        var logger = new CapturingLogger();
+        var purge = CreatePurgeService(isReplicationConfigured: () => true, logger: logger);
+        purge.RunPurge();
+
+        Assert.Equal(1, GetEventCount());
+        Assert.DoesNotContain(logger.Entries, e => e.Message.Contains(BacklogNoteFragment, StringComparison.Ordinal));
+    }
+
+    [Fact]
+    public void PurgeByStorageCap_OnReplicatedNode_LogsTheOplogBacklogNote()
+    {
+        // A cap-driven trim deletes rows through the same replicated table and is usually
+        // the larger of the two bursts, so it carries the same note.
+        for (int i = 0; i < 100; i++)
+        {
+            InsertEventWithTimestamp(DateTimeOffset.UtcNow);
+        }
+
+        var capOptions = new SiteEventLogOptions
+        {
+            DatabasePath = _dbPath,
+            RetentionDays = 30,
+            MaxStorageMb = 0  // 0 MB cap forces the cap purge; retention deletes nothing here
+        };
+
+        var logger = new CapturingLogger();
+        var purge = CreatePurgeService(capOptions, isReplicationConfigured: () => true, logger: logger);
+        purge.RunPurge();
+
+        Assert.Equal(0, GetEventCount());
+        var note = Assert.Single(logger.Entries, e => e.Message.Contains(BacklogNoteFragment, StringComparison.Ordinal));
+        Assert.Equal(LogLevel.Information, note.Level);
+    }
+
+    [Fact]
+    public void RunPurge_WhenReplicationCheckThrows_StillPurgesAndSwallows()
+    {
+        // Defensive: the replication predicate only selects log wording. A throw from it
+        // must never escape the purge — the rows are already deleted by the time it is
+        // consulted, and an exception here would surface as "Error during event log purge"
+        // for a purge that in fact succeeded.
+        InsertEventWithTimestamp(DateTimeOffset.UtcNow.AddDays(-31));
+
+        var logger = new CapturingLogger();
+        var purge = CreatePurgeService(
+            isReplicationConfigured: () => throw new InvalidOperationException("boom"),
+            logger: logger);
+        purge.RunPurge();
+
+        Assert.Equal(0, GetEventCount());
+        Assert.DoesNotContain(logger.Entries, e => e.Level == LogLevel.Error);
+        Assert.DoesNotContain(logger.Entries, e => e.Message.Contains(BacklogNoteFragment, StringComparison.Ordinal));
+    }
+
+    /// <summary>
+    /// Minimal <see cref="ILogger{TCategoryName}"/> that records level + rendered message.
+    /// The purge service has no other observable output for a log-only change, and the
+    /// suite has no shared capturing logger to reuse.
+    /// </summary>
+    private sealed class CapturingLogger : ILogger<EventLogPurgeService>
+    {
+        public List<(LogLevel Level, string Message)> Entries { get; } = [];
+
+        public IDisposable? BeginScope<TState>(TState state) where TState : notnull => null;
+
+        public bool IsEnabled(LogLevel logLevel) => true;
+
+        public void Log<TState>(
+            LogLevel logLevel,
+            EventId eventId,
+            TState state,
+            Exception? exception,
+            Func<TState, Exception?, string> formatter)
+        {
+            Entries.Add((logLevel, formatter(state, exception)));
+        }
+    }
 }