Merge fix/localdb-phase1-followups: close both LocalDb Phase 1 live-gate follow-ups

- Oplog growth on default-OFF nodes, fixed at the source: the artifact cache
  now skips the store entirely when the pointer already names this
  deployment/revision, the SHA matches the bytes, and the expected chunk count
  is present. Both extra conditions are load-bearing — guard tests go RED
  against a naive pointer-only skip.
- Wiped-node back-fill, fixed upstream in ZB.MOM.WW.LocalDb 0.1.2 + 0.1.3 and
  pinned here.

Live-gated on docker-dev (docs/plans/2026-07-21-localdb-followups-live-gate.md).
The gate found a third defect that offline tests could not have found first,
because it lived behind the one being fixed: once back-fill worked, the rebuilt
node's own writes turned out to go nowhere. It also surfaced one unrelated
pre-existing bug, filed as issue #485.

Claude-Session: https://claude.ai/code/session_01GASWkNEi68FSCtvr6rLoEW

Directory.Packages.props

@@ -127,8 +127,8 @@
       package floors the native SQLitePCLRaw.lib.e_sqlite3 at 2.1.12, so consumers inherit the
       CVE-2025-6965-fixed native binary without needing the surgical pin below.
     -->
-    <PackageVersion Include="ZB.MOM.WW.LocalDb" Version="0.1.1" />
-    <PackageVersion Include="ZB.MOM.WW.LocalDb.Replication" Version="0.1.1" />
+    <PackageVersion Include="ZB.MOM.WW.LocalDb" Version="0.1.3" />
+    <PackageVersion Include="ZB.MOM.WW.LocalDb.Replication" Version="0.1.3" />
     <PackageVersion Include="ZB.MOM.WW.Health.Akka" Version="0.1.0" />
     <PackageVersion Include="ZB.MOM.WW.Health.EntityFrameworkCore" Version="0.1.0" />
     <PackageVersion Include="ZB.MOM.WW.Telemetry" Version="0.1.0" />

docs/operations/2026-07-20-localdb-pair-replication.md

@@ -72,6 +72,18 @@ The replication library's passive endpoint verifies **no** authentication — th
   If a node is offline **longer than the tombstone-retention window**, a delete that happened during
   its outage may no longer be expressible as a tombstone on rejoin — a pruned deployment could
   briefly reappear until the next deploy re-prunes it. Keep pair outages well inside that window.
+- **A rebuilt node is back-filled by its peer.** If a node loses its LocalDb file — a wiped volume,
+  a re-imaged host, a fresh container — it rejoins with an empty database and its peer snapshots the
+  cached configuration back to it, with no new deploy and no central SQL. Two things are worth
+  knowing: this needs **`ZB.MOM.WW.LocalDb` ≥ 0.1.3**, and it heals the *cache*, so the node regains
+  boot-from-cache for future outages. (On `0.1.1` a converged pair has pruned every oplog row on ack
+  and the wiped node was never snapshotted — it stayed empty until the next deploy. On `0.1.2` it is
+  back-filled but its OWN writes are silently dropped until its restarted seq counter climbs past the
+  peer's stale watermark, so the pair looks converged right up until the moment only the rebuilt node
+  applies a deploy.) A node
+  wiped **during** a central outage is still repopulated by its peer under 0.1.2; a node with **no**
+  peer replication configured self-heals only from central on its next successful apply.
+
 - **What boot-from-cache does NOT cover.** The cache is a *fallback for central-SQL outages at
   boot*, not a replacement for central. A **new deployment still requires central SQL** — the cache
   is only read when the central fetch fails at startup. When central returns, the node resumes

docs/plans/2026-07-20-localdb-phase1-live-gate.md

@@ -33,11 +33,11 @@ Rig: 6 nodes rebuilt from branch `feat/localdb-phase1` @ `ce9fa07f`, all up.
 | 1 | Rig up, site-a pair healthy, `/metrics` shows `localdb_` series | ✅ PASS | Both site-a nodes `/healthz` 200 Healthy; `/metrics` shows `localdb_oplog_depth` + `localdb_sync_reconnects_total` on meter `ZB.MOM.WW.LocalDb.Replication` (allowlist works); site-a-2 served `/localdb_sync.v1.LocalDbSync/Sync` with routing `match_status=success` (interceptor passed, session established); both oplogs depth 0 |
 | 2 | Deploy → both site-a nodes' pointer + artifacts byte-identical, same HLC + origin | ✅ PASS | Deploy `6e687451…` (rev `efb04c79…`) via `POST :9200/api/deployments` → both site-a-1 and site-a-2 `deployment_pointer` = `SITE-A / 6e687451… / efb04c79… / sha EFB04C79…` (identical); artifact 1 chunk each; **pointer `__localdb_row_version` identical on both: hlc `116955620225646592`, origin node_id `a0576df6-…`, tombstone 0** — one row replicated, not two derivations; both oplogs depth 0 |
 | 3 | Boot-from-cache: SQL down, restart site-a-2, serves cached config w/ signal | ✅ PASS (found + fixed a defect) | With SQL stopped, site-a-2 restart logged `RUNNING FROM CACHE — … booted deployment 6e687451…`; **address space materialised from the cached blob**: `AddressSpaceApplier: applied plan (added=18)` / `OpcUaPublish: applied rebuild (added=18)`. Browse confirms **16 nodes on site-a-2 = 16 on site-a-1**. **Defect the gate caught (`9137cb41`):** pre-fix the rebuild re-read the artifact from the down ConfigDb and no-op'd, so the cache-booted node served **1** node vs the healthy peer's 16 — it logged success but browsed empty. Fixed by passing the in-hand cached blob to `RebuildAddressSpace`; regression test added |
-| 4 | Replicated-cache payoff: wipe a-2 DB, repopulate, then SQL-down boot | ⚠️ PARTIAL — 1 limitation documented, 1 defect fixed | Wiped a-2's LocalDb volume, restarted with **SQL down**: sync session re-established (a-2 served a `Sync` call) but a-2's cache stayed **0/0/0** and a-1's oplog was **0** — the deploy's rows had been acked by the old a-2 and pruned, so there was no delta to send and **no snapshot-resync fired**. ⇒ **Limitation: pure replication does not back-fill a fully-wiped, already-converged node** (library delta-replication; snapshot-resync is gated on the oplog cap, which the running engine never hits). Separately found a **defect (`c6a9f93a`)**: recovery via `RestoreApplied` never wrote the cache (only fresh `ApplyAndAck` did), so a wiped node stayed cache-less. Fixed: `RestoreApplied` now re-caches. Verified — after the fix, wiped a-2 restarted with SQL up: `restored served state … on bootstrap` → cache repopulated `SITE-A / 6e687451`, chunks=1. **Net: a wiped node self-heals its cache from central on next boot (regaining boot-from-cache for future outages); it is not back-filled by its peer during the same outage.** |
+| 4 | Replicated-cache payoff: wipe a-2 DB, repopulate, then SQL-down boot | ⚠️ PARTIAL — 1 defect fixed; the limitation is since **CLOSED** (LocalDb 0.1.2 — see Summary) | Wiped a-2's LocalDb volume, restarted with **SQL down**: sync session re-established (a-2 served a `Sync` call) but a-2's cache stayed **0/0/0** and a-1's oplog was **0** — the deploy's rows had been acked by the old a-2 and pruned, so there was no delta to send and **no snapshot-resync fired**. ⇒ **Limitation: pure replication does not back-fill a fully-wiped, already-converged node** (library delta-replication; snapshot-resync is gated on the oplog cap, which the running engine never hits). Separately found a **defect (`c6a9f93a`)**: recovery via `RestoreApplied` never wrote the cache (only fresh `ApplyAndAck` did), so a wiped node stayed cache-less. Fixed: `RestoreApplied` now re-caches. Verified — after the fix, wiped a-2 restarted with SQL up: `restored served state … on bootstrap` → cache repopulated `SITE-A / 6e687451`, chunks=1. **Net: a wiped node self-heals its cache from central on next boot (regaining boot-from-cache for future outages); it is not back-filled by its peer during the same outage.** |
 | 5 | Transport kill: pause a-2, oplog rises; unpause, drains to 0 | ✅ PASS | `docker pause` on a-2 severed the sync transport; during the window a-2 accumulated **oplog=2** unacked local writes (its re-cache) while a-1 held 0. `docker unpause` → a-2's oplog **drained to 0 within 5s**, both converged on the same pointer. Note: the rig's redeploys carry an identical revision, so a fresh cache write on a-1 short-circuits (no-op) — the store-during-partition variant is covered by the automated harness scenario `WritesWhileTransportDown_SurviveRejoin` (proven red without `RegisterReplicated`) |
 | 6 | Retention: 3rd deploy prunes oldest; chunks gone + tombstones on BOTH nodes | ✅ PASS | Cached 3 distinct deployment-ids (via deploy + restart re-cache). Both nodes retain exactly the newest 2 (`475df87a`, `962962dc`); the oldest `6e687451`'s chunks are **0 on both** (pruned); **1 artifact tombstone on both** — the prune replicated as a tombstone, not left as a live chunk |
 | 7 | Both-nodes-together restart: clean rejoin, identical counts, no I/O errors | ✅ PASS | Restarted site-a-1 + site-a-2 together: **0** `disk I/O error`/`SQLITE_IOERR`/`corrupt` lines in either log; both re-cached and converged to **identical** counts (ptr `962962dc…`, chunks=2, rowver=3). Byte-identical convergence preserved across a simultaneous restart |
-| 8 | site-b default-OFF pin: cache works locally, Healthy, no sync listener on 9001 | ✅ PASS (+1 finding) | site-b-1 cache works locally (`SITE-B / 6e687451`, chunks=1, from its own apply); listeners **`[::]:8080` only — no 9001**; port 9001 **not bound** (`/proc/net/tcp`); `/healthz` **200**; no sync/reconnect metric series. **Finding (follow-up, not a blocker):** `localdb_oplog_depth=5` — because `OnReady` registers replication unconditionally, a default-OFF node captures cache writes into its oplog but has no peer to ack/prune them, so the oplog grows slowly (≈2 rows/deploy, amplified by the re-cache-on-restore fix). Bounded by the library `MaxOplogRows` cap (1M); worth a periodic pruner or skip-if-unchanged in a follow-up |
+| 8 | site-b default-OFF pin: cache works locally, Healthy, no sync listener on 9001 | ✅ PASS (+1 finding, since **CLOSED** by the skip-if-unchanged cache write — see Summary) | site-b-1 cache works locally (`SITE-B / 6e687451`, chunks=1, from its own apply); listeners **`[::]:8080` only — no 9001**; port 9001 **not bound** (`/proc/net/tcp`); `/healthz` **200**; no sync/reconnect metric series. **Finding (follow-up, not a blocker):** `localdb_oplog_depth=5` — because `OnReady` registers replication unconditionally, a default-OFF node captures cache writes into its oplog but has no peer to ack/prune them, so the oplog grows slowly (≈2 rows/deploy, amplified by the re-cache-on-restore fix). Bounded by the library `MaxOplogRows` cap (1M); worth a periodic pruner or skip-if-unchanged in a follow-up |
 
 ## Summary
 
@@ -56,13 +56,38 @@ missed**, each now fixed on-branch with a regression test:
 4. **Cache not repopulated on `RestoreApplied`** (`c6a9f93a`) — a wiped/fresh-volume node recovered its
    served state from central but stayed cache-less, unable to boot-from-cache on the next outage.
 
-**Two documented limitations (follow-ups, not blockers):**
+**Two documented limitations (follow-ups, not blockers) — both since CLOSED:**
 
 - **No replication back-fill of a fully-wiped node** (check 4): a peer's already-acked cache rows are
   pruned from its oplog and snapshot-resync is gated on the (never-hit) oplog cap, so a wiped node is
   not healed by its peer — it self-heals from central on next boot instead.
+  → **FIXED in `ZB.MOM.WW.LocalDb` 0.1.2 + 0.1.3** (scadaproj `cad3bcb` + `3b7489a`), live-gated in
+  `2026-07-21-localdb-followups-live-gate.md`. The diagnosis in this doc was
+  slightly off: the cap was not the gate. `ComputeSnapshotRequiredAsync` measured the peer's gap
+  against the *oldest surviving oplog row* and read an empty oplog as "no gap possible" — and an
+  empty oplog is the **steady state of a converged pair**, since ack-pruning deletes everything the
+  peer has confirmed. So the healthy state was the one state that could not heal a wiped peer. With
+  an empty oplog the gap is now measured against `last_acked_seq`. Covered at both levels:
+  `SnapshotResyncTests.WipedPeer_AfterEverythingWasAckedAndPruned_StillGetsSnapshot` in the library,
+  and `LocalDbPairConvergenceTests.WipedNode_IsBackFilledByItsPeer_WithoutAnyNewDeploy` here —
+  the latter verified RED against the pinned 0.1.1 and green on 0.1.2. **0.1.3 closes a second,
+  deeper half that only became reachable once back-fill worked:** the rebuilt node's OWN writes were
+  silently dropped, because `last_applied_remote_seq` is a watermark in the *peer's* seq space and a
+  rebuilt peer numbers from 1 again. The follow-up live gate caught that one — offline tests could
+  not have, since the scenario had no test while it was still a documented limitation.
 - **Oplog growth on default-OFF nodes** (check 8): `OnReady` registers replication unconditionally, so
   a node with no peer accumulates unacked/unpruned oplog rows (~2/deploy). Slow, bounded by the 1M cap;
   a periodic pruner or a skip-if-unchanged cache write would close it.
+  → **FIXED at the source**: `LocalDbDeploymentArtifactCache.StoreAsync` now **skips entirely** when
+  the pointer already names this deployment/revision, the SHA matches the bytes, and the expected
+  chunk count is present — so re-caching an artifact the node already holds (every restart's
+  boot-from-cache, every `RestoreApplied`) writes nothing and mints no oplog rows. Two guard tests
+  pin what the skip must NOT do (stale bytes under an unchanged revision; a matching pointer over
+  missing chunks), both verified RED against a naive pointer-only skip.
+  Also correcting this row's bound: the growth was never headed for the 1M cap. `AddZbLocalDbReplication`
+  is registered unconditionally, so `MaintenanceBackgroundService` runs on default-OFF nodes too and
+  the **7-day `MaxOplogAge`** cap prunes — the true bound was ~7 days of writes, not 1M rows.
 
-Not merged, per the plan. The two limitations are recorded as Phase 1 follow-ups.
+Merged to `master` as `c957db52` (2026-07-21) and pushed. Both follow-ups closed on
+`fix/localdb-phase1-followups` (2026-07-21) and live-gated — see
+`2026-07-21-localdb-followups-live-gate.md`, which found a third defect in the process.

docs/plans/2026-07-21-localdb-followups-live-gate.md

@@ -0,0 +1,68 @@
+# LocalDb Phase 1 follow-ups — live gate (docker-dev rig)
+
+> Closes the two limitations left open by `2026-07-20-localdb-phase1-live-gate.md` (checks 4 and 8).
+> Rig: local `docker-dev/docker-compose.yml`, 6 nodes rebuilt from `fix/localdb-phase1-followups`.
+> site-a pair replicates; central + site-b stay default-OFF.
+>
+> **Safety rules followed** (same as the Phase 1 gate): all DB inspection via `docker cp` of the
+> `db`/`-wal`/`-shm` triplet, querying the copy — never host `sqlite3` on the live WAL file.
+
+## Result
+
+**Both follow-ups closed, and the gate found a third defect** — one that was *unreachable* until the
+first fix landed. Library `ZB.MOM.WW.LocalDb` went `0.1.1` → `0.1.2` → `0.1.3` in the course of this
+gate; both consumers (OtOpcUa, ScadaBridge) are pinned to `0.1.3`.
+
+| # | Check | Result | Evidence |
+|---|---|---|---|
+| 8 | Oplog stops growing on a default-OFF node | ✅ PASS | site-b-1 restarted twice with an unchanged cached artifact: `oplog_depth` **7 → 7 → 7**, `deployment_pointer.applied_at_utc` frozen at `05:34:58` across both — the re-cache was skipped outright. Repeated on the `0.1.3` build after a real deploy: **13 → 13**. Pre-fix this grew ~2 rows per restart forever, because a default-OFF node has no peer to ack and prune them. |
+| 8b | …but a genuinely new deployment still writes (positive control) | ✅ PASS | A real config change (renamed tag `SITEA-tag`) produced rev `822bdf34`, then `002c16b3`. site-b-1's oplog **7 → 10 → 13** and its pointer advanced each time. The skip suppresses *identical* re-writes only; it is not a dead cache-write path. |
+| 4 | A wiped node is back-filled by its peer, no new deploy, central down | ✅ PASS (was ⚠️ PARTIAL) | Precondition asserted first — site-a-1's oplog **0**, `needs_snapshot 0`: the exact state that used to make back-fill impossible. Then SQL stopped, site-a-2's LocalDb **volume destroyed**, container recreated. a-1 logged `Snapshot sent (as_of_seq 0, 4 rows)` — a line that could not appear on `0.1.1`. a-2 came back with pointer `SITE-A / 475df87a @ 03:09:12` (a-1's **original** timestamp, not a re-derived one), chunks 2, and a `__localdb_row_version` dump **byte-identical to a-1's**, carrying origin node ids `6812ca9c` and `935659a5` — the rebuilt node's own id appears nowhere, so the rows were replicated, not recreated. |
+| 4b | …and the back-filled node then boots from cache | ✅ PASS | With SQL still down, site-a-2 logged `RUNNING FROM CACHE — … booted deployment 475df87a… cached at 2026-07-21T03:09:12`, `raw subtree materialised (containers=16, tags=1)`. OPC UA browse of both nodes: **17 ns=2 nodes each, diff-identical** — the rebuilt node serves exactly what its healthy peer does, from a configuration it never applied itself and could not have fetched. |
+| 4c | The rebuilt node's OWN writes reach its peer | ✅ PASS **after a third fix** (see below) | On `0.1.2` this failed: a-1 held `last_applied_remote = 10` while the rebuilt a-2's oplog was seqs **1,2,3** and its `last_acked` stayed **0** — never accepted. On `0.1.3`, a-1 logs `Replication peer identity changed (ebb10854 -> c85d8b1d): the peer was rebuilt … Resetting the inbound watermark`, and after the next deploy a-1 shows `last_applied_remote = 3` / a-2 shows `last_acked = 3` with a drained oplog and identical row_versions. |
+
+## The third defect (the reason this gate was worth running)
+
+`0.1.2` made back-fill work, which for the first time let a rebuilt node be observed *writing*. It
+turned out its writes went nowhere.
+
+`last_applied_remote_seq` is a watermark in the **peer's** seq space, and a rebuilt peer is a new
+database whose oplog numbers from 1. Two independent bugs stacked, and fixing either alone leaves the
+data stuck:
+
+1. The healthy node kept advertising the **old** peer's watermark. The *sending* side seeds its pump
+   directly from that number (`_sentThruSeq`), so the rebuilt node skipped its entire oplog.
+   → the watermark (and the observed peer clock) is now reset when the peer's node id changes.
+   `last_acked_seq` is deliberately **not** reset: it describes our own oplog's pruned horizon and is
+   exactly what `0.1.2` keys the snapshot decision off — clearing it would switch back-fill back off.
+2. A peer claiming to have applied more of our stream than we have ever produced can only be
+   remembering a previous incarnation of *us*. → that claim is now rejected in favour of pumping from
+   the start; LWW makes the re-send harmless. This mirrors the clamp `RecordPeerAckAsync` already
+   applies to acks.
+
+Both were reproduced offline as a RED test before fixing
+(`SnapshotResyncTests.RebuiltPeer_OwnWritesReachTheHealthyNode_DespiteTheOldPeersWatermark`).
+
+**Why offline tests could not have found it first:** the wiped-peer scenario had no test because the
+scenario was believed impossible to recover from — it was a *documented limitation*, not a bug. The
+gate is what turned the limitation into a reproducible case, and the second defect only existed
+behind the first.
+
+## Unrelated finding (NOT a follow-up of this work — pre-existing; filed as `lmxopcua` issue #485)
+
+While SQL was being stopped and started to drive check 4, site-b-1 took a deploy whose artifact load
+hit a transient ConfigDb error, and **emptied its served address space**:
+
+```
+[06:11:20 ERR] An error occurred using the connection to database 'OtOpcUa' on server 'sql,1433'.
+[06:11:20 WRN] OpcUaPublish: failed to load artifact for deployment 43d54397…; rebuild becomes no-op
+[06:11:20 INF] AddressSpaceApplier: applied plan (kind=PureRemove, added=0, removed=16, rebuild=True)
+```
+
+The log says "rebuild becomes no-op" and the applier then removed all 16 nodes — those two disagree.
+The cache layer behaved correctly and refused to store the bad artifact ("not caching deployment … its
+artifact was empty or could not be loaded, so it is not a usable last-known-good configuration"), and
+the node recovered fully on restart (`containers=16`). But a transient ConfigDb blip emptying the
+served address space instead of holding last-known-good is the same failure *class* as the Phase 1
+gate's check-3 defect. Untouched by this work — site-b-1 has replication off entirely — and induced by
+this gate's own SQL flapping.

src/Server/ZB.MOM.WW.OtOpcUa.Runtime/Deployment/LocalDbDeploymentArtifactCache.cs

@@ -63,6 +63,14 @@ public sealed class LocalDbDeploymentArtifactCache : IDeploymentArtifactCache
         var cachedAtUtc = FormatTimestamp(DateTimeOffset.UtcNow);
         var chunkCount = (artifact.Length + ChunkSize - 1) / ChunkSize;
 
+        if (await IsAlreadyCachedAsync(clusterId, deploymentId, revisionHash, sha, chunkCount, ct))
+        {
+            _logger.LogDebug(
+                "Deployment {DeploymentId} (revision {RevisionHash}) is already cached intact for cluster {ClusterId}; skipping the re-store.",
+                deploymentId, revisionHash, clusterId);
+            return;
+        }
+
         // One transaction for the whole store. A pointer that commits without its chunks — or
         // chunks that commit without the pointer — is a cache that reads back as a corrupt hit
         // rather than a clean miss, which is the one outcome this type exists to prevent.
@@ -155,6 +163,74 @@ public sealed class LocalDbDeploymentArtifactCache : IDeploymentArtifactCache
         await tx.CommitAsync(ct);
     }
 
+    /// <summary>
+    ///     Whether this exact artifact is already cached <b>and readable</b> for the cluster, so
+    ///     storing it again would rewrite every row to its current value.
+    /// </summary>
+    /// <remarks>
+    ///     <para>
+    ///         <b>Why this exists.</b> Every write to a replicated table fires a capture trigger and
+    ///         mints an oplog row. Re-caching an artifact the node already holds — which is what a
+    ///         restart's boot-from-cache and every <c>RestoreApplied</c> recovery does — would
+    ///         therefore cost a delete plus an insert per chunk plus a pointer update, all carrying
+    ///         values identical to what is already there. On a replicating pair that is pure churn;
+    ///         on an unpaired (default-OFF) node there is no peer to ack those rows, so they
+    ///         accumulate in the oplog until the library's age cap prunes them. The Phase 1 live gate
+    ///         saw exactly that (check 8).
+    ///     </para>
+    ///     <para>
+    ///         <b>Identity is over the bytes, not the ids.</b> The pointer must match on
+    ///         <c>artifact_sha256</c> as well as the deployment id and revision hash: a re-composed
+    ///         artifact can legitimately carry the same ids with different bytes, and keeping the
+    ///         stale copy would serve a boot-from-cache node the wrong address space while every
+    ///         id-shaped signal claimed it was right.
+    ///     </para>
+    ///     <para>
+    ///         <b>The chunk count is load-bearing.</b> A pointer can name a deployment whose chunks
+    ///         are missing or partial — a half-replicated artifact, or a prune that raced the
+    ///         pointer. Skipping on the pointer alone would make that state permanent, because the
+    ///         re-store that repairs it is the call being skipped.
+    ///     </para>
+    ///     <para>
+    ///         <b>Accepted consequence:</b> a skipped store leaves <c>applied_at_utc</c> at the
+    ///         moment the artifact was <i>first</i> cached rather than last confirmed. Refreshing it
+    ///         would mint the very oplog row this check exists to avoid. Nothing reads it as a
+    ///         liveness signal; retention ranks distinct deployments, which are unaffected.
+    ///     </para>
+    /// </remarks>
+    private async Task<bool> IsAlreadyCachedAsync(
+        string clusterId, string deploymentId, string revisionHash, string sha, int chunkCount,
+        CancellationToken ct)
+    {
+        var matches = await _db.QueryAsync(
+            """
+            SELECT (
+                SELECT COUNT(*)
+                FROM deployment_artifacts
+                WHERE cluster_id = @ClusterId
+                  AND deployment_id = @DeploymentId
+                  AND chunk_count = @ChunkCount
+            )
+            FROM deployment_pointer
+            WHERE cluster_id = @ClusterId
+              AND deployment_id = @DeploymentId
+              AND revision_hash = @RevisionHash
+              AND artifact_sha256 = @Sha
+            """,
+            r => r.GetInt32(0),
+            new
+            {
+                ClusterId = clusterId,
+                DeploymentId = deploymentId,
+                RevisionHash = revisionHash,
+                Sha = sha,
+                ChunkCount = chunkCount,
+            },
+            ct);
+
+        return matches.Count == 1 && matches[0] == chunkCount;
+    }
+
     /// <inheritdoc/>
     public async Task<CachedDeploymentArtifact?> GetCurrentAsync(
         string clusterId, CancellationToken ct = default)

tests/Server/ZB.MOM.WW.OtOpcUa.Host.IntegrationTests/LocalDb/LocalDbPairConvergenceTests.cs

@@ -103,6 +103,54 @@ public sealed class LocalDbPairConvergenceTests
             "node B to prune dep1 (with tombstones) while keeping dep3");
     }
 
+    [Fact]
+    public async Task WipedNode_IsBackFilledByItsPeer_WithoutAnyNewDeploy()
+    {
+        // The Phase 1 live gate's check 4, as an offline scenario. A node whose LocalDb volume is
+        // lost used to come back permanently empty: the pair had already converged, so every oplog
+        // row was acked and pruned, and the healthy node had no delta left to send. Nothing short of
+        // a fresh deploy could repopulate it — which is the worst possible moment to depend on
+        // central being reachable, since the cache exists precisely for when it is not.
+        //
+        // Fixed in ZB.MOM.WW.LocalDb 0.1.2 (snapshot resync now measures the peer's gap against
+        // last_acked_seq when the oplog is empty). Pin the payoff at this level, not just the
+        // library's: what matters here is that the deployment artifact itself comes back.
+        await using var harness = new LocalDbPairHarness();
+        await harness.StartAsync();
+
+        var deploymentId = Guid.NewGuid().ToString("N");
+        var artifact = MultiChunkArtifact(seed: 20);
+        await harness.CacheA.StoreAsync(Cluster, deploymentId, "rev-1", artifact,
+            TestContext.Current.CancellationToken);
+
+        await LocalDbPairHarness.WaitUntilAsync(
+            async () => await ReadArtifactAsync(harness.CacheB, Cluster) is { } b && b.AsSpan().SequenceEqual(artifact),
+            "the artifact to converge before the wipe");
+
+        // Convergence alone is not the precondition — an emptied oplog on A is. That is the state
+        // that used to make back-fill impossible, so assert it rather than assume it.
+        await LocalDbPairHarness.WaitUntilAsync(
+            async () => await LocalDbPairHarness.OplogDepthAsync(harness.A) == 0,
+            "node A's oplog to be fully acked and pruned");
+
+        await harness.WipePassiveAsync();
+        Assert.Null(await ReadArtifactAsync(harness.CacheB, Cluster));
+
+        await harness.RestartPairAsync();
+
+        // No new deploy, no central: the rebuilt node gets its cached configuration back from its
+        // peer alone, byte-identical, and can boot from cache again.
+        await LocalDbPairHarness.WaitUntilAsync(
+            async () =>
+            {
+                var cached = await harness.CacheB.GetCurrentAsync(Cluster, TestContext.Current.CancellationToken);
+                return cached is not null
+                       && cached.DeploymentId == deploymentId
+                       && cached.Artifact.AsSpan().SequenceEqual(artifact);
+            },
+            "the wiped node to be back-filled from its peer");
+    }
+
     [Fact]
     public async Task WritesWhileTransportDown_SurviveRejoin()
     {

tests/Server/ZB.MOM.WW.OtOpcUa.Host.IntegrationTests/LocalDb/LocalDbPairHarness.cs

@@ -69,10 +69,13 @@ public sealed class LocalDbPairHarness : IAsyncDisposable
     private readonly string _apiKeyB;
 
     private readonly string _pathA = Path.Combine(Path.GetTempPath(), $"otopcua-pairA-{Guid.NewGuid():N}.db");
-    private readonly string _pathB = Path.Combine(Path.GetTempPath(), $"otopcua-pairB-{Guid.NewGuid():N}.db");
+
+    // Not readonly: WipePassiveAsync replaces node B's database wholesale, which is what a rebuilt
+    // node with a lost volume actually is — a new file, and with it a new node id.
+    private string _pathB = Path.Combine(Path.GetTempPath(), $"otopcua-pairB-{Guid.NewGuid():N}.db");
 
     private readonly ServiceProvider _dbProviderA;
-    private readonly ServiceProvider _dbProviderB;
+    private ServiceProvider _dbProviderB;
 
     private IHost? _serverHost;      // node B — passive
     private IHost? _initiatorHost;   // node A — dials the peer
@@ -125,6 +128,30 @@ public sealed class LocalDbPairHarness : IAsyncDisposable
         _serverHost = null;
     }
 
+    /// <summary>
+    ///     Destroys node B's database and replaces it with an empty one, leaving the pairing config
+    ///     untouched — the rebuilt-node case: a lost volume, a re-imaged host, a fresh container.
+    /// </summary>
+    /// <remarks>
+    ///     A new file, not a truncated one, because that is what the real thing is: the node id lives
+    ///     in the database, so a rebuilt node comes back with a new identity and a zero watermark. The
+    ///     caller follows with <see cref="RestartPairAsync"/> to bring the wiped node back online.
+    /// </remarks>
+    public async Task WipePassiveAsync()
+    {
+        await StopPassiveAsync();
+        await _dbProviderB.DisposeAsync();
+
+        // Pooled connections outlive the provider; without this the delete silently no-ops on
+        // Windows and the "wiped" node would still hold its rows.
+        Microsoft.Data.Sqlite.SqliteConnection.ClearAllPools();
+        DeleteDatabaseFiles(_pathB);
+
+        _pathB = Path.Combine(Path.GetTempPath(), $"otopcua-pairB-{Guid.NewGuid():N}.db");
+        _dbProviderB = BuildDatabaseProvider(_pathB);
+        _ = B;
+    }
+
     /// <summary>
     ///     Brings node B back on a NEW loopback port and re-dials from A. The initiator re-reads the
     ///     peer address on each reconnect, so this is a genuine rejoin over the same databases.
@@ -201,6 +228,17 @@ public sealed class LocalDbPairHarness : IAsyncDisposable
         return rows[0];
     }
 
+    /// <summary>
+    ///     Counts oplog rows on a node. Zero means every local change has been acked by the peer and
+    ///     pruned — the steady state of a converged pair, and the state from which a wiped peer can
+    ///     only be healed by a snapshot.
+    /// </summary>
+    public static async Task<long> OplogDepthAsync(ILocalDb db)
+    {
+        var rows = await db.QueryAsync("SELECT COUNT(*) FROM __localdb_oplog", static r => r.GetInt64(0));
+        return rows[0];
+    }
+
     /// <summary>Counts tombstone rows in <c>__localdb_row_version</c> for one table on a node.</summary>
     public static async Task<long> TombstoneCountAsync(ILocalDb db, string table)
     {
@@ -313,12 +351,16 @@ public sealed class LocalDbPairHarness : IAsyncDisposable
         await _dbProviderB.DisposeAsync();
 
         Microsoft.Data.Sqlite.SqliteConnection.ClearAllPools();
-        foreach (var path in new[] { _pathA, _pathB })
+        DeleteDatabaseFiles(_pathA);
+        DeleteDatabaseFiles(_pathB);
+    }
+
+    /// <summary>Deletes a SQLite database and its WAL/shm sidecars, best effort.</summary>
+    private static void DeleteDatabaseFiles(string path)
+    {
+        foreach (var suffix in new[] { "", "-wal", "-shm" })
         {
-            foreach (var suffix in new[] { "", "-wal", "-shm" })
-            {
-                try { File.Delete(path + suffix); } catch { /* best effort */ }
-            }
+            try { File.Delete(path + suffix); } catch { /* best effort */ }
         }
     }
 }

tests/Server/ZB.MOM.WW.OtOpcUa.Runtime.Tests/Deployment/LocalDbDeploymentArtifactCacheTests.cs

@@ -90,6 +90,23 @@ public sealed class LocalDbDeploymentArtifactCacheTests : IDisposable
             "SELECT DISTINCT deployment_id FROM deployment_artifacts ORDER BY deployment_id",
             r => r.GetString(0));
 
+    /// <summary>
+    ///     Total capture-trigger rows in the replication oplog — the direct measure of how much
+    ///     replication traffic (and, on an unpaired node, how much permanent oplog growth) a store
+    ///     costs.
+    /// </summary>
+    private async Task<long> OplogDepthAsync()
+        => (await _db.QueryAsync("SELECT COUNT(*) FROM __localdb_oplog", r => r.GetInt64(0)))[0];
+
+    /// <summary>
+    ///     The LWW version stamps of every replicated row, as a stable ordered projection. Comparing
+    ///     it across a call proves whether that call touched replicated state at all.
+    /// </summary>
+    private async Task<IReadOnlyList<string>> RowVersionsAsync()
+        => await _db.QueryAsync(
+            "SELECT table_name, pk_json, hlc FROM __localdb_row_version ORDER BY table_name, pk_json",
+            r => $"{r.GetString(0)}|{r.GetString(1)}|{r.GetInt64(2)}");
+
     [Fact]
     public async Task StoreThenGetCurrent_RoundTripsAMultiChunkArtifactByteForByte()
     {
@@ -221,6 +238,77 @@ public sealed class LocalDbDeploymentArtifactCacheTests : IDisposable
         cached.Artifact.ShouldBe(second);
     }
 
+    [Fact]
+    public async Task Store_WritesNothingWhenTheCachedArtifactIsAlreadyIdentical()
+    {
+        // Phase 1 live gate, check 8: every store rewrote the chunks and the pointer unconditionally,
+        // so re-caching an artifact the node already holds — which is what boot-from-cache and
+        // RestoreApplied both do on every restart — minted fresh oplog rows for a row nobody changed.
+        // On a replicating pair that is pure churn; on a default-OFF node there is no peer to ack
+        // them, so they sit in the oplog until the age cap eventually prunes them.
+        var cache = CreateCache();
+        var artifact = RandomArtifact(300 * 1024);
+
+        await cache.StoreAsync(ClusterA, "dep-1", "rev-1", artifact);
+
+        var oplogDepth = await OplogDepthAsync();
+        var rowVersions = await RowVersionsAsync();
+
+        await cache.StoreAsync(ClusterA, "dep-1", "rev-1", artifact);
+
+        (await OplogDepthAsync()).ShouldBe(oplogDepth);
+        (await RowVersionsAsync()).ShouldBe(rowVersions);
+
+        // Skipping must not cost readability: the artifact still reads back intact.
+        var cached = await cache.GetCurrentAsync(ClusterA);
+        cached.ShouldNotBeNull();
+        cached.DeploymentId.ShouldBe("dep-1");
+        cached.Artifact.ShouldBe(artifact);
+    }
+
+    [Fact]
+    public async Task Store_RewritesWhenTheCachedChunksAreIncompleteDespiteAMatchingPointer()
+    {
+        // The dangerous half of skip-if-unchanged. A pointer can name a deployment whose chunks are
+        // gone or partial — a half-replicated artifact, or a prune that raced the pointer. Trusting
+        // the pointer alone would turn a repairable cache into a permanently unreadable one, because
+        // the re-store that would have fixed it is exactly the call being skipped.
+        var cache = CreateCache();
+        var artifact = RandomArtifact(300 * 1024);
+
+        await cache.StoreAsync(ClusterA, "dep-1", "rev-1", artifact);
+        await _db.ExecuteAsync(
+            "DELETE FROM deployment_artifacts WHERE deployment_id = @DeploymentId AND chunk_index = 2",
+            new { DeploymentId = "dep-1" });
+        (await cache.GetCurrentAsync(ClusterA)).ShouldBeNull();
+
+        await cache.StoreAsync(ClusterA, "dep-1", "rev-1", artifact);
+
+        (await CountChunksAsync("dep-1")).ShouldBe(3);
+        var cached = await cache.GetCurrentAsync(ClusterA);
+        cached.ShouldNotBeNull();
+        cached.Artifact.ShouldBe(artifact);
+    }
+
+    [Fact]
+    public async Task Store_RewritesWhenTheArtifactBytesChangedUnderAnUnchangedRevision()
+    {
+        // The identity check must be over the bytes, not just the ids. A rebuilt artifact carrying
+        // the same deployment id and revision hash is a real case (the same revision re-composed),
+        // and silently keeping the stale bytes would serve a boot-from-cache node the wrong address
+        // space with every id-shaped signal saying it was correct.
+        var cache = CreateCache();
+
+        await cache.StoreAsync(ClusterA, "dep-1", "rev-1", RandomArtifact(1024));
+
+        var rebuilt = RandomArtifact(1024);
+        await cache.StoreAsync(ClusterA, "dep-1", "rev-1", rebuilt);
+
+        var cached = await cache.GetCurrentAsync(ClusterA);
+        cached.ShouldNotBeNull();
+        cached.Artifact.ShouldBe(rebuilt);
+    }
+
     [Fact]
     public async Task GetCurrentUnkeyed_ReturnsTheOnlyCachedArtifact()
     {