# LocalDb pair replication — operations runbook

> **Phase 1 scope.** Every driver-role OtOpcUa node keeps a consolidated
> [`ZB.MOM.WW.LocalDb`](../../CLAUDE.md) SQLite database. Today it caches exactly one thing: the
> **deployed-configuration artifact**, chunked, so a node can **boot from cache when central SQL
> Server is unreachable**. That cache can *optionally* replicate to the node's redundant pair peer
> over the library's gRPC sync. **Replication is default-OFF and fail-closed.** This runbook covers
> enabling it, the operational rules that keep a pair converging, and the DB-inspection safety
> rules.

## What replicates, and what it buys you

- **Replicated tables:** `deployment_artifacts` (the artifact, split into base64 chunks) and
  `deployment_pointer` (one current-deployment pointer per cluster). Registered — in this order,
  after the DDL — by `LocalDbSetup.OnReady`.
- **The payoff:** in a redundant driver pair, *either* node can be the one that applied the latest
  deploy and wrote the cache. With replication on, the peer holds a byte-identical copy. So a node
  that restarts into a central-SQL outage can boot the current config **even if it never applied
  that deploy itself** — its peer did, and the row replicated.
- **Convergence model:** last-writer-wins per primary key, HLC-stamped. Both nodes also store
  locally after every apply, so the pair converges to identical content regardless of which node
  deployed. Retention (newest-2 deployments per cluster) prunes on one node and the deletes
  replicate as tombstones.

## Enabling replication on a pair

Replication has two knobs, both under `LocalDb`:

| Key | Role | Notes |
|---|---|---|
| `LocalDb:SyncListenPort` | binds the dedicated **h2c** sync listener | `0` (default) = no listener, replication off. Set the **same** non-zero port on **both** nodes of the pair. |
| `LocalDb:Replication:PeerAddress` | the address this node **dials** | Set on **one** node (the initiator); leave empty on the other (passive). The stream is bidirectional — one dial carries both directions. |
| `LocalDb:Replication:ApiKey` | bearer token for the sync stream | **Must be byte-identical on both nodes.** See fail-closed rule below. Supply via `${secret:...}` / env — never a cleartext literal in production config. |
| `LocalDb:Replication:MaxBatchSize` | rows per replication batch | `16` for the artifact cache. Batching is **row-count-only** against gRPC's 4 MB message cap; artifact chunk rows are ≈171 KB, so 16 × 171 KB ≈ 2.7 MB stays under the cap. Raising it risks tripping the cap. |

Example (site-a-1 initiator, site-a-2 passive):

```
site-a-1:  LocalDb__SyncListenPort=9001
           LocalDb__Replication__PeerAddress=http://site-a-2:9001
           LocalDb__Replication__ApiKey=${secret:site-a-localdb-sync-key}
           LocalDb__Replication__MaxBatchSize=16
site-a-2:  LocalDb__SyncListenPort=9001
           LocalDb__Replication__ApiKey=${secret:site-a-localdb-sync-key}   # no PeerAddress
           LocalDb__Replication__MaxBatchSize=16
```

> **Kestrel note.** Setting `SyncListenPort` makes the Host add an explicit `Listen*` for the h2c
> sync listener. An explicit `Listen*` makes Kestrel **ignore `ASPNETCORE_URLS` entirely**, so the
> Host also re-binds the primary HTTP port in the same block. If you change the primary port,
> verify both `:<httpPort>` and `:<syncPort>` appear in the startup "Now listening on" lines.

### The ApiKey rule (fail-closed)

The replication library's passive endpoint verifies **no** authentication — the host
`LocalDbSyncAuthInterceptor` is the only gate, and it is **fail-closed**:

- **No key configured ⇒ every sync call is refused** (`PermissionDenied`). "No key" is never
  "no auth required".
- **A key mismatch ⇒ the pair silently stops converging.** The initiator's stream is rejected at
  the peer; nothing errors loudly. A typo in one node's key looks exactly like "replication is
  broken". If a pair is not converging, **check the keys match first.**

## Operational rules

- **Stop / start the pair together where you can.** Each node keeps working (and caching locally)
  while its peer is down; the outage is not a data-loss event — the surviving node accumulates
  writes and the peer catches up on rejoin. But a long-lived solo node drifts further from its
  peer, so avoid leaving a pair split for extended periods.
- **Tombstone-retention resurrection window.** Retention prunes to the newest 2 deployments and
  replicates the prune as tombstones. Tombstones are themselves retained only for a bounded window.
  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` ≥ 0.1.2** (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), and it heals the *cache*, so the node regains boot-from-cache for future outages. 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
  fresh-config behavior. Boot-from-cache logs a running-from-cache signal; treat a node that stays
  on it as a central-connectivity incident, not steady state.

## Inspecting the LocalDb file — safety rules

The database runs in **WAL mode**. These rules exist because violating them corrupted a live DB in
the 2026-07-20 ScadaBridge incident:

- **Never run `sqlite3` on the live file** (host-side, against a bind-mounted or container path).
  Opening a live WAL DB from a second process across virtiofs poisons the WAL. **Always copy the
  triplet first** and query the copy:

  ```bash
  docker cp <node>:/app/data/otopcua-localdb.db      /tmp/localdb.db
  docker cp <node>:/app/data/otopcua-localdb.db-wal  /tmp/localdb.db-wal
  docker cp <node>:/app/data/otopcua-localdb.db-shm  /tmp/localdb.db-shm
  sqlite3 /tmp/localdb.db 'SELECT cluster_id, deployment_id FROM deployment_pointer;'
  ```

- **Metrics** come from the container, not a host curl (`aspnet:10.0` has no `curl`):

  ```bash
  docker run --rm --network container:<node> curlimages/curl:latest -s localhost:<httpPort>/metrics | grep localdb_
  ```

- **If an anomaly appears within seconds of your own measurement, suspect the measurement.**
  Restart both nodes and re-observe untouched before blaming the library.

## Useful queries (on a copied triplet)

```sql
-- What each node thinks the current deployment is
SELECT cluster_id, deployment_id, revision_hash, applied_at_utc FROM deployment_pointer;

-- Convergence check: the pointer's origin stamp should match on both nodes
SELECT pk_json, hlc, node_id, is_tombstone
FROM __localdb_row_version WHERE table_name = 'deployment_pointer' ORDER BY pk_json;

-- Replication backlog (should drain to 0 when a pair is caught up)
SELECT COUNT(*) FROM __localdb_oplog;
```

## See also

- [`docs/Redundancy.md`](../Redundancy.md) — the pair-local config cache section.
- [`docs/Configuration.md`](../Configuration.md) — the `LocalDb` appsettings section.
- The two-node convergence harness: `tests/Server/ZB.MOM.WW.OtOpcUa.Host.IntegrationTests/LocalDb/`.