Joseph Doherty
f587e7972e
The gate closed both follow-ups on the docker-dev rig and found a third defect that offline tests could not have found first: it lived behind the one being fixed. Once 0.1.2 made back-fill work, a rebuilt node could be observed writing for the first time — and its writes went nowhere, because the healthy node kept the old peer's seq watermark. Library 0.1.1 -> 0.1.2 -> 0.1.3 over the course of the gate; both consumers now pinned to 0.1.3. Check 4 (was PARTIAL, now PASS): with SQL stopped and site-a-2's LocalDb volume destroyed, a-1 logged "Snapshot sent (as_of_seq 0, 4 rows)" — a line that could not appear on 0.1.1 — and a-2 came back with a row_version dump byte-identical to a-1's, carrying a-1's origin node ids and its ORIGINAL applied_at_utc. It then booted from cache and served 17 ns=2 nodes, diff-identical to its peer, from a configuration it never applied and could not have fetched. Check 8 (PASS): two restarts with an unchanged artifact left the oplog at 7 and the pointer timestamp frozen — the re-cache was skipped outright. Positive control: a real config change still wrote, 7 -> 10 -> 13. Also records one unrelated pre-existing finding the gate's SQL flapping surfaced: a transient ConfigDb error during artifact load empties the served address space while logging "rebuild becomes no-op". The cache layer correctly refused to store the bad artifact and the node recovered on restart, but the two logs disagree and it is the same class as the Phase 1 gate's check-3 defect. Untouched by this work; wants its own issue. Claude-Session: https://claude.ai/code/session_01GASWkNEi68FSCtvr6rLoEW
137 lines
8.1 KiB
Markdown
137 lines
8.1 KiB
Markdown
# LocalDb pair replication — operations runbook
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> **Phase 1 scope.** Every driver-role OtOpcUa node keeps a consolidated
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> [`ZB.MOM.WW.LocalDb`](../../CLAUDE.md) SQLite database. Today it caches exactly one thing: the
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> **deployed-configuration artifact**, chunked, so a node can **boot from cache when central SQL
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> Server is unreachable**. That cache can *optionally* replicate to the node's redundant pair peer
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> over the library's gRPC sync. **Replication is default-OFF and fail-closed.** This runbook covers
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> enabling it, the operational rules that keep a pair converging, and the DB-inspection safety
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> rules.
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## What replicates, and what it buys you
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- **Replicated tables:** `deployment_artifacts` (the artifact, split into base64 chunks) and
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`deployment_pointer` (one current-deployment pointer per cluster). Registered — in this order,
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after the DDL — by `LocalDbSetup.OnReady`.
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- **The payoff:** in a redundant driver pair, *either* node can be the one that applied the latest
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deploy and wrote the cache. With replication on, the peer holds a byte-identical copy. So a node
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that restarts into a central-SQL outage can boot the current config **even if it never applied
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that deploy itself** — its peer did, and the row replicated.
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- **Convergence model:** last-writer-wins per primary key, HLC-stamped. Both nodes also store
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locally after every apply, so the pair converges to identical content regardless of which node
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deployed. Retention (newest-2 deployments per cluster) prunes on one node and the deletes
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replicate as tombstones.
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## Enabling replication on a pair
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Replication has two knobs, both under `LocalDb`:
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| Key | Role | Notes |
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|---|---|---|
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| `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. |
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| `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. |
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| `LocalDb:Replication:ApiKey` | bearer token for the sync stream | **Must be byte-identical on both nodes.** See fail-closed rule below. Supply via `${secret:...}` / env — never a cleartext literal in production config. |
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| `LocalDb:Replication:MaxBatchSize` | rows per replication batch | `16` for the artifact cache. Batching is **row-count-only** against gRPC's 4 MB message cap; artifact chunk rows are ≈171 KB, so 16 × 171 KB ≈ 2.7 MB stays under the cap. Raising it risks tripping the cap. |
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Example (site-a-1 initiator, site-a-2 passive):
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```
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site-a-1: LocalDb__SyncListenPort=9001
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LocalDb__Replication__PeerAddress=http://site-a-2:9001
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LocalDb__Replication__ApiKey=${secret:site-a-localdb-sync-key}
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LocalDb__Replication__MaxBatchSize=16
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site-a-2: LocalDb__SyncListenPort=9001
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LocalDb__Replication__ApiKey=${secret:site-a-localdb-sync-key} # no PeerAddress
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LocalDb__Replication__MaxBatchSize=16
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```
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> **Kestrel note.** Setting `SyncListenPort` makes the Host add an explicit `Listen*` for the h2c
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> sync listener. An explicit `Listen*` makes Kestrel **ignore `ASPNETCORE_URLS` entirely**, so the
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> Host also re-binds the primary HTTP port in the same block. If you change the primary port,
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> verify both `:<httpPort>` and `:<syncPort>` appear in the startup "Now listening on" lines.
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### The ApiKey rule (fail-closed)
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The replication library's passive endpoint verifies **no** authentication — the host
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`LocalDbSyncAuthInterceptor` is the only gate, and it is **fail-closed**:
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- **No key configured ⇒ every sync call is refused** (`PermissionDenied`). "No key" is never
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"no auth required".
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- **A key mismatch ⇒ the pair silently stops converging.** The initiator's stream is rejected at
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the peer; nothing errors loudly. A typo in one node's key looks exactly like "replication is
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broken". If a pair is not converging, **check the keys match first.**
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## Operational rules
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- **Stop / start the pair together where you can.** Each node keeps working (and caching locally)
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while its peer is down; the outage is not a data-loss event — the surviving node accumulates
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writes and the peer catches up on rejoin. But a long-lived solo node drifts further from its
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peer, so avoid leaving a pair split for extended periods.
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- **Tombstone-retention resurrection window.** Retention prunes to the newest 2 deployments and
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replicates the prune as tombstones. Tombstones are themselves retained only for a bounded window.
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If a node is offline **longer than the tombstone-retention window**, a delete that happened during
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its outage may no longer be expressible as a tombstone on rejoin — a pruned deployment could
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briefly reappear until the next deploy re-prunes it. Keep pair outages well inside that window.
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- **A rebuilt node is back-filled by its peer.** If a node loses its LocalDb file — a wiped volume,
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a re-imaged host, a fresh container — it rejoins with an empty database and its peer snapshots the
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cached configuration back to it, with no new deploy and no central SQL. Two things are worth
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knowing: this needs **`ZB.MOM.WW.LocalDb` ≥ 0.1.3**, and it heals the *cache*, so the node regains
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boot-from-cache for future outages. (On `0.1.1` a converged pair has pruned every oplog row on ack
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and the wiped node was never snapshotted — it stayed empty until the next deploy. On `0.1.2` it is
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back-filled but its OWN writes are silently dropped until its restarted seq counter climbs past the
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peer's stale watermark, so the pair looks converged right up until the moment only the rebuilt node
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applies a deploy.) A node
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wiped **during** a central outage is still repopulated by its peer under 0.1.2; a node with **no**
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peer replication configured self-heals only from central on its next successful apply.
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- **What boot-from-cache does NOT cover.** The cache is a *fallback for central-SQL outages at
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boot*, not a replacement for central. A **new deployment still requires central SQL** — the cache
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is only read when the central fetch fails at startup. When central returns, the node resumes
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fresh-config behavior. Boot-from-cache logs a running-from-cache signal; treat a node that stays
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on it as a central-connectivity incident, not steady state.
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## Inspecting the LocalDb file — safety rules
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The database runs in **WAL mode**. These rules exist because violating them corrupted a live DB in
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the 2026-07-20 ScadaBridge incident:
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- **Never run `sqlite3` on the live file** (host-side, against a bind-mounted or container path).
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Opening a live WAL DB from a second process across virtiofs poisons the WAL. **Always copy the
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triplet first** and query the copy:
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```bash
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docker cp <node>:/app/data/otopcua-localdb.db /tmp/localdb.db
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docker cp <node>:/app/data/otopcua-localdb.db-wal /tmp/localdb.db-wal
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docker cp <node>:/app/data/otopcua-localdb.db-shm /tmp/localdb.db-shm
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sqlite3 /tmp/localdb.db 'SELECT cluster_id, deployment_id FROM deployment_pointer;'
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```
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- **Metrics** come from the container, not a host curl (`aspnet:10.0` has no `curl`):
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```bash
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docker run --rm --network container:<node> curlimages/curl:latest -s localhost:<httpPort>/metrics | grep localdb_
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```
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- **If an anomaly appears within seconds of your own measurement, suspect the measurement.**
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Restart both nodes and re-observe untouched before blaming the library.
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## Useful queries (on a copied triplet)
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```sql
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-- What each node thinks the current deployment is
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SELECT cluster_id, deployment_id, revision_hash, applied_at_utc FROM deployment_pointer;
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-- Convergence check: the pointer's origin stamp should match on both nodes
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SELECT pk_json, hlc, node_id, is_tombstone
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FROM __localdb_row_version WHERE table_name = 'deployment_pointer' ORDER BY pk_json;
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-- Replication backlog (should drain to 0 when a pair is caught up)
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SELECT COUNT(*) FROM __localdb_oplog;
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```
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## See also
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- [`docs/Redundancy.md`](../Redundancy.md) — the pair-local config cache section.
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- [`docs/Configuration.md`](../Configuration.md) — the `LocalDb` appsettings section.
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- The two-node convergence harness: `tests/Server/ZB.MOM.WW.OtOpcUa.Host.IntegrationTests/LocalDb/`.
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