# ZB.MOM.WW.LocalDb

An embedded **SQLite local cache** with **optional bidirectional async 2-node
replication over gRPC**, for the ZB.MOM.WW SCADA / OT family. It gives an application a
fast, crash-safe local store that can optionally keep a second node in sync
asynchronously — local writes **never** block on the peer, and a node that never enables
replication is simply a fast local DB.

Change capture uses SQLite triggers writing a single HLC-stamped oplog inside the
consumer's own write transaction (crash-consistent, non-bypassable). Convergence is
last-writer-wins over a hybrid logical clock. All timestamps are UTC.

## Packages (all `0.1.0`)

| Package | Contents |
|---|---|
| `ZB.MOM.WW.LocalDb` | Embedded DB host: `ILocalDb` SQL surface (`ExecuteAsync` / `QueryAsync` / transactions), `RegisterReplicated(table)` (PK validation + trigger install + oplog schema), `HybridLogicalClock`, `AddZbLocalDb()` DI. |
| `ZB.MOM.WW.LocalDb.Contracts` | `localdb_sync.v1` proto + generated gRPC types (packable; a future peer could be non-.NET). |
| `ZB.MOM.WW.LocalDb.Replication` | gRPC sync engine: passive `LocalDbSyncService`, initiating `SyncBackgroundService`, `MaintenanceBackgroundService`, LWW apply, watermark acks + pruning, snapshot resync, `ISyncStatus`, `LocalDbMetrics`, `AddZbLocalDbReplication()` / `MapZbLocalDbSync()`. |

> Status: **built and published to the Gitea feed at `0.1.0` (2026-07-18); no app has adopted it yet.**

## Quickstart

### Local-only (no replication)

```csharp
builder.Services.AddZbLocalDb(builder.Configuration, db =>
{
    // onReady: create your own tables, then opt them into (future) replication.
    db.ExecuteAsync(
        "CREATE TABLE IF NOT EXISTS widget (id INTEGER PRIMARY KEY, name TEXT, qty INTEGER)")
      .GetAwaiter().GetResult();
    db.RegisterReplicated("widget");   // PK required; installs capture triggers
});

// Consume ILocalDb anywhere:
await db.ExecuteAsync("INSERT INTO widget (id, name, qty) VALUES (@id, @n, @q)",
    new { id = 1, n = "bolt", q = 42 });
var rows = await db.QueryAsync("SELECT name, qty FROM widget",
    r => (Name: r.GetString(0), Qty: r.GetInt32(1)));
```

`AddZbLocalDb` is first-registration-wins: a second call is a no-op. `RegisterReplicated`
installs capture triggers even without the replication package present — you can register
tables now and turn on sync later.

### Replicated (2 nodes)

Every node registers the passive side (so either can accept a stream) and the initiator
(idle unless a `PeerAddress` is set). Configure exactly **one** side with the peer's
address; the other stays passive.

```csharp
// Both nodes:
builder.Services.AddZbLocalDb(builder.Configuration, db => { /* create + RegisterReplicated */ });
builder.Services.AddGrpc();
builder.Services.AddZbLocalDbReplication(builder.Configuration);   // passive host + initiator + maintenance

var app = builder.Build();
app.MapZbLocalDbSync();   // passive gRPC endpoint — host must have AddGrpc()
app.Run();
```

```jsonc
// Node A — the INITIATOR: dials node B
"LocalDb": {
  "Path": "nodeA.db",
  "Replication": { "PeerAddress": "https://nodeB:5001", "ApiKey": "…" }
}
// Node B — PASSIVE: no PeerAddress, its SyncBackgroundService idles and only serves the endpoint
"LocalDb": { "Path": "nodeB.db", "Replication": { } }
```

Both directions flow on the single bidirectional stream the initiator opens; the passive
node still pushes its own deltas back. Only one side needs `PeerAddress`.

## Configuration

### `LocalDb` section (`LocalDbOptions`)

| Key | Default | Meaning |
|---|---|---|
| `Path` | `""` (required) | Filesystem path to the SQLite database file. |
| `BusyTimeoutMs` | `5000` | SQLite `busy_timeout` pragma, in milliseconds. |
| `Synchronous` | `"NORMAL"` | SQLite `synchronous` pragma (`OFF` / `NORMAL` / `FULL`). |

### `LocalDb:Replication` section (`ReplicationOptions`)

| Key | Default | Meaning |
|---|---|---|
| `PeerAddress` | `""` | gRPC address to dial. Empty ⇒ passive (initiator idles). |
| `ApiKey` | `null` | Bearer token the initiator presents on its channel; `null` ⇒ no key header. |
| `FlushInterval` | `250 ms` | How often the change pump flushes pending oplog deltas. |
| `MaxBatchSize` | `500` | Max oplog entries per outbound batch (also the snapshot page size). |
| `MaxOplogRows` | `1_000_000` | Backlog ceiling; exceeding it flags a snapshot resync and prunes oldest rows. |
| `MaxOplogAge` | `7 days` | Age ceiling; an unpruned entry older than this flags a snapshot resync. |
| `TombstoneRetention` | `7 days` | How long deleted-row tombstones are kept before pruning. |
| `ReconnectBackoffMax` | `60 s` | Upper bound on exponential reconnect backoff (validator caps ≤ 1 day). |
| `MaintenanceInterval` | `60 s` | How often maintenance enforces caps + prunes expired tombstones. |
| `MaxHlcDriftAhead` | `5 min` | Max tolerated HLC drift where the peer's clock runs ahead of ours. |
| `FailClosedOnDrift` | `false` | When true, an over-drift entry is dead-lettered instead of applied. |

All options are validated at startup (`ZB.MOM.WW.Configuration` validators).

## How it works

- **Capture** — three AFTER triggers per registered table write each change into one
  shared, HLC-stamped `__localdb_oplog` plus a `__localdb_row_version` ledger, in the
  consumer's own transaction. A DB-scoped `__localdb_applying` guard makes replicated
  applies skip re-capture.
- **Convergence** — last-writer-wins: compare incoming HLC, then `node_id` ordinal as the
  tie-break; the strict-greater winner is applied, the loser discarded (but still acked).
- **Streaming** — one long-lived bidirectional gRPC stream. The initiator dials; both
  sides push oplog batches above the peer's watermark and ack the applied watermark back,
  which drives pruning. Outbound uses a single writer fed by two lanes (an unbounded
  control lane for handshake/acks that always preempts a bounded data lane for
  delta/snapshot batches), so receiving never blocks on the ability to ack.
- **Resync** — when a peer's watermark falls below the pruned oplog horizon, it is sent a
  full snapshot of the registered tables (streamed, keyset-paged) and deltas resume above
  the snapshot's as-of seq. **Each node decides from its OWN state** whether it owes the
  peer a snapshot.
- **Maintenance** — a background service on **both** roles enforces the oplog row/age caps
  and prunes expired tombstones every `MaintenanceInterval`.

## Semantics worth knowing

- **PK-column updates** are captured as delete-of-old-PK + insert-of-new — a tombstone on
  the old key plus a fresh insert (full delete+insert support).
- **Upserts** (`INSERT … ON CONFLICT DO UPDATE`) are fully supported (the row-version
  trigger uses an explicit `ON CONFLICT` clause so it is not rewritten by the outer
  statement's conflict algorithm).
- **Clock-drift guard** — warn-once per session when a peer entry's HLC physical time is
  more than `MaxHlcDriftAhead` ahead. With `FailClosedOnDrift`, each over-drift entry is
  dead-lettered; otherwise it is applied (the HLC merge absorbs the skew). The **session
  never dies from drift** — no livelock on reconnect.
- **Writes never block on the peer**; local commit is independent of replication.
- **All timestamps are UTC** (HLC physical component is UTC Unix milliseconds).

## Limitations (honest list)

- **Exactly two nodes.** No mesh / N-node topologies.
- **LWW only.** No per-table pluggable conflict resolvers.
- **BLOB columns are rejected at registration** (a column declared `BLOB` fails
  `RegisterReplicated`, because `json_object` cannot capture it). A *typeless* column can
  still hold a BLOB value at runtime, which then surfaces as a write-time error, not a
  registration error.
- **CDC semantics** — rows that existed **before** `RegisterReplicated` are not captured
  and are not snapshotted; only changes after registration are replicated. Re-baselining
  pre-existing rows requires re-writing them.
- **REAL ±Infinity** is carried in SQLite's JSON dialect on the wire (fine for the
  SQLite↔SQLite peers this library targets).
- `AddZbLocalDb` / `AddZbLocalDbReplication` are **first-registration-wins** (idempotent).
- A **mutual simultaneous snapshot** buffers the peer's inbound snapshot in memory while
  this node is still sending its own (acceptable for the small datasets this lib targets).

## Observability

Metrics publish under the single meter **`ZB.MOM.WW.LocalDb.Replication`** (all instruments
null-guarded — replication runs identically whether or not metrics are collected):

| Instrument | Kind | Notes |
|---|---|---|
| `localdb.oplog.depth` | ObservableGauge | Unacked oplog backlog (polled cheaply at scrape time). |
| `localdb.sync.lag.seconds` | ObservableGauge | Seconds since the last successful sync (no measurement when never synced). |
| `localdb.sync.applied` | Counter | Rows applied from inbound **delta** batches. |
| `localdb.sync.conflicts_discarded` | Counter | Inbound rows that lost LWW. |
| `localdb.sync.dead_lettered` | Counter | Poison / over-drift rows routed to the dead-letter table. |
| `localdb.sync.reconnects` | Counter | Reconnect attempts (initiator). |
| `localdb.sync.snapshots` | Counter | Snapshots, tagged `direction=sent`/`received` (one increment per snapshot). |

**Counter split to note:** rows merged from a **snapshot** are *not* counted under
`localdb.sync.applied`; a completed snapshot instead increments `localdb.sync.snapshots`
once (with its `direction` tag). Only delta-batch rows increment `localdb.sync.applied`.

`ISyncStatus` (registered singleton) surfaces live state for dashboards / health:
`Connected` (true while **any** session is active — a node can run both roles at once),
`PeerNodeId`, `LastSyncUtc`, `OplogBacklog` (**nullable** — null means unknown because no
provider is wired or the poll threw, so a DB failure never reads as "0 backlog"), and
`ConnectionAttempts`.

## Ops notes

- When the oplog row/age caps are exceeded, maintenance prunes to the ceiling and flags
  `needs_snapshot` (disk safety over delta continuity); the peer then snapshot-resyncs.
- The **`__localdb_dead_letter`** table holds poison rows (bad JSON, null-row non-tombstone
  entries) and over-drift rejects (when `FailClosedOnDrift`). Inspect it to diagnose
  rejects.
- **Peer auth**: `ApiKey` is sent by the initiator as a static `Authorization: Bearer …`
  header on its channel. The library's passive service does **not** itself verify the key
  — the passive host enforces whatever gRPC auth the host application configures (e.g.
  `ZB.MOM.WW.Auth`). Configure host-side authorization to actually gate inbound streams.

## Testing

144 tests, all offline / no live dependencies — unit (HLC ordering + restart
persistence, trigger SQL generation, LWW decision table, watermark/pruning, digest
computation), integration over two real SQLite DBs on an in-memory (and real) gRPC pipe
(bidirectional convergence, delete/update races, offline-accumulate → reconnect,
prune → snapshot resync), plus seeded randomized property tests (random interleaved op
sequences → both DBs identical after quiesce). Family conventions apply
(`TreatWarningsAsErrors`).

Build / test from this directory:

```bash
dotnet build ZB.MOM.WW.LocalDb.slnx
dotnet test  ZB.MOM.WW.LocalDb.slnx
```

See the design doc: [`docs/plans/2026-07-17-localdb-design.md`](../docs/plans/2026-07-17-localdb-design.md).
