# Alarm Historian — store-and-forward SQLite sink

Reference for `ZB.MOM.WW.OtOpcUa.Core.AlarmHistorian`
([`src/Core/ZB.MOM.WW.OtOpcUa.Core.AlarmHistorian/`](../src/Core/ZB.MOM.WW.OtOpcUa.Core.AlarmHistorian/)),
the durable local queue that historizes alarm transitions to AVEVA Historian
without ever blocking the alarm engine or operator actions.

This is the *sink mechanics* doc. For how the three alarm sources converge on
the OPC UA Part 9 surface and which alarms route here, see
[AlarmTracking.md](AlarmTracking.md). For the historian client that drains this
queue, see [DriverLifecycle.md](DriverLifecycle.md#ihistoriandatasource--server-side-historian-read-surface)
and [ServiceHosting.md](ServiceHosting.md).

---

## Why store-and-forward

Scripted alarms (and any future non-Galaxy `IAlarmSource`, e.g. AB CIP ALMD)
must reach AVEVA Historian, but the historian sidecar can be slow, busy, or
disconnected. The sink decouples the alarm engine from historian reachability:
every qualifying transition is committed to a **local SQLite queue first**, and
a background drain worker forwards rows to the historian on a backoff-aware
cadence. Operator acks and alarm-state transitions are never blocked waiting on
the historian.

> Galaxy-native alarms with `$Alarm*` extensions reach AVEVA Historian directly
> via System Platform's `HistorizeToAveva` toggle — they do **not** flow through
> this sink. This path is exclusively for non-Galaxy alarm producers.

---

## Contracts

All in
[`IAlarmHistorianSink.cs`](../src/Core/ZB.MOM.WW.OtOpcUa.Core.AlarmHistorian/IAlarmHistorianSink.cs)
unless noted.

- **`IAlarmHistorianSink`** — the intake contract. `EnqueueAsync(evt, ct)`
  durably enqueues an event and returns as soon as the queue row is committed
  (fire-and-forget from the engine's perspective; the sink must not block the
  emitting thread). `GetStatus()` returns a `HistorianSinkStatus` snapshot.
- **`NullAlarmHistorianSink`** — the no-op default for tests and deployments
  that don't historize alarms. It is the default DI binding (registered in the
  Runtime's `AddOtOpcUaRuntime`); production overrides it with
  `SqliteStoreAndForwardSink`.
- **`AlarmHistorianEvent`**
  ([`AlarmHistorianEvent.cs`](../src/Core/ZB.MOM.WW.OtOpcUa.Core.AlarmHistorian/AlarmHistorianEvent.cs))
  — the source-agnostic event record: `AlarmId`, `EquipmentPath` (UNS path,
  doubles as Historian's SourceNode), `AlarmName`, `AlarmTypeName` (Part 9
  subtype), `Severity`, `EventKind` (free-form transition string —
  "Activated"/"Cleared"/"Acknowledged"/etc.), `Message`, `User`, `Comment`,
  `TimestampUtc`.
- **`IAlarmHistorianWriter`** — what the drain worker delegates writes to.
  `WriteBatchAsync(batch, ct)` returns one `HistorianWriteOutcome` per event,
  in order. Production binds this to `WonderwareHistorianClient` (the AVEVA
  Historian sidecar IPC client).
- **`HistorianWriteOutcome`** — per-event drain result: `Ack` (persisted,
  remove from queue), `RetryPlease` (transient failure — leave queued, retry
  after backoff), `PermanentFail` (malformed/unrecoverable — move to
  dead-letter).
- **`HistorianSinkStatus`** — diagnostic snapshot surfaced to the AdminUI and
  `/healthz`: `QueueDepth`, `DeadLetterDepth`, `LastDrainUtc`, `LastSuccessUtc`,
  `LastError`, `DrainState`, and `EvictedCount`.
- **`HistorianDrainState`** — `Disabled` / `Idle` / `Draining` / `BackingOff`.

---

## SqliteStoreAndForwardSink

[`SqliteStoreAndForwardSink.cs`](../src/Core/ZB.MOM.WW.OtOpcUa.Core.AlarmHistorian/SqliteStoreAndForwardSink.cs)
is the production `IAlarmHistorianSink`. Construction takes a SQLite database
path, an `IAlarmHistorianWriter`, a logger, and optional `batchSize` (default
100), `capacity` (default 1,000,000), `deadLetterRetention` (default 30 days),
and a test clock.

### Queue table

The sink owns one SQLite table (created on construction, WAL journal mode):

```sql
CREATE TABLE Queue (
    RowId          INTEGER PRIMARY KEY AUTOINCREMENT,
    AlarmId        TEXT    NOT NULL,
    EnqueuedUtc    TEXT    NOT NULL,
    PayloadJson    TEXT    NOT NULL,   -- JSON-serialized AlarmHistorianEvent
    AttemptCount   INTEGER NOT NULL DEFAULT 0,
    LastAttemptUtc TEXT    NULL,
    LastError      TEXT    NULL,
    DeadLettered   INTEGER NOT NULL DEFAULT 0
);
CREATE INDEX IX_Queue_Drain ON Queue (DeadLettered, RowId);
```

`EnqueueAsync` does a single `INSERT` on the hot path. To avoid a
`SELECT COUNT(*)` on every enqueue, the sink keeps an in-memory non-dead-lettered
row counter (seeded at startup, kept current by every mutation, and re-synced
from storage every 10,000 enqueues to defend against drift). SQLite writer
contention is handled via `PRAGMA busy_timeout=5000` + WAL so an enqueue/drain
collision waits out the file lock instead of failing fast.

### Drain worker

`StartDrainLoop(tickInterval)` starts a **self-rescheduling one-shot
`System.Threading.Timer`** (not started automatically — tests drive
`DrainOnceAsync` deterministically). Each tick:

1. Purges aged dead-lettered rows past the retention window.
2. Reads up to `batchSize` non-dead-lettered rows in `RowId` order.
3. Rows with un-deserializable payloads are dead-lettered immediately (by their
   own `RowId`) so they can't stall the queue head.
4. The remaining batch is handed to `IAlarmHistorianWriter.WriteBatchAsync`, and
   each outcome is applied in one transaction: `Ack` deletes the row,
   `PermanentFail` flips its `DeadLettered` flag, `RetryPlease` bumps its attempt
   count and leaves it queued.
5. The timer re-arms its next due-time to `max(tickInterval, currentBackoff)`.

**Backoff ladder** (applied to the timer's next due-time, so a historian outage
genuinely slows the drain cadence): 1s → 2s → 5s → 15s → 60s cap. Any
`RetryPlease` outcome — or a writer exception, or a writer cardinality violation
(outcome count ≠ event count) — bumps the backoff and sets `DrainState =
BackingOff`; a clean batch resets it. The async-void timer callback is fully
guarded: a fault is logged and recorded into `GetStatus()` rather than lost as
an unobserved task exception.

### Durability bound (important)

**The durability guarantee is bounded by `capacity` (default 1,000,000 rows).**
When the non-dead-lettered queue reaches capacity, `EnqueueAsync` evicts the
oldest non-dead-lettered rows (oldest `RowId` first) to make room, logs a WARN,
and increments `HistorianSinkStatus.EvictedCount`. Under a sustained historian
outage, accepted alarm events can therefore be dropped before delivery. A
non-zero `EvictedCount` is a data-loss signal that requires operator attention —
it surfaces silent loss without log scraping.

### Dead-letter + operator recovery

`PermanentFail` and corrupt-payload rows are retained in-place with
`DeadLettered = 1` for the retention window (default 30 days) so operators can
inspect them before the sweeper purges them. `RetryDeadLettered()` is the
operator action (from the AdminUI) that clears the dead-letter flag and attempt
count on every dead-lettered row, returning them to the regular queue with a
fresh backoff.

---

## Runtime wiring

Production routes alarm transitions through the Akka cluster. The
`HistorianAdapterActor`
([`Runtime/Historian/HistorianAdapterActor.cs`](../src/Server/ZB.MOM.WW.OtOpcUa.Runtime/Historian/HistorianAdapterActor.cs))
bridges messages from the scripted-alarm actor into the sink's `EnqueueAsync`,
fire-and-forget so the actor loop is never blocked on historian reachability.
The `WonderwareHistorianClient` is the `IAlarmHistorianWriter` the drain worker
delegates to. See [ServiceHosting.md](ServiceHosting.md) for the sidecar setup.

---

## See also

- [AlarmTracking.md](AlarmTracking.md) — the three alarm sources and the OPC UA
  Part 9 surface; which alarms route to this sink.
- [DriverLifecycle.md](DriverLifecycle.md) — `IHistorianDataSource` (the
  historian *read* surface; this page covers the *write* path) and the
  `WonderwareHistorianClient`.
- [ScriptedAlarms.md](ScriptedAlarms.md) — the scripted-alarm engine that emits
  most events into this sink.
- [ServiceHosting.md](ServiceHosting.md) — the optional Wonderware historian
  sidecar.
