lmxopcua/docs/DriverLifecycle.md

# Driver Lifecycle & Server Infrastructure Contracts

Reference for the server-side infrastructure interfaces that surround a
driver but are **not** driver *capabilities* (read/write/subscribe/etc.,
documented in [ReadWriteOperations.md](ReadWriteOperations.md) and the
per-driver pages). These contracts live in
[`src/Core/ZB.MOM.WW.OtOpcUa.Core.Abstractions/`](../src/Core/ZB.MOM.WW.OtOpcUa.Core.Abstractions/)
so they carry no behavior — concrete implementations live in the driver
projects, the Runtime, and the ControlPlane. Each subsection below gives the
purpose, the key members, and where it is implemented/used.

The capability interfaces a driver opts into (`IReadable`, `IWritable`,
`ITagDiscovery`, `ISubscribable`, `IAlarmSource`, `IHistoryProvider`,
`IHostConnectivityProbe`, `IPerCallHostResolver`, `IRediscoverable`) are
covered elsewhere and discovered by the server via `is`-checks on the
`IDriver` instance. The interfaces here are the *plumbing* the server uses to
**create**, **probe**, **supervise**, **report on**, and **configure** those
drivers, plus the server-side historian read surface.

---

## IDriverFactory — creating drivers from config rows

[`Core.Abstractions/IDriverFactory.cs`](../src/Core/ZB.MOM.WW.OtOpcUa.Core.Abstractions/IDriverFactory.cs)

Abstraction over the process-wide driver registry. The Runtime consumes this
instead of the concrete registry so the Runtime project does not pull in
`ZB.MOM.WW.OtOpcUa.Core` (which would drag in Polly + driver hosting).

Members:

- `IDriver? TryCreate(string driverType, string driverInstanceId, string driverConfigJson)`
  — returns a new driver for the given type, or `null` when no factory is
  registered for that type (missing assembly, typo). The `DriverHostActor`
  logs and skips the row rather than failing the whole apply.
- `IReadOnlyCollection<string> SupportedTypes` — driver-type names this
  factory can materialise; mostly for diagnostics and logs.

Implementations:

- `NullDriverFactory` (same file) returns `null` from every `TryCreate` and
  exposes zero supported types. Bound when no concrete driver assemblies have
  been registered (Mac dev path, smoke tests); the deployment becomes a no-op.
- `DriverFactoryRegistry`
  ([`Core/Hosting/DriverFactoryRegistry.cs`](../src/Core/ZB.MOM.WW.OtOpcUa.Core/Hosting/DriverFactoryRegistry.cs))
  is the real process-singleton registry keyed by `DriverInstance.DriverType`
  (case-insensitive). Each driver project ships a `Register(...)` extension;
  `Register` records the factory **and** the driver's stability
  [`DriverTier`](../src/Core/ZB.MOM.WW.OtOpcUa.Core.Abstractions/DriverTier.cs)
  (defaults to Tier A). Registering the same type twice throws.
- `DriverFactoryRegistryAdapter`
  ([`Core/Hosting/DriverFactoryRegistryAdapter.cs`](../src/Core/ZB.MOM.WW.OtOpcUa.Core/Hosting/DriverFactoryRegistryAdapter.cs))
  bridges the registry to the `IDriverFactory` abstraction.

Wiring: `DriverFactoryBootstrap.AddOtOpcUaDriverFactories`
([`Host/Drivers/DriverFactoryBootstrap.cs`](../src/Server/ZB.MOM.WW.OtOpcUa.Host/Drivers/DriverFactoryBootstrap.cs))
registers the singleton registry, runs every driver assembly's `Register`
extension, then binds `IDriverFactory` to the adapter. It must run **before**
`AddAkka` so the Runtime can resolve `IDriverFactory` when spawning the
`DriverHostActor`
([`Runtime/Drivers/DriverHostActor.cs`](../src/Server/ZB.MOM.WW.OtOpcUa.Runtime/Drivers/DriverHostActor.cs)).
The registry is skipped on admin-only nodes (they never run drivers); the
probe set is the exception — see [IDriverProbe](#idriverprobe--test-connect).

---

## IDriverProbe — Test Connect

[`Core.Abstractions/IDriverProbe.cs`](../src/Core/ZB.MOM.WW.OtOpcUa.Core.Abstractions/IDriverProbe.cs)

A cheap test-connect probe for one driver type, backing the AdminUI **Test
Connect** button. An implementation deserializes a driver-config JSON, attempts
a cheap connection (TCP open, OPC UA session, gRPC ping — whatever the driver's
native protocol supports), and reports success/failure with latency. **Probes
must not mutate persistent state**: the AdminUI invokes them against the
transient config in the typed form, not against the persisted `DriverInstance`
row.

Members:

- `string DriverType { get; }` — the `DriverInstance.DriverType` string this
  probe handles; used for DI lookup.
- `Task<DriverProbeResult> ProbeAsync(string configJson, TimeSpan timeout, CancellationToken ct)`
  — never throws on connection failure; returns a result with `Ok = false`
  and a message instead.
- `DriverProbeResult(bool Ok, string? Message, TimeSpan? Latency)` — outcome
  record (`Message` is `null` on success; `Latency` is `null` on failure).

Implementations: every driver ships a `*DriverProbe` in its driver project
(e.g.
[`Driver.Modbus/ModbusDriverProbe.cs`](../src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Modbus/ModbusDriverProbe.cs)
does a bare socket open/close). The historian backend is the external
HistorianGateway (consumed as a gRPC client package, not an `IDriver`), so it
has no driver probe.

Flow: the AdminUI's `AdminProbeService`
([`AdminUI/Clients/AdminProbeService.cs`](../src/Server/ZB.MOM.WW.OtOpcUa.AdminUI/Clients/AdminProbeService.cs))
dispatches a `TestDriverConnect` message through `IAdminOperationsClient` to the
cluster-singleton `AdminOperationsActor`
([`ControlPlane/AdminOperations/AdminOperationsActor.cs`](../src/Server/ZB.MOM.WW.OtOpcUa.ControlPlane/AdminOperations/AdminOperationsActor.cs)),
which holds the probes keyed by `DriverType` and invokes the matching one
(timeout clamped to `[1, 60]` seconds). Because the admin singleton is
admin-pinned, the probe set must be registered on admin nodes too — `Program.cs`
calls `AddOtOpcUaDriverProbes` in the `hasAdmin` block, and
`AddOtOpcUaDriverFactories` registers it for fused admin+driver nodes.

---

## IDriverSupervisor / process recycle — REMOVED (Gitea #522)

`IDriverSupervisor`, `MemoryRecycle`, `MemoryTracking` and
`ScheduledRecycleScheduler` **no longer exist.** This section used to describe
them as the Tier C protection layer.

They were removed because the thing they acted on is gone. "Tier C" meant a
driver running **out-of-process behind a supervisor that could restart its Host**
without tearing down the OPC UA session or co-hosted drivers. No such process
remains anywhere: Galaxy reaches MXAccess over gRPC to the external
`mxaccessgw` sidecar (PR 7.2 retired the in-process `Galaxy.Host` / `Proxy` /
`Shared` projects), and FOCAS has run in-process since its managed wire client
landed 2026-04-24. Consistent with that, `IDriverSupervisor` had **zero
implementations**, and `MemoryTracking` / `MemoryRecycle` /
`ScheduledRecycleScheduler` were constructed **only in their own unit tests** —
so this was not dormant-but-ready code, it was code with no path to running.

The operator-facing half went too: `RecycleIntervalSeconds` was authorable
through the AdminUI's driver resilience section and validated by the parser
while configuring nothing. A stored `ResilienceConfig` still carrying the key
parses cleanly (unknown keys are ignored) and the AdminUI preserves it rather
than silently rewriting an operator's config.

**`DriverTier` itself survives and is load-bearing** — do not delete it while
following this note. `DriverResilienceOptions.GetTierDefaults(tier)` supplies
the real per-capability timeout / retry / breaker policies, resolved via
`DriverFactoryRegistry.GetTier` and applied by `DriverCapabilityInvokerFactory`.
Every driver runs Tier A because no factory passes a tier, so today that means
one uniform policy set.

`IDriver` **no longer carries `GetMemoryFootprint()` or
`FlushOptionalCachesAsync()`** (Gitea **#525**). They existed to feed
`MemoryTracking`, deleted with the rest of the driver-tier recycle machinery in
**#522**; after that nothing called them, and their doc-comments still claimed
Core polled the footprint every 30 s, which had not been true since v3. The
sweep removed them from the interface, all 12 drivers, and every test stub,
along with the support code that existed only to feed them — MTConnect's
per-entry byte weights and its cached data-item count, OpcUaClient's
`_discoveredNodeCount`, and MQTT's per-tag byte estimate.

Two consequences worth knowing:

- `AbCipTemplateCache` now has **no production reset path**. Its only caller was
  AB CIP's flush — which itself had no caller, so nothing has actually changed
  in behaviour, only in how visible the fact is. A UDT redefined on a live
  controller keeps its stale shape until the driver instance is rebuilt.
- MTConnect's `/probe` cache is still dropped, by `InvalidateProbeModel()` on an
  Agent restart. The tests that used to reach it through the flush now drive
  that path instead, so they cover a case that actually occurs.

If per-driver memory protection is wanted again, build it for the architecture
as it is now: a process-wide watchdog that can ask drivers to flush, not a
per-driver tier whose only remedy requires a supervisor that cannot exist
in-process. The deleted per-driver implementations are in git history and were
mostly reusable — what was always missing is the trigger.

---

## IDriverHealthPublisher — health pub/sub sink

[`Core.Abstractions/IDriverHealthPublisher.cs`](../src/Core/ZB.MOM.WW.OtOpcUa.Core.Abstractions/IDriverHealthPublisher.cs)

A sink for driver-health state-change notifications. Implementations must be
non-blocking and safe to call from any thread.

Member:

- `void Publish(string clusterId, string driverInstanceId, DriverHealth health, int errorCount5Min)`

Implementations:

- `NullDriverHealthPublisher` (same file) is the drop-in no-op for tests and
  dev-stub paths. A `DriverInstanceActor` defaults to it when no publisher is
  supplied.
- `AkkaDriverHealthPublisher`
  ([`Runtime/Drivers/AkkaDriverHealthPublisher.cs`](../src/Server/ZB.MOM.WW.OtOpcUa.Runtime/Drivers/AkkaDriverHealthPublisher.cs))
  is the production binding: it forwards each transition as a
  `DriverHealthChanged` message onto the cluster-wide `driver-health`
  Akka DistributedPubSub topic.

Producer: `DriverInstanceActor`
([`Runtime/Drivers/DriverInstanceActor.cs`](../src/Server/ZB.MOM.WW.OtOpcUa.Runtime/Drivers/DriverInstanceActor.cs))
calls `Publish` when a driver's health transitions. The published snapshot is
consumed AdminUI-side and surfaced through the driver-status panel (read
in-process by the AdminUI bridge rather than dialing its own hub).

### Writing health around a teardown (Gitea #517)

Two rules, and the second is the counter-intuitive one:

1. **`GetHealth()` must report actual state, never a constant.** A hard-coded `Healthy` reports a
   healthy driver before initialize and after shutdown alike. `CalculationDriver` did exactly this —
   it was the fleet's one driver reporting Healthy with its timers stopped, deterministically rather
   than as a race. A driver with no transport still has a running / not-running distinction.

2. **Land the terminal health value *after* the teardown — and, where the window is observable, write
   it *before* as well.** #517's original suggestion was to move the write earlier so the race window
   leans conservative. Moving it is wrong: teardown drains in-flight poll/probe loops, and those loops
   write health as they unwind, so a driver that only wrote first could have a late loop overwrite the
   shutdown state with `Healthy` and leave *that* as the resting value — with every connection closed.
   The safe shape is a **bracket**: conservative value before, terminal value after.

Where the window is actually observable is worth knowing before adding brackets everywhere:

| Path | Caller | Can `GetHealth()` land mid-teardown? |
|---|---|---|
| `ShutdownAsync` | `DriverInstanceActor.PostStop` | **No** — actor thread, after the mailbox drains, so the health poll is serialised against it. |
| `InitializeAsync` failure | `DriverInstanceActor.InitializeAsync` | **Yes** — invoked on a fire-and-forget `Task.Run`, so the health-poll tick keeps calling `GetHealth()` throughout. |

So a driver whose `InitializeAsync` disposes anything on its failure path must bracket; only
`MTConnectDriver` does today, because it is the only one that tears down inside a `catch`. The
shutdown paths deliberately keep the single trailing write.

### The comms-loss watchdog (Gitea #527) — `GetHealth()` is now load-bearing

`GetHealth()` used to be **reporting-only**: `DriverInstanceActor.PublishHealthSnapshot` read it every 30 s
and forwarded it to the AdminUI, and nothing ever acted on a driver that reported itself unhealthy. That
left `DisconnectObserved` with **no production sender at all** — the only paths into `Reconnecting` were a
failed *initial* connect and an operator's Force Reconnect. A driver that connected and then lost its
device stayed `Connected` forever: publishing Bad for every tag, never re-initialising, and **not
recovering when the device came back** (live-confirmed against a stopped Modbus fixture, which needed a
node restart). Both comms-loss quality paths — #477's native-condition annotation and #481's
scripted-alarm input fan-out — hang off `ConnectivityChanged`, so both were wired to a signal that never
fired.

The `Connected`-state health tick now also evaluates a watchdog, and self-sends `DisconnectObserved` when
**both** of these hold:

1. the driver reports a state other than `Healthy`, **and**
2. `DriverHealth.LastSuccessfulRead` has not advanced for `DefaultCommsLossTimeout` (90 s = three ticks),
   measured from the Connected entry when the driver has never reported a successful read.

**The conjunction is the whole design, and "non-Healthy for N polls" is the wrong rule.** `Degraded` means
*connected but some data is bad* — the enum says so. Several drivers publish health **per tag inside their
read loop** (`ModbusDriver.ReadAsync` writes `Healthy` or `Degraded` once per reference), so a single
unreadable address leaves the driver latched `Degraded` whenever it happens to be read last, indefinitely,
against a device that is perfectly reachable. A state-only watchdog would tear that driver down every 90 s
forever — strictly worse than the bug it fixes. `LastSuccessfulRead` is the honest liveness evidence: every
driver advances it on any successful read and *preserves* it across `Degraded`/`Faulted` writes, so one bad
tag among good ones keeps it moving while a device that is genuinely gone freezes it. The state check is
still needed as the second conjunct, because a `Healthy` driver with nothing subscribed legitimately never
reads and must not be declared dead for being idle. Both halves are pinned by their own test in
`DriverInstanceActorCommsLossWatchdogTests`, each of which goes red when the *other* conjunct is deleted.

Two consequences worth knowing:

- **`CalculationDriver` can never trip it, correctly.** It reports `Healthy` whenever it is running and has
  no transport to be degraded about. Every other driver reaches a non-Healthy state on sustained failure:
  the nine poll drivers via `Degraded`, MQTT via its live connection state, Galaxy via its reconnect
  supervisor. This was surveyed across all 12 before relying on it — it is the assumption the whole
  mechanism rests on.
- **A device that accepts a connection but serves no data will cycle**, reconnecting once per threshold.
  That is the correct reading of a broken device, it is rate-limited, and it is visible in the health
  snapshot's error count — but do not mistake it for a bug when reading logs.

**The reconnect retry now tears down before re-initialising.** `Reconnecting`'s `RetryConnect` calls
`ShutdownAsync` before `InitializeAsync` — the sequence every driver's own `ReinitializeAsync` already
uses — because several drivers **overwrite** an `IAsyncDisposable` transport field inside `InitializeAsync`
without disposing the previous one (`ModbusDriver._transport`, `MqttDriver._connection`). Re-initialising
without a teardown leaks one socket per attempt. That was latent while only a manual Force Reconnect
re-entered this path; the watchdog makes it automatic and unbounded, so a device down for an hour would
have leaked ~360 transports at the 10 s retry interval. The shutdown is best-effort: a driver that throws
on it still gets its initialise attempt.

### The connect path feeds the circuit breaker (R2-10 T10 closure)

`IDriver.InitializeAsync` is routed through the capability invoker as
**`DriverCapability.Initialize`**, keyed on the driver-instance id (init spans the whole driver,
like a bulk subscribe). Before this, the connect path was the ONE capability call the invoker never
saw: a driver that could not connect contributed **zero** breaker input, so the AdminUI resilience
panel read *"All targets healthy"* against a dead endpoint — live-proven during the R2-10 T10 gate,
and the reason that gate was structurally unsatisfiable for a year of attempts. Things to know
before touching it:

- **The Initialize policy never retries** — this actor's `RetryConnect` timer *is* the retry loop,
  and pipeline retries would stack inside each timer tick (and stretch one attempt past the timer
  interval so attempts overlap). The parser forces a `retryCount` override back to 0, same funnel
  as the write-shaped rule.
- ⚠️ **The breaker threshold is 2, and raising it silently re-breaks the feature.** The breaker
  samples over 30 s and the reconnect timer fires every 10 s, so at most **three** init attempts fit
  one sampling window — a Read-sized threshold of 5 can literally never open on connect failures,
  which reads exactly like "the panel doesn't work" with nothing logged. Pinned by
  `Initialize_BreakerThreshold_IsReachableAtTheReconnectCadence` and, behaviourally, by
  `Initialize_BreakerOpens_AtTierDefaults_AndFastFailsTheNextAttempt`.
- **An open breaker cannot wedge the reconnect loop.** A fast-failed attempt
  (`BrokenCircuitException`) lands in the same catch as a real connect failure →
  `InitializeFailed` → the timer keeps ticking. The 15 s break window is shorter than two 10 s
  ticks, so recovery costs at most one extra tick. The teardown `ShutdownAsync` stays deliberately
  UNWRAPPED — best-effort cleanup of an already-dead transport must not count toward the breaker.
- **A respawn resets the instance's tracker snapshots** (`DriverResilienceStatusTracker.Invalidate`,
  called by `DriverCapabilityInvokerFactory.Create` alongside the pipeline invalidation). The
  pipelines carry the actual breaker state; once they are discarded, a retained "Breaker OPEN"
  snapshot could never self-correct — only the discarded pipeline's `OnClosed` stamps the close —
  so a deploy during an outage would otherwise strand a false OPEN chip forever. This was
  unreachable while breakers never opened; the Initialize wrap made it reachable.
- `ConsecutiveFailures` stays 0 during a connect outage **by design** — the tracker counts retry
  attempts, and Initialize has none. The breaker-OPEN chip is the operator signal.

---

## IDriverConfigEditor — custom AdminUI config editor (plug-point)

[`Core.Abstractions/IDriverConfigEditor.cs`](../src/Core/ZB.MOM.WW.OtOpcUa.Core.Abstractions/IDriverConfigEditor.cs)

An **optional** plug-point a driver can implement to provide a custom AdminUI
editor for its `DriverConfig` JSON. Drivers that don't implement it fall back to
the generic JSON editor with schema-driven validation. This is the contract
between the driver and the Admin Blazor app; the Admin app discovers
implementations and slots them into the Driver Detail screen.

Members:

- `string DriverType { get; }` — the driver type this editor handles.
- `Type EditorComponentType { get; }` — the Razor component type that renders
  the editor (returned as `Type` so `Core.Abstractions` needs no Blazor
  reference).

Status: this is a forward-looking plug-point. No driver ships a concrete
`IDriverConfigEditor` today — every driver uses the generic JSON editor — so
the interface currently has the contract defined but no implementations.

---

## IHistorianDataSource — server-side historian read surface

[`Core.Abstractions/Historian/IHistorianDataSource.cs`](../src/Core/ZB.MOM.WW.OtOpcUa.Core.Abstractions/Historian/IHistorianDataSource.cs)

The server-side historian read surface. Registered with the server's history
router and resolved **per OPC UA namespace**, independent of any driver's
lifecycle. This is distinct from the driver capability `IHistoryProvider`:

- `IHistoryProvider` is a *driver capability* — the server dispatches to it via
  the driver instance.
- `IHistorianDataSource` is a *server registration* — the server resolves it by
  namespace and calls it directly, so one historian (the HistorianGateway) can
  serve many drivers' nodes, and drivers can restart without dropping history
  availability.

The interface is `: IDisposable` and declares the full read surface as
**required** members (unlike `IHistoryProvider`, where at-time/event reads are
optional default-impl methods so legacy drivers can stay raw-only):

- `ReadRawAsync(fullReference, startUtc, endUtc, maxValuesPerNode, ct)` — raw
  historical samples over a time range.
- `ReadProcessedAsync(fullReference, startUtc, endUtc, interval, aggregate, ct)`
  — interval-bucketed aggregates (average/min/max/count); an empty bucket
  returns a `BadNoData` sample.
- `ReadAtTimeAsync(fullReference, timestampsUtc, ct)` — one sample per requested
  timestamp (OPC UA HistoryReadAtTime); the returned list matches the requested
  length and order, gaps as Bad-quality snapshots.
- `ReadEventsAsync(sourceName, startUtc, endUtc, maxEvents, ct)` — historical
  alarm/event records (OPC UA HistoryReadEvents); `sourceName` is `null` to
  return all sources. `maxEvents` is a signed `int` so a non-positive value is a
  "use the backend's default cap" sentinel.
- `GetHealthSnapshot()` — point-in-time health snapshot for diagnostics and
  dashboards; pure observation, never blocks on backend I/O.

All values use the shared `DataValueSnapshot` / `HistoricalEvent` shapes;
backend-specific quality/type encodings are translated to OPC UA `StatusCode`
uints inside the data source.

Implementation:

- `GatewayHistorianDataSource`
  ([`Driver.Historian.Gateway/GatewayHistorianDataSource.cs`](../src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Historian.Gateway/GatewayHistorianDataSource.cs))
  — the read backend that talks gRPC to the external `ZB.MOM.WW.HistorianGateway`
  (via the `ZB.MOM.WW.HistorianGateway.Client` package, behind the
  `IHistorianGatewayClient` seam). The alarm-event drain target is the separate
  `GatewayAlarmHistorianWriter` (the gateway `SendEvent` path; see
  [AlarmHistorian.md](AlarmHistorian.md)).

The HistorianGateway is the sole historian backend; its config keys and
deployment prerequisites are in [Historian.md](Historian.md).

---

## Commons — shared cross-cutting primitives

[`src/Core/ZB.MOM.WW.OtOpcUa.Commons/`](../src/Core/ZB.MOM.WW.OtOpcUa.Commons/)

`ZB.MOM.WW.OtOpcUa.Commons` is the low-level shared library that the Runtime,
ControlPlane, AdminUI, and OPC UA server projects all reference. It holds
cross-cutting primitives with no driver- or host-specific behavior, so the
heavier projects can share message contracts and value types without taking a
dependency on each other. It references only `Akka` and the internal
`ZB.MOM.WW.Audit` package.

Folders:

- **`Messages/`** — Akka message contracts grouped by concern (`Admin`,
  `Alerts`, `Deploy`, `Drivers`, `Fleet`, `Logging`, `Redundancy`). These are
  the wire/inter-actor messages — e.g. `Messages/Admin/TestDriverConnect.cs`
  (Test Connect request, see [IDriverProbe](#idriverprobe--test-connect)) and
  `Messages/Drivers/DriverHealthChanged.cs` (the driver-health pub/sub payload,
  see [IDriverHealthPublisher](#idriverhealthpublisher--health-pubsub-sink)).
- **`Interfaces/`** — cluster-facing client contracts such as
  `IAdminOperationsClient`, `IClusterRoleInfo`, and `IFleetDiagnosticsClient`.
- **`Types/`** — strongly-typed identifier value types: `CorrelationId`,
  `DeploymentId`, `ExecutionId`, `NodeId`, `RevisionHash`.
- **`Browsing/`** — live-browse abstractions (`BrowseNode`, `IBrowseSession`,
  `IDriverBrowser`) backing the AdminUI address pickers.
- **`Engines/`** — evaluator seams (`IScriptedAlarmEvaluator`,
  `IVirtualTagEvaluator`, `IAlarmActorStateStore`) consumed by the
  [VirtualTags](VirtualTags.md) / [ScriptedAlarms](ScriptedAlarms.md) engines.
- **`OpcUa/`** — deferred-publish seams (`IOpcUaAddressSpaceSink`,
  `IServiceLevelPublisher` and their `Deferred*` no-op stand-ins) so address-space
  and [ServiceLevel](Redundancy.md) writes can be wired late.
- **`Observability/`** — `OtOpcUaTelemetry` (the shared ActivitySource/metrics
  surface).

---

## See also

- [ReadWriteOperations.md](ReadWriteOperations.md) — the driver *capability*
  interfaces (read/write/subscribe) and resilience pipeline.
- [ServiceHosting.md](ServiceHosting.md) — role gating, the Akka cluster, and
  the external HistorianGateway backend.
- [AlarmHistorian.md](AlarmHistorian.md) — the store-and-forward SQLite alarm
  sink that drains to `IAlarmHistorianWriter`.
- [Redundancy.md](Redundancy.md) — driver stability tiers in the redundancy
  context.