# 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 — Tier C out-of-process recycle

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

The process-level supervisor contract a **Tier C** (out-of-process) driver's
topology provides. Its concern is restarting the out-of-process Host when a
hard fault is detected (memory breach, wedge, scheduled recycle window). Tier
A/B drivers run in-process and do **not** have a supervisor — recycling them
would kill every OPC UA session and every co-hosted driver. The Core.Stability
layer only invokes this interface after asserting the tier.

Members:

- `string DriverInstanceId { get; }` — the driver instance this supervisor
  governs.
- `Task RecycleAsync(string reason, CancellationToken cancellationToken)` —
  request a terminate+restart of the Host process; implementations are
  expected to be idempotent under repeat calls during an in-flight recycle.

Callers (both in
[`Core/Stability/`](../src/Core/ZB.MOM.WW.OtOpcUa.Core/Stability/)):

- `ScheduledRecycleScheduler`
  ([`Core/Stability/ScheduledRecycleScheduler.cs`](../src/Core/ZB.MOM.WW.OtOpcUa.Core/Stability/ScheduledRecycleScheduler.cs))
  — opt-in periodic recycle. A `TickAsync` method advanced by the caller's
  ambient scheduler decides whether the configured interval has elapsed and, if
  so, drives `RecycleAsync`. Its constructor throws unless the tier is C, making
  in-process misuse structurally impossible.
- `MemoryRecycle`
  ([`Core/Stability/MemoryRecycle.cs`](../src/Core/ZB.MOM.WW.OtOpcUa.Core/Stability/MemoryRecycle.cs))
  — on a memory hard-breach, calls `RecycleAsync` (when a supervisor is wired).

---

## 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).

---

## 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.
