62 lines
3.2 KiB
Markdown
62 lines
3.2 KiB
Markdown
# OPC UA Client driver
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Tier-A in-process driver that opens a `Session` against a remote OPC UA server
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and re-exposes its address space through the local OtOpcUa server. The
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"gateway / aggregation" direction — opposite to the usual "server exposes PLC
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data" flow.
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For the test fixture (opc-plc) see [`OpcUaClient-Test-Fixture.md`](OpcUaClient-Test-Fixture.md).
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For the configuration surface see `OpcUaClientDriverOptions` in
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[`src/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient/OpcUaClientDriverOptions.cs`](../../src/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient/OpcUaClientDriverOptions.cs).
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## Auto re-import on `ModelChangeEvent`
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The driver subscribes to `BaseModelChangeEventType` (and its subtype
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`GeneralModelChangeEventType`) on the upstream `Server` node (`i=2253`) at
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the end of `InitializeAsync`. When the upstream server advertises a
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topology change, the driver coalesces events over a debounce window and
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runs a single re-import (equivalent to calling `ReinitializeAsync` —
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internally `ShutdownAsync` + `InitializeAsync`).
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### Configuration
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| Option | Default | Notes |
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| --- | --- | --- |
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| `WatchModelChanges` | `true` | Disable to skip the watch entirely (no extra subscription, no re-import on topology change). |
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| `ModelChangeDebounce` | `5s` | Coalescing window. The first event starts the timer; further events extend it; when it elapses with no new events, the driver fires one re-import. |
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### Behaviour
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- One model-change subscription per driver instance, separate from the
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data + alarm subscriptions. Created best-effort: a server that doesn't
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advertise the event types or rejects the `EventFilter` falls through to
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no-watch — `InitializeAsync` still succeeds.
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- The `EventFilter` selects only the `EventType` field (a `WhereClause`
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constrains by `OfType BaseModelChangeEventType`). Payload fields like
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`Changes[]` are intentionally ignored: the driver always re-imports the
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full upstream root, so per-event delta tracking would just add wire
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overhead.
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- Debounce is implemented via a single-shot `Timer`; every event calls
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`Timer.Change(window, Infinite)` so a burst of N events triggers exactly
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one re-import after the window elapses with no further events.
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- The re-import path acquires the same `_gate` semaphore that `ReadAsync`
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/ `WriteAsync` / `BrowseAsync` / `SubscribeAsync` use. Downstream callers
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see a brief browse-gap (≈ the upstream `DiscoverAsync` duration) while
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the gate is held — but no torn reads or split-batch writes.
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- Failure during the re-import is best-effort: the next `ModelChangeEvent`
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triggers another attempt, and the keep-alive watchdog covers permanent
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upstream loss. Operators see failures through `DriverHealth.LastError`
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+ the diagnostics counters.
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### When to disable
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Flip `WatchModelChanges` to `false` when:
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- The upstream topology is known-static (e.g. firmware-pinned PLC) and
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the driver should never run a re-import unprompted.
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- The brief browse-gap during re-import is unacceptable and a manual
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`ReinitializeAsync` call from the operator is preferred.
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- The upstream server fires spurious `ModelChangeEvent`s that don't
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reflect real topology changes, causing wasted re-imports. Tighten or
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disable rather than chasing the noise downstream.
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