# Drivers

OtOpcUa is a multi-driver OPC UA server. The Core (`ZB.MOM.WW.OtOpcUa.Core` + `Core.Abstractions` + `Server`) owns the OPC UA stack, address space, session/security/subscription machinery, resilience pipeline, and namespace kinds (Equipment + SystemPlatform). Drivers plug in through **capability interfaces** defined in `src/Core/ZB.MOM.WW.OtOpcUa.Core.Abstractions/`:

- `IDriver` — lifecycle (`InitializeAsync`, `ReinitializeAsync`, `ShutdownAsync`, `GetHealth`)
- `IReadable` / `IWritable` — one-shot reads and writes
- `ITagDiscovery` — address-space enumeration
- `ISubscribable` — driver-pushed data-change streams
- `IHostConnectivityProbe` — per-host reachability events
- `IPerCallHostResolver` — multi-host drivers that route each call to a target endpoint at dispatch time
- `IAlarmSource` — driver-emitted OPC UA A&C events
- `IHistoryProvider` — driver-side raw / processed / at-time / events HistoryRead (see [HistoricalDataAccess.md](../v1/HistoricalDataAccess.md))
- `IRediscoverable` — driver-initiated address-space rebuild notifications
- `IHistorianDataSource` — server-side historian read backend registration (the HistorianGateway backend), distinct from the driver-side `IHistoryProvider` HistoryRead path

Each driver opts into only the capabilities it supports. Every async capability call at the Server dispatch layer goes through `CapabilityInvoker` (`Core/Resilience/CapabilityInvoker.cs`), which wraps it in a Polly pipeline keyed on `(DriverInstanceId, HostName, DriverCapability)`. The `OTOPCUA0001` analyzer enforces the wrap at build time. Drivers themselves never depend on Polly; they just implement the capability interface and let the Core wrap it.

Driver type metadata is registered at startup in `DriverTypeRegistry` (`src/Core/ZB.MOM.WW.OtOpcUa.Core.Abstractions/DriverTypeRegistry.cs`). The registry records each type's allowed namespace kinds (`Equipment` / `SystemPlatform` / `Simulated`), its JSON Schema for `DriverConfig` / `DeviceConfig` / `TagConfig` columns, and its stability tier per [docs/v2/driver-stability.md](../v2/driver-stability.md).

## Ground-truth driver list

| Driver | Project path | Tier | Wire / library | Capabilities | Notable quirk |
|--------|--------------|:----:|----------------|--------------|---------------|
| [Galaxy](Galaxy.md) | `Driver.Galaxy` (+ `.Browser`, `.Contracts`) | A | gRPC to the external `mxaccessgw` gateway (the gateway owns MXAccess COM + the Galaxy Repository SQL reader) | IDriver, ITagDiscovery, IReadable, IWritable, ISubscribable, IAlarmSource, IRediscoverable, IHostConnectivityProbe | In-process .NET 10 driver — the COM bitness constraint lives in the gateway's x86 net48 worker, not here. PR 7.2 retired the legacy in-process `Galaxy.{Shared, Host, Proxy}` + named-pipe Windows service. Native MxAccess alarms work end-to-end |
| [Modbus TCP](Modbus.md) | `Driver.Modbus` | A | NModbus-derived in-house client | IDriver, ITagDiscovery, IReadable, IWritable, ISubscribable, IHostConnectivityProbe, IPerCallHostResolver | Polled subscriptions via the shared `PollGroupEngine`. DL205 PLCs are covered by `AddressFormat=DL205` (octal V/X/Y/C/T/CT translation) — no separate driver |
| [Siemens S7](S7.md) | `Driver.S7` | A | [S7netplus](https://github.com/S7NetPlus/s7netplus) | IDriver, ITagDiscovery, IReadable, IWritable, ISubscribable, IHostConnectivityProbe | Single S7netplus `Plc` instance per PLC serialized with `SemaphoreSlim` — the S7 CPU's comm mailbox is scanned at most once per cycle, so parallel reads don't help |
| [AB CIP](AbCip.md) | `Driver.AbCip` | A | libplctag CIP | IDriver, ITagDiscovery, IReadable, IWritable, ISubscribable, IHostConnectivityProbe, IPerCallHostResolver, IAlarmSource | ControlLogix / CompactLogix. Tag discovery uses the `@tags` walker to enumerate controller-scoped + program-scoped symbols; UDT member resolution via the UDT template reader |
| [AB Legacy](AbLegacy.md) | `Driver.AbLegacy` | A | libplctag PCCC | IDriver, ITagDiscovery, IReadable, IWritable, ISubscribable, IHostConnectivityProbe, IPerCallHostResolver | SLC 500 / MicroLogix. File-based addressing (`N7:0`, `F8:0`) — no symbol table, tag list is user-authored in the config DB |
| [TwinCAT](TwinCAT.md) | `Driver.TwinCAT` | B | Beckhoff `TwinCAT.Ads` (`TcAdsClient`) | IDriver, ITagDiscovery, IReadable, IWritable, ISubscribable, IHostConnectivityProbe, IPerCallHostResolver, IRediscoverable | The only native-notification driver outside Galaxy — ADS delivers `ValueChangedCallback` events the driver forwards straight to `ISubscribable.OnDataChange` without polling. Symbol tree uploaded via `SymbolLoaderFactory` |
| [FOCAS](FOCAS.md) | `Driver.FOCAS` | A | Pure-managed `FocasWireClient` — FOCAS/2 Ethernet binary protocol on TCP:8193, inlined into the driver assembly | IDriver, ITagDiscovery, IReadable, IWritable, ISubscribable, IHostConnectivityProbe, IPerCallHostResolver, IAlarmSource | `IWritable` is implemented but read-only by design — `WriteAsync` returns `BadNotWritable` for every point. CNC-shaped data model (axes, spindle, PMC, macros, alarms) not a flat tag map. Previously Tier-C (Host + P/Invoke + shim DLL); retired in the 2026-04-24 migration when the managed wire client landed |
| [OPC UA Client](OpcUaClient.md) | `Driver.OpcUaClient` | B | OPCFoundation `Opc.Ua.Client` | IDriver, ITagDiscovery, IReadable, IWritable, ISubscribable, IAlarmSource, IHistoryProvider, IHostConnectivityProbe | Gateway/aggregation driver — the only driver implementing driver-side `IHistoryProvider` (forwards HistoryRead to the upstream server). Opens a single `Session` against a remote OPC UA server and re-exposes its address space. Owns its own `ApplicationConfiguration` (distinct from `Client.Shared`) because it's always-on with keep-alive + `TransferSubscriptions` across SDK reconnect, not an interactive CLI |
| [Historian.Gateway](../Historian.md) | `Driver.Historian.Gateway` | — | `ZB.MOM.WW.HistorianGateway.Client` gRPC (`historian_gateway.v1`) | IHistorianDataSource (server-side read backend) + alarm `SendEvent` writer + `WriteLiveValues` recorder + `IHistorianProvisioning` | Not a tag driver — the sole historian backend. Registers `GatewayHistorianDataSource : IHistorianDataSource` for HistoryRead and serves alarm-write + continuous historization through the gateway. No `IDriver`/`ITagDiscovery` surface. (The retired Wonderware sidecar backend it replaced is documented at [Historian.Wonderware.md](Historian.Wonderware.md).) |

## Per-driver documentation

- **Galaxy** has its own docs in this folder because the gRPC-to-gateway architecture + MXAccess rules (owned by the gateway) + Galaxy Repository SQL + Historian + runtime probe manager don't fit a single table row:
  - [Galaxy.md](Galaxy.md) — gateway gRPC bridge, hierarchy source, runtime probes
  - [Galaxy-Repository.md](../v1/drivers/Galaxy-Repository.md) — ZB SQL reader, `LocalPlatform` scope filter, change detection (v1 archive)

- **FOCAS** has a short getting-started doc because the backend-selection env var + alarm projection opt-in need explaining up front:
  - [FOCAS.md](FOCAS.md) — deployment, config, capability surface, alarm projection, troubleshooting

- **Modbus TCP**, **AB CIP**, **AB Legacy**, **Siemens S7**, **TwinCAT**, and **OPC UA Client** each have a per-driver overview page:
  - [Modbus.md](Modbus.md) — in-process Modbus-TCP driver: address formats, polled subscription model, DL205 octal mapping
  - [AbCip.md](AbCip.md) — AB CIP / EtherNet-IP driver (ControlLogix / CompactLogix / Micro800 / GuardLogix): tag discovery, UDT resolution, alarm source
  - [AbLegacy.md](AbLegacy.md) — AB Legacy PCCC driver (SLC 500 / MicroLogix / PLC-5): file-based addressing, user-authored tag list
  - [S7.md](S7.md) — Siemens S7 driver (S7-300/400/1200/1500 + S7-200): getting started, config, data-block addressing, serialized single-connection model
  - [TwinCAT.md](TwinCAT.md) — Beckhoff TwinCAT (ADS) driver: getting started, native-notification subscription, symbol-tree upload
  - [OpcUaClient.md](OpcUaClient.md) — OPC UA Client (gateway/aggregation) driver: remote-server session, driver-side HistoryRead forwarding, reconnect behaviour

- **Historian.Gateway** (server-side historian backend, not a tag driver) is documented in the main guide:
  - [../Historian.md](../Historian.md) — HistorianGateway backend: read-path registration, HistoryRead dispatch, alarm store-and-forward (`SendEvent`), continuous historization (`WriteLiveValues`), `EnsureTags` provisioning, config keys, deployment prerequisites. (The retired Wonderware sidecar backend it replaced: [Historian.Wonderware.md](Historian.Wonderware.md).)

- The full per-field spec (capability surface, config schema, addressing, data-type maps, connection settings, quirks for every driver) lives in [docs/v2/driver-specs.md](../v2/driver-specs.md). The overview pages above are the short path; that file is the authoritative per-driver reference.

## Test-fixture coverage maps

Each driver has a dedicated fixture doc that lays out what the integration / unit harness actually covers vs. what's trusted from field deployments. Read the relevant one before claiming "green suite = production-ready" for a driver.

- [AB CIP](AbServer-Test-Fixture.md) — Dockerized `ab_server` (multi-stage build from libplctag source); atomic-read smoke across 4 families; UDT / ALMD / family quirks unit-only
- [Modbus](Modbus-Test-Fixture.md) — Dockerized `pymodbus` + per-family JSON profiles (4 compose profiles); best-covered driver, gaps are error-path-shaped
- [Siemens S7](S7-Test-Fixture.md) — Dockerized `python-snap7` server; DB/MB read + write round-trip verified end-to-end on `:1102`
- [AB Legacy](AbLegacy-Test-Fixture.md) — Dockerized `ab_server` PCCC mode across SLC500 / MicroLogix / PLC-5 profiles (task #224); N/F/L-file round-trip verified end-to-end. `/1,0` cip-path required for the Docker fixture; real hardware uses empty. Residual gap: bit-file writes (`B3:0/5`) still surface BadState — real HW / RSEmulate 500 for those
- [TwinCAT](TwinCAT-Test-Fixture.md) — XAR-VM integration scaffolding (task #221); three smoke tests skip when VM unreachable. Unit via `FakeTwinCATClient` with native-notification harness
- [FOCAS](FOCAS-Test-Fixture.md) — no integration fixture, unit-only via `FakeFocasClient`; Tier C out-of-process isolation scoped but not shipped
- [OPC UA Client](OpcUaClient-Test-Fixture.md) — Dockerized `opc-plc` integration suite (task #215): real Secure Channel + Session, read + subscribe verified end-to-end; write not yet exercised in the integration suite; exhaustive capability matrix (reconnect, failover, cert-auth, history, alarms) via unit suite with mocked `Session`
- [Galaxy](../v1/drivers/Galaxy-Test-Fixture.md) — richest harness: gateway E2E + ZB SQL live-smoke + MXAccess opt-in (v1 archive)

## Related cross-driver docs

- [HistoricalDataAccess.md](../v1/HistoricalDataAccess.md) — `IHistoryProvider` dispatch, aggregate mapping, continuation points. The OPC UA Client driver is the only driver that implements driver-side `IHistoryProvider` (it forwards HistoryRead to the upstream server); the AVEVA Historian path is served server-side by the HistorianGateway-backed `IHistorianDataSource` instead. Other drivers do not implement the interface and return `BadHistoryOperationUnsupported`.
- [AlarmTracking.md](../AlarmTracking.md) — `IAlarmSource` event model and filtering. Implemented by Galaxy (native MxAccess alarms, working end-to-end), OPC UA Client, AB CIP, and FOCAS; AB Legacy, Modbus, S7, and TwinCAT have no alarm source.
- [Subscriptions.md](../v1/Subscriptions.md) — how the Server multiplexes subscriptions onto `ISubscribable.OnDataChange`.
- [docs/v2/driver-stability.md](../v2/driver-stability.md) — tier system (A / B / C), shared `CapabilityPolicy` defaults per tier × capability, `MemoryTracking` hybrid formula, and process-level recycle rules.
- [docs/v2/plan.md](../v2/plan.md) — authoritative vision, architecture decisions, migration strategy.
