lmxopcua/docs/drivers/README.md

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/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 — raw / processed / at-time / events HistoryRead (see HistoricalDataAccess.md)
• IRediscoverable — driver-initiated address-space rebuild notifications

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/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.

Ground-truth driver list

Driver	Project path	Tier	Wire / library	Capabilities	Notable quirk
Galaxy	Driver.Galaxy.{Shared, Host, Proxy}	C	MXAccess COM + aahClientManaged + SqlClient	IDriver, ITagDiscovery, IReadable, IWritable, ISubscribable, IAlarmSource, IHistoryProvider, IRediscoverable, IHostConnectivityProbe	Out-of-process — Host is its own Windows service (.NET 4.8 x86 for the COM bitness constraint); Proxy talks to Host over a named pipe
Modbus TCP	Driver.Modbus	A	NModbus-derived in-house client	IDriver, ITagDiscovery, IReadable, IWritable, ISubscribable, IHostConnectivityProbe	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	Driver.S7	A	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	Driver.AbCip	A	libplctag CIP	IDriver, ITagDiscovery, IReadable, IWritable, ISubscribable, IHostConnectivityProbe, IPerCallHostResolver	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	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	Driver.TwinCAT	B	Beckhoff TwinCAT.Ads (TcAdsClient)	IDriver, ITagDiscovery, IReadable, IWritable, ISubscribable, IHostConnectivityProbe, IPerCallHostResolver	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	Driver.FOCAS	C	FANUC FOCAS2 (Fwlib32.dll P/Invoke)	IDriver, ITagDiscovery, IReadable, IWritable, ISubscribable, IHostConnectivityProbe, IPerCallHostResolver	Tier C — FOCAS DLL has crash modes that warrant process isolation. CNC-shaped data model (axes, spindle, PMC, macros, alarms) not a flat tag map
OPC UA Client	Driver.OpcUaClient	B	OPCFoundation Opc.Ua.Client	IDriver, ITagDiscovery, IReadable, IWritable, ISubscribable, IAlarmSource, IHistoryProvider, IHostConnectivityProbe	Gateway/aggregation driver. 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

Per-driver documentation

• Galaxy has its own docs in this folder because the out-of-process architecture + MXAccess COM rules + Galaxy Repository SQL + Historian + runtime probe manager don't fit a single table row:

  • Galaxy.md — COM bridge, STA pump, IPC, runtime probes
  • Galaxy-Repository.md — ZB SQL reader, LocalPlatform scope filter, change detection

• All other drivers share a single per-driver specification in docs/v2/driver-specs.md — addressing, data-type maps, connection settings, and quirks live there. That file is the authoritative per-driver reference; this index points at it rather than duplicating.

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 — Dockerized ab_server (multi-stage build from libplctag source); atomic-read smoke across 4 families; UDT / ALMD / family quirks unit-only
• Modbus — Dockerized pymodbus + per-family JSON profiles (4 compose profiles); best-covered driver, gaps are error-path-shaped
• Siemens S7 — Dockerized python-snap7 server; DB/MB read + write round-trip verified end-to-end on :1102
• AB Legacy — no integration fixture, unit-only via FakeAbLegacyTag (libplctag PCCC)
• TwinCAT — no integration fixture, unit-only via FakeTwinCATClient with native-notification harness
• FOCAS — no integration fixture, unit-only via FakeFocasClient; Tier C out-of-process isolation scoped but not shipped
• OPC UA Client — no integration fixture, unit-only via mocked Session; loopback against this repo's own server is the obvious next step
• Galaxy — richest harness: E2E Host subprocess + ZB SQL live-smoke + MXAccess opt-in

Related cross-driver docs

• HistoricalDataAccess.md — IHistoryProvider dispatch, aggregate mapping, continuation points. The Galaxy driver's Aveva Historian implementation is the first; OPC UA Client forwards to the upstream server; other drivers do not implement the interface and return BadHistoryOperationUnsupported.
• AlarmTracking.md — IAlarmSource event model and filtering.
• Subscriptions.md — how the Server multiplexes subscriptions onto ISubscribable.OnDataChange.
• docs/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 — authoritative vision, architecture decisions, migration strategy.