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Joseph Doherty 1219b7b13e fix(opcuaclient): review fixes - drop driver-side OperationLimits chunking (SDK batches), diagnostics as OTel counter, safe discovery selection, options validation at load (#273 #275 #276 #277 #278 )

Claude-Session: https://claude.ai/code/session_01Ef2ozxS7QPanGT8uoATbTe

2026-10-08 14:55:53 -04:00

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OPC UA Client (Gateway) Driver

Getting-started guide for the OPC UA Client driver. This is the short path — for the full per-field spec read docs/v2/driver-specs.md §8, and for the test-harness map read OpcUaClient-Test-Fixture.md.

What it talks to

A remote OPC UA server. This driver runs the opposite direction from the usual "server exposes PLC data" flow: it acts as an OPC UA client, opens a Session against an upstream server, and re-exposes that server's address space through the local OtOpcUa server. Browse, read, write, subscribe, alarm, and history calls are passed through to the upstream endpoint.

It is built on the OPC Foundation UA .NET Standard reference SDK and runs in-process in the OtOpcUa server's .NET 10 AnyCPU host — pure managed, no out-of-process isolation.

There is no standalone driver CLI for the OPC UA Client driver. To exercise a remote OPC UA endpoint by hand, point the general-purpose Client CLI at it directly.

Project split

Project	Target	Role
`src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient/`	net10.0	In-process driver — session lifetime, read / write / subscribe / alarm / history passthrough
`src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Browser/`	net10.0	`IDriverBrowser` — live address-picker browse used by the AdminUI
`src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Contracts/`	net10.0	Config records + enums bound from `DriverConfig` JSON

Minimum deployment

"Drivers": {
  "upstream-1": {
    "Type": "OpcUaClient",
    "Config": {
      "EndpointUrl": "opc.tcp://plc.internal:4840",
      "SecurityPolicy": "None",
      "SecurityMode": "None",
      "AuthType": "Anonymous",
      "TargetNamespaceKind": "Equipment"
    }
  }
}

EndpointUrls (a list) takes precedence over the single-URL EndpointUrl and provides ordered failover — the driver tries each candidate in turn at init and on session drop, and the first to connect wins (e.g. a hot-standby pair on 4840 / 4841). See src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Contracts/OpcUaClientDriverOptions.cs for every field (security policy/mode, auth type, session timeout, keep-alive, reconnect period, browse root, node/depth caps).

Session lifetime

A single Session per driver instance; subscriptions multiplex onto it. The SDK reconnect handler takes the session down and brings it back on remote-server restart, re-sending subscriptions on reconnect so monitored-item handles don't dangle. Stored NodeIds embed the server-stable namespace URI (not the session-relative ns=N index) so a remote namespace-table reorder across a restart doesn't silently re-point references at the wrong namespace.

Namespace assignment

This is the only driver that gateways into either namespace kind, decided per instance via TargetNamespaceKind:

Equipment — the remote server exposes raw equipment data; remote browse paths are remapped to UNS via a required UnsMappingTable.
SystemPlatform — the remote server exposes processed/derived data; the remote hierarchy is preserved with no UNS conversion (and the mapping table must be empty).

The choice is enforced at startup so a misconfiguration fails draft validation rather than surfacing as a runtime surprise.

Capability surface

OpcUaClientDriver : IDriver, ITagDiscovery, IReadable, IWritable, ISubscribable, IHostConnectivityProbe, IAlarmSource, IHistoryProvider (src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient/OpcUaClientDriver.cs:31).

Capability	Path	Notes
`ITagDiscovery`	`DiscoverAsync` (recursive browse)	Mirrors the upstream tree from `BrowseRoot` (default `ObjectsFolder` i=85), bounded by `MaxDiscoveredNodes` / `MaxBrowseDepth`
`IReadable`	`ReadAsync` → `Session.ReadAsync`	Upstream `StatusCode`s pass through verbatim (cascading-quality rule)
`IWritable`	`WriteAsync` → `Session.WriteAsync`	Passthrough write
`ISubscribable`	native OPC UA subscriptions / monitored items	The remote server pushes data changes
`IHostConnectivityProbe`	session keep-alive	Host key is the endpoint URL actually connected to after the failover sweep
`IAlarmSource`	`SubscribeAlarmsAsync` (EventFilter) + `AcknowledgeAsync`	Subscribes to upstream alarm/condition events and forwards acks
`IHistoryProvider`	`ReadRawAsync` / `ReadProcessedAsync` / `ReadAtTimeAsync` → `Session.HistoryReadAsync`	Unique to this driver — passthrough history read against the upstream server

This driver does not implement IRediscoverable — there is no push-driven rediscovery signal from a remote OPC UA server in this driver. IHistoryProvider is implemented by no other driver; history reads for every other source route server-side through IHistoryRouter.

History passthrough

IHistoryProvider forwards HistoryRead to the upstream server's own historian. Raw, processed (the full HistoryAggregateType set — Average / Minimum / Maximum / Total / Count plus Interpolative, TimeAverage, Start, End, Delta, Range, Min/Max ActualTime, Start/EndBound, DeltaBounds, DurationGood/Bad, PercentGood/Bad, WorstQuality, NumberOfTransitions, AnnotationCount and the population StandardDeviation/Variance — each mapped to its OPC UA Part 13 standard aggregate NodeId; #285), and at-time reads are supported; each returned DataValue keeps its upstream StatusCode and timestamps verbatim. Event-history (ReadEventsAsync) is left at the interface default — the interface doesn't yet carry the EventFilter surface needed to forward it.

Certificate trust

AutoAcceptCertificates accepts any self-signed / untrusted server certificate. It is dev-only — leave it false in production so a MITM against the opc.tcp channel fails closed. The SDK rejects a revoked certificate (BadCertificateRevoked / BadCertificateIssuerRevoked) before the auto-accept handler runs, and the handler also refuses them explicitly — defence in depth (#277), not a behaviour change.

CertificateValidation (defaults change nothing):

Key	Default	Meaning
`rejectUnknownRevocationStatus`	`false`	Reject a server certificate whose revocation status cannot be determined (no CRL for its issuer, or a stale one). Leave `false` for brownfield sites without CRL infrastructure. Only effective with `autoAcceptCertificates` off — the auto-accept handler accepts every suppressible error, including revocation-unknown.
`issuerStorePath`	`null` (= `%LocalAppData%/OtOpcUa/pki/issuers`)	Directory used as the trusted-issuer store. The SDK reads issuer CRLs (`*.crl`) from its `crl/` sub-folder, so this is how an operator points the driver at a managed CRL set.

Subscription tuning (#273)

The per-subscription parameters that used to be hardcoded are driver options; the defaults are the old values.

Key	Default	Notes
`subscriptionPublishingInterval`	`null`	A .NET `TimeSpan` string (`"00:00:02"`), not a number. `null` = the interval the runtime requests per subscribe (floored at 50 ms). When set it overrides that value (floor still applies); monitored-item sampling follows it.
`subscriptionKeepAliveCount`	`10`	`>= 1`.
`subscriptionLifetimeCount`	`1000`	Must be `>= 3 x` keep-alive count (Part 4 §5.13.2).
`subscriptionMaxNotificationsPerPublish`	`0`	`0` = server decides.
`subscriptionPriority`	`0`	`0`-`255`.

Out-of-range values (and a non-absolute discoveryUrl) are rejected when the options are loaded — driver creation / InitializeAsync fails once with a clear message rather than the driver faulting and retrying forever. The alarm subscription uses the same counts. Per-tag tuning (deadband / sampling interval / queue size, #274) is not implemented — it needs an ISubscribable signature change on Core.

Server OperationLimits (#275)

Nothing driver-side: the SDK session already honours the remote server's ServerCapabilities/OperationLimits. Opc.Ua.Client.Session derives from Opc.Ua.SessionClientBatched, fetches the limits on open (FetchOperationLimitsAsync) and splits Read / Write / CreateMonitoredItems / Browse / HistoryRead requests that exceed them (a limit of 0 = unlimited). The driver issues one logical call per operation and a unit test pins the Session : SessionClientBatched relationship so an SDK change that dropped it is caught. (A driver-side splitter was tried and removed: it duplicated the SDK and, on a stalled link, held the driver gate for chunks x timeout.)

Discovery URL (#278)

discoveryUrl (default null, e.g. opc.tcp://lds:4840): at InitializeAsync the driver runs FindServers on it and GetEndpoints on every returned server, keeping the endpoints matching securityPolicy + securityMode. Discovered URLs are tried after endpointUrls/endpointUrl (case-insensitive de-dup) — discovery can add fallbacks but never outranks or replaces the configured server. Because an LDS commonly registers several servers (identical line PLCs, a gateway plus a historian) and the first to accept a connection would be bound silently:

discoveryServerUri set: only endpoints of the server with that ApplicationUri are used.
unset and exactly one distinct server discovered: that server is used.
unset and more than one: the connect attempt fails with an error naming the discovered ApplicationUris and telling the operator to set discoveryServerUri.

A discovery network failure (LDS down) is logged and non-fatal — the static list is used (note it is retried on every InitializeAsync, including comms-loss reconnects). The discovery channel itself is always unsecured (spec).

Diagnostics counters (#276)

Session/publish events are published on the shared OTel meter as the counter otopcua.driver.opcuaclient.events, tagged driver_instance (the driver instance id) and event = publish_response, notification (items), data_change, bad_status_value, publish_error, keepalive_miss, reconnect_attempt, reconnect_success, reconnect_exhausted. A climbing keepalive_miss / reconnect_attempt rate is the signature of a flapping upstream. They are not part of DriverHealth or the /hosts panel.

Testing

Unit tests — tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Tests/ cover the session lifecycle, namespace remapping, alarm/history passthrough, and config binding against a faked SDK session.
Integration fixture — exercises the driver against a reference OPC UA server (opc-plc) on the shared docker host; see OpcUaClient-Test-Fixture.md for the coverage map.

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