# OPC UA Server

The OPC UA server component (`src/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer/`) hosts the OPC UA stack and exposes a browsable address space built from the registered drivers. The server itself is driver-agnostic — Galaxy/MXAccess, Modbus, S7, AB CIP, AB Legacy, TwinCAT, FOCAS, and OPC UA Client are all plugged in as `IDriver` implementations via the capability interfaces in `src/Core/ZB.MOM.WW.OtOpcUa.Core.Abstractions/`.

In v2 the Server and Admin processes were fused into a single role-gated `ZB.MOM.WW.OtOpcUa.Host` binary. Which subsystems start (OPC UA endpoint, Admin UI, control plane, driver runtime) is decided by the `OTOPCUA_ROLES` gate, not by running separate executables. See `docs/ServiceHosting.md` for the role model.

## Composition

`OtOpcUaSdkServer` (`src/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer/OtOpcUaSdkServer.cs`) subclasses the OPC Foundation `StandardServer` and wires a single custom node manager:

- `CreateMasterNodeManager` constructs one `OtOpcUaNodeManager` (`src/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer/OtOpcUaNodeManager.cs`) — a `CustomNodeManager2` subclass that owns the writable address space under the namespace `https://zb.com/otopcua/ns` and a single `OtOpcUa` root folder organized under the standard `Objects` folder. It is wrapped in a `MasterNodeManager` with no additional core managers.
- `OtOpcUaSdkServer.NodeManager` exposes the live node manager after `StartAsync`, so the hosting layer can wrap it in a `SdkAddressSpaceSink` (`src/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer/SdkAddressSpaceSink.cs`) and hand it to `OpcUaPublishActor`.

Address-space population is push-driven: drivers stream discovery and data-change events through the Akka actor system (`DriverInstanceActor` → `OpcUaPublishActor`), and `OpcUaPublishActor` writes them into the node manager through the `IOpcUaAddressSpaceSink` seam. `OtOpcUaNodeManager.EnsureFolder` / `EnsureVariable` materialize the UNS folder + variable hierarchy; `WriteValue` / `WriteAlarmState` push runtime values and fire `ClearChangeMasks` so subscribed clients see updates.

The driver-agnostic walk that turns a driver's discovery into folder/variable calls lives in `GenericDriverNodeManager` (`src/Core/ZB.MOM.WW.OtOpcUa.Core/OpcUa/GenericDriverNodeManager.cs`): it walks `ITagDiscovery.DiscoverAsync` into an `IAddressSpaceBuilder`, captures alarm-condition sinks for variables flagged via `IVariableHandle.MarkAsAlarmCondition`, subscribes to `IAlarmSource.OnAlarmEvent`, and routes each alarm transition to the sink registered for its `SourceNodeId`.

The lifecycle facade `OpcUaApplicationHost` (`src/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer/OpcUaApplicationHost.cs`) owns the `ApplicationInstance` + `ApplicationConfiguration` lifetime, starts the `StandardServer`, and attaches the `ImpersonateUser` hook (see Session impersonation).

## Resilience and capability dispatch

Driver-capability calls (`IReadable.ReadAsync`, `IWritable.WriteAsync`, `ITagDiscovery.DiscoverAsync`, `ISubscribable.SubscribeAsync/UnsubscribeAsync`, the `IHostConnectivityProbe` probe loop, `IAlarmSource` surfaces, and the four `IHistoryProvider` reads) are routed through a `CapabilityInvoker` (`src/Core/ZB.MOM.WW.OtOpcUa.Core/Resilience/CapabilityInvoker.cs`) so the Polly resilience pipeline (retry / timeout / breaker / bulkhead) applies. There is one invoker per `(DriverInstance, IDriver)` pair; all invokers share the process-singleton `DriverResiliencePipelineBuilder`, which keys pipelines on `(DriverInstanceId, hostName, DriverCapability)`. Per-instance resilience options come from `DriverTypeRegistry` (the driver's tier) plus per-instance JSON overrides parsed from `DriverInstance.ResilienceConfig` by `DriverResilienceOptionsParser`.

The `OTOPCUA0001` Roslyn analyzer (`src/Tooling/ZB.MOM.WW.OtOpcUa.Analyzers/UnwrappedCapabilityCallAnalyzer.cs`, category `OtOpcUa.Resilience`, severity Warning) flags direct driver-capability calls that bypass the invoker.

| Capability | Surface | Invoker entry point |
|---|---|---|
| Read | `IReadable.ReadAsync` | `ExecuteAsync(DriverCapability.Read, host, …)` |
| Write | `IWritable.WriteAsync` | `ExecuteWriteAsync(host, isIdempotent, …)` — disables retries for non-idempotent writes per `WriteIdempotentAttribute` / decisions #44-45, #143 |
| Discovery | `ITagDiscovery.DiscoverAsync` | `ExecuteAsync(DriverCapability.Discover, host, …)` |
| Subscribe / Unsubscribe | `ISubscribable.SubscribeAsync/UnsubscribeAsync` | `ExecuteAsync(DriverCapability.Subscribe, host, …)` |
| HistoryRead (raw / processed / at-time / events) | `IHistoryProvider.*Async` | `ExecuteAsync(DriverCapability.HistoryRead, host, …)` |
| Alarm subscribe / unsubscribe / acknowledge | `IAlarmSource.SubscribeAlarmsAsync/UnsubscribeAlarmsAsync/AcknowledgeAsync` | via `AlarmSurfaceInvoker` (`src/Core/ZB.MOM.WW.OtOpcUa.Core/Resilience/AlarmSurfaceInvoker.cs`), which fans out per host |

The host name fed to the invoker comes from `IPerCallHostResolver.ResolveHost(fullReference)` when the driver implements it (multi-host drivers: AB CIP, Modbus, FOCAS, TwinCAT, AB Legacy resolve per device). Single-host drivers fall back to `DriverInstanceId`, preserving the per-instance pipeline-key semantics (decision #144).

## Configuration

Tenant-scoped server wiring flows from the SQL Server **Config DB**, not from `appsettings.json`: `ServerInstance` + `DriverInstance` + `Tag` + `NodeAcl` rows are published as a *generation* by `sp_PublishGeneration` and loaded into the running process by the generation applier. The Admin UI (Blazor Server, `docs/v2/admin-ui.md`) is the operator surface — drafts accumulate edits and `sp_ComputeGenerationDiff` drives the DiffViewer preview before publish. Optimistic concurrency uses each entity's `RowVersion`; a stale edit fails the publish/save rather than silently overwriting. See `docs/v2/config-db-schema.md` for the schema.

Environmental knobs that aren't per-tenant — bind address, port, PKI store root, security profiles — are supplied to `OpcUaApplicationHostOptions` and resolved from `appsettings.json` on the Host project.

## Transport

The server binds a TCP endpoint at `opc.tcp://{PublicHostname}:{OpcUaPort}/OtOpcUa` (defaults `0.0.0.0:4840`). The `ApplicationConfiguration` is built programmatically in `OpcUaApplicationHost.BuildConfigurationAsync` — there are no UA XML files unless `ApplicationConfigPath` is set. Security profiles are listed in `OpcUaApplicationHostOptions.EnabledSecurityProfiles`; by default all three baseline profiles are exposed (`None`, `Basic256Sha256` + Sign, `Basic256Sha256` + SignAndEncrypt) and the SDK publishes one endpoint descriptor per profile. Production deployments typically drop `None`. User token policies (`Anonymous`, `UserName`) are always attached; the `UserName` policy is SDK-encrypted with the server certificate so it works on `None` endpoints too. See `docs/security.md` for hardening.

## Session impersonation

`OpcUaApplicationHost` subscribes to `SessionManager.ImpersonateUser` after `ApplicationInstance.Start`. The handler (`HandleImpersonation`) deals with the token types as follows:

- `UserNameIdentityToken` → the password is decrypted, then `IOpcUaUserAuthenticator.AuthenticateUserNameAsync` validates the credential (`LdapUserAuthenticator` in production, a stub in tests). On success a `UserIdentity` carrying the token is attached and the LDAP-derived roles are logged; on failure `ImpersonateEventArgs.IdentityValidationError` is set to `BadIdentityTokenRejected`.
- `AnonymousIdentityToken` and X.509 tokens → the handler returns without intervening, so the SDK's default validation stands.

Decryption failures and authenticator exceptions also map to `BadIdentityTokenRejected`.

## Authorization

Node-level authorization is backed by a permission trie under `src/Core/ZB.MOM.WW.OtOpcUa.Core/Authorization/` (`PermissionTrie`, `PermissionTrieBuilder`, `PermissionTrieCache`, `TriePermissionEvaluator`, `NodeScope`, `UserAuthorizationState`, `AuthorizationDecision`). The trie is built from `NodeAcl` rows and a session's `UserAuthorizationState`, and an `IPermissionEvaluator` can return a per-tag `AuthorizationDecision` for Read / HistoryRead / Write / Browse independently. See `docs/v2/acl-design.md`.

## Redundancy

`Redundancy.Enabled = true` on the `ServerInstance` activates the `RedundancyStateActor` + `ServiceLevelCalculator` (`src/Server/ZB.MOM.WW.OtOpcUa.ControlPlane/Redundancy/`). The OPC UA `Server/ServiceLevel` node (`VariableIds.Server_ServiceLevel`) is recomputed and republished via `SdkServiceLevelPublisher` (`src/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer/SdkServiceLevelPublisher.cs`, wired as `IServiceLevelPublisher`) whenever role or driver-health changes; `ServiceLevelCalculator` produces a 0–255 value where higher means more authoritative, so the primary advertises a higher ServiceLevel than the secondary. Clients also read the standard `Server/ServerRedundancy/RedundancySupport` and `Server/ServerRedundancy/ServerUriArray` properties the SDK exposes on the ServerObject. An apply-lease prevents two instances from concurrently applying a generation. See `docs/Redundancy.md`.

Peer endpoints are advertised through the standard `Server.ServerArray` property: `OpcUaApplicationHost` appends `OpcUaApplicationHostOptions.PeerApplicationUris` to `IServerInternal.ServerUris` after start so warm-redundancy clients can discover the partner.

## Server class hierarchy

### OtOpcUaSdkServer extends StandardServer

- **`CreateMasterNodeManager`** — Constructs the single `OtOpcUaNodeManager` and wraps it in a `MasterNodeManager` with no extra core managers.
- **`NodeManager`** — Public accessor exposing the live `OtOpcUaNodeManager` once the SDK has bootstrapped (null until `CreateMasterNodeManager` runs).

`ApplicationName`, `ApplicationUri` (`urn:OtOpcUa`), and `ProductUri` (`https://zb.com/otopcua`) come from `OpcUaApplicationHostOptions`, which the `ApplicationConfiguration` is built from in `OpcUaApplicationHost`.

## Certificate handling

Certificate stores are directory-based under `OpcUaApplicationHostOptions.PkiStoreRoot` (default `pki`, relative to the host's working directory):

| Store | Path suffix |
|---|---|
| Own (application certificate) | `pki/own` |
| Trusted issuers | `pki/issuer` |
| Trusted peers | `pki/trusted` |
| Rejected | `pki/rejected` |

`OpcUaApplicationHostOptions.AutoAcceptUntrustedClientCertificates` (default `false`) controls whether unknown client certificates are auto-trusted on first connection; production deployments leave it off and operators promote peers via the Admin UI. The application instance certificate is auto-created (SDK defaults: 2048-bit, 12-month lifetime) on first start against a fresh PKI tree, and the server certificate is always created — even for `None`-only deployments — because `UserName` token encryption needs it.

## Key source files

- `src/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer/OtOpcUaSdkServer.cs` — `StandardServer` subclass wiring the single node manager
- `src/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer/OpcUaApplicationHost.cs` — programmatic `ApplicationConfiguration` + lifecycle + `ImpersonateUser` hook + ServerArray population
- `src/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer/OtOpcUaNodeManager.cs` — `CustomNodeManager2` owning the writable address space
- `src/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer/SdkAddressSpaceSink.cs` — `IOpcUaAddressSpaceSink` adapter the actor system pushes into
- `src/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer/SdkServiceLevelPublisher.cs` — publishes the redundancy `ServiceLevel` node
- `src/Core/ZB.MOM.WW.OtOpcUa.Core/OpcUa/GenericDriverNodeManager.cs` — driver-agnostic discovery walk + alarm routing
- `src/Core/ZB.MOM.WW.OtOpcUa.Core/Hosting/DriverHost.cs` — process-local driver registration + lifecycle
- `src/Core/ZB.MOM.WW.OtOpcUa.Core/Resilience/CapabilityInvoker.cs` — Polly pipeline entry point for capability calls
- `src/Core/ZB.MOM.WW.OtOpcUa.Core/Resilience/AlarmSurfaceInvoker.cs` — per-host fan-out wrapper for `IAlarmSource`
- `src/Core/ZB.MOM.WW.OtOpcUa.Core/Authorization/` — permission trie + evaluator (`PermissionTrie`, `PermissionTrieCache`, `TriePermissionEvaluator`)