# OPC UA Server

The OPC UA server component hosts the Galaxy-backed namespace on a configurable TCP endpoint and exposes deployed System Platform objects and attributes to OPC UA clients.

## Configuration

`OpcUaConfiguration` defines the server endpoint and session settings. All properties have sensible defaults:

| Property | Default | Description |
|----------|---------|-------------|
| `Port` | `4840` | TCP port the server listens on |
| `EndpointPath` | `/LmxOpcUa` | URI path appended to the base address |
| `ServerName` | `LmxOpcUa` | Application name presented to clients |
| `GalaxyName` | `ZB` | Galaxy name used in the namespace URI |
| `MaxSessions` | `100` | Maximum concurrent client sessions |
| `SessionTimeoutMinutes` | `30` | Idle session timeout |
| `AlarmTrackingEnabled` | `false` | Enables `AlarmConditionState` nodes for alarm attributes |

The resulting endpoint URL is `opc.tcp://0.0.0.0:{Port}{EndpointPath}`, e.g., `opc.tcp://0.0.0.0:4840/LmxOpcUa`.

The namespace URI follows the pattern `urn:{GalaxyName}:LmxOpcUa` and serves as both the `ApplicationUri` and `ProductUri`.

## Programmatic ApplicationConfiguration

`OpcUaServerHost` builds the entire `ApplicationConfiguration` in code. There are no XML configuration files. This keeps deployment simple on factory floor machines where editing XML is error-prone.

The configuration covers:

- **ServerConfiguration** -- base address, session limits, security policies, and user token policies
- **SecurityConfiguration** -- certificate store paths under `%LOCALAPPDATA%\OPC Foundation\pki\`, auto-accept enabled
- **TransportQuotas** -- 4 MB max message/string/byte-string size, 120-second operation timeout, 1-hour security token lifetime
- **TraceConfiguration** -- OPC Foundation SDK tracing is disabled (output path `null`, trace masks `0`); all logging goes through Serilog instead

## Security Policy

The server runs with `MessageSecurityMode.None` and `SecurityPolicies.None`. Only anonymous user tokens are accepted:

```csharp
SecurityPolicies = { new ServerSecurityPolicy
{
    SecurityMode = MessageSecurityMode.None,
    SecurityPolicyUri = SecurityPolicies.None
} },
UserTokenPolicies = { new UserTokenPolicy(UserTokenType.Anonymous) }
```

This is intentional for plant-floor deployments where the server sits on an isolated OT network. Galaxy-level security classification controls write access per attribute rather than at the transport layer.

## Certificate handling

On startup, `OpcUaServerHost.StartAsync` calls `CheckApplicationInstanceCertificate(false, 2048)` to locate or create a 2048-bit self-signed certificate. The certificate subject follows the format `CN={ServerName}, O=ZB MOM, DC=localhost`. Certificate stores use the directory-based store type under `%LOCALAPPDATA%\OPC Foundation\pki\`:

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

`AutoAcceptUntrustedCertificates` is set to `true` so the server does not reject client certificates.

## Server class hierarchy

### LmxOpcUaServer extends StandardServer

`LmxOpcUaServer` inherits from the OPC Foundation `StandardServer` base class and overrides two methods:

- **`CreateMasterNodeManager`** -- Instantiates `LmxNodeManager` with the Galaxy namespace URI, the `IMxAccessClient` for runtime I/O, performance metrics, and an optional `HistorianDataSource`. The node manager is wrapped in a `MasterNodeManager` with no additional core node managers.
- **`LoadServerProperties`** -- Returns server metadata: manufacturer `ZB MOM`, product `LmxOpcUa Server`, and the assembly version as the software version.

### Session tracking

`LmxOpcUaServer` exposes `ActiveSessionCount` by querying `ServerInternal.SessionManager.GetSessions().Count`. `OpcUaServerHost` surfaces this for status reporting.

## Startup and Shutdown

`OpcUaServerHost.StartAsync` performs the following sequence:

1. Build `ApplicationConfiguration` programmatically
2. Validate the configuration via `appConfig.Validate(ApplicationType.Server)`
3. Create `ApplicationInstance` and check/create the application certificate
4. Instantiate `LmxOpcUaServer` and start it via `ApplicationInstance.Start`

`OpcUaServerHost.Stop` calls `_server.Stop()` and nulls both the server and application instance references. The class implements `IDisposable`, delegating to `Stop`.

## Key source files

- `src/ZB.MOM.WW.LmxOpcUa.Host/OpcUa/OpcUaServerHost.cs` -- Application lifecycle and programmatic configuration
- `src/ZB.MOM.WW.LmxOpcUa.Host/OpcUa/LmxOpcUaServer.cs` -- StandardServer subclass and node manager creation
- `src/ZB.MOM.WW.LmxOpcUa.Host/Configuration/OpcUaConfiguration.cs` -- Configuration POCO