# Proposed shared library: `ZB.MOM.WW.Telemetry`

A contract on paper — the public surface to extract so the three projects stop implementing
observability separately. Realizes [`../spec/SPEC.md`](../spec/SPEC.md) and
[`../spec/METRIC-CONVENTIONS.md`](../spec/METRIC-CONVENTIONS.md). **Not yet created.**
Reference implementations already exist: OtOpcUa `ObservabilityExtensions.cs` (OTel + Serilog),
ScadaBridge `LoggerConfigurationFactory.cs` (Serilog enrichers), MxGateway
`GatewayMetrics.cs` + `GatewayLogRedactor.cs`.

## Packages (.NET 10)

```
ZB.MOM.WW.Telemetry          # OTel bootstrap: Resource, metrics, traces, exporters
ZB.MOM.WW.Telemetry.Serilog  # Serilog bootstrap: enrichers, TraceContextEnricher, ILogRedactor
```

Both packages are .NET 10 — all three logging-bearing processes are .NET 10 (OtOpcUa server,
mxaccessgw gateway, ScadaBridge central). The x86 net48 mxaccessgw worker uses a bespoke
`IWorkerLogger` (stderr key=value); net48 multi-targeting is **not** required. Published to
the Gitea NuGet feed; SemVer; lockstep to start.

## Packaging & distribution

**Two NuGet packages, one DLL each**, on the Gitea NuGet feed. Libraries linked into each
app — there is no central telemetry service. Both packages are consumed by all three apps
after adoption:

| Package (→ DLL) | Transitive deps | OtOpcUa | MxGateway | ScadaBridge |
|---|---|---|---|---|
| `…Telemetry` | OpenTelemetry SDK, `OpenTelemetry.Exporter.Prometheus.AspNetCore`, `OpenTelemetry.Exporter.OpenTelemetryProtocol`, standard instrumentation packages | ✅ | ✅ | ✅ |
| `…Telemetry.Serilog` | Serilog, `Serilog.Extensions.Hosting`, `Serilog.AspNetCore` (version note below) | ✅ | ✅ | ✅ |

> **`Serilog.AspNetCore` version split (open convergence note):** OtOpcUa and ScadaBridge
> target .NET 10 and may use `Serilog.AspNetCore` 9.x; MxGateway's adoption starts from
> `Serilog.AspNetCore` 9.x as well. If a project remains on .NET 8 ASP.NET Core for any
> reason, the compatible version is `Serilog.AspNetCore` 8.x. Coordinate the version floor
> when the first app takes a dependency and pin it in `Directory.Packages.props`.

---

## `ZB.MOM.WW.Telemetry`

```csharp
namespace ZB.MOM.WW.Telemetry;

/// Selects how instrumentation data is exported.
public enum ZbExporter
{
    /// Prometheus scrape endpoint (default). Call app.MapZbMetrics() to mount /metrics.
    Prometheus,

    /// OTLP gRPC export. Set OtlpEndpoint (e.g. "http://collector:4317").
    /// Coexists with Prometheus when both endpoints are desired.
    Otlp,
}

/// Options for AddZbTelemetry. All properties feed the shared OTel Resource and
/// Serilog enrichers (via AddZbSerilog in the .Serilog package).
public sealed class ZbTelemetryOptions
{
    /// Required. Short lower-case app identifier — e.g. "otopcua", "mxgateway", "scadabridge".
    /// Populates OTel Resource service.name.
    public string ServiceName { get; set; } = "";

    /// Fleet-wide namespace. Default "ZB.MOM.WW". Do not override per-app.
    /// Populates OTel Resource service.namespace.
    public string ServiceNamespace { get; set; } = "ZB.MOM.WW";

    /// Optional. Populate from AssemblyInformationalVersion.
    /// Populates OTel Resource service.version.
    public string? ServiceVersion { get; set; }

    /// Optional. Physical or logical site identifier.
    /// Populates OTel Resource site.id and Serilog property SiteId.
    public string? SiteId { get; set; }

    /// Optional. Node function: "central", "site", "hub", "standalone".
    /// Populates OTel Resource node.role and Serilog property NodeRole.
    public string? NodeRole { get; set; }

    /// App-specific Meter names to register with the OTel MeterProvider.
    /// Always register the app's primary Meter here. Standard instrumentation meters are
    /// added automatically (ASP.NET Core, HttpClient, runtime, process).
    public string[] Meters { get; set; } = [];

    /// App-specific ActivitySource names to register with the OTel TracerProvider.
    public string[] ActivitySources { get; set; } = [];

    /// Export path. Default Prometheus; use Otlp for a real collector.
    public ZbExporter Exporter { get; set; } = ZbExporter.Prometheus;

    /// Required when Exporter = ZbExporter.Otlp.
    /// OTLP gRPC endpoint, e.g. "http://collector:4317".
    public string? OtlpEndpoint { get; set; }
}

/// Extension point for configuring the OTel bootstrap on an IHostApplicationBuilder.
public static class ZbTelemetryExtensions
{
    /// Configures the OpenTelemetry MeterProvider and TracerProvider with the shared Resource,
    /// standard instrumentation (ASP.NET Core, HttpClient, gRPC client, runtime, process),
    /// the app's own Meters and ActivitySources, and the selected exporter.
    /// Does NOT configure Serilog — call AddZbSerilog() in the .Serilog package for that.
    public static IHostApplicationBuilder AddZbTelemetry(
        this IHostApplicationBuilder builder,
        Action<ZbTelemetryOptions>   configure);

    /// IServiceCollection overload for contexts where IHostApplicationBuilder is not available.
    /// Requires the caller to supply a pre-built ZbTelemetryOptions (Resource attributes must
    /// be populated before DI composition, so the options-object overload is preferred).
    public static IServiceCollection AddZbTelemetry(
        this IServiceCollection    services,
        ZbTelemetryOptions         options);

    /// IServiceCollection convenience overload that accepts a configure delegate.
    /// Equivalent to calling BuildOptions(configure) then AddZbTelemetry(services, options).
    /// Use when only an IServiceCollection is available but the lambda form is preferred.
    public static IServiceCollection AddZbTelemetry(
        this IServiceCollection    services,
        Action<ZbTelemetryOptions> configure);
}

/// Builds the shared OTel ResourceBuilder from ZbTelemetryOptions.
/// Used internally by AddZbTelemetry. Exposed for tests and custom pipelines.
public static class ZbResource
{
    /// Deterministic, process-stable service instance identifier, formatted as
    /// "MachineName:ProcessId". Populates OTel Resource service.instance.id on every signal
    /// (metrics, traces, logs). The OTel SDK's random-GUID default is disabled in favour of this
    /// value so all signals from one process share one restart-stable instance id, enabling
    /// cross-signal correlation. Always present (not optional).
    public static string InstanceId { get; }

    /// Returns a ResourceBuilder pre-populated with service.name, service.namespace,
    /// service.version, service.instance.id (always — see InstanceId), site.id, node.role, and
    /// host.name (always Environment.MachineName). Attributes with null/empty values are omitted.
    public static ResourceBuilder Build(ZbTelemetryOptions options);

    /// The single source of truth for the shared Resource attribute set. Both the OTel SDK
    /// metrics/traces pipeline and the Serilog OTLP log sink derive their attributes from this one
    /// map, so logs cannot drift from metrics/traces. service.name/namespace/instance.id/host.name
    /// are always present; service.version/site.id/node.role are included only when the option is
    /// non-null/non-empty.
    public static IReadOnlyDictionary<string, object> BuildAttributes(ZbTelemetryOptions options);
}

/// Endpoint extension for mounting the Prometheus /metrics scrape endpoint.
public static class ZbMetricsEndpointExtensions
{
    /// Mounts the Prometheus /metrics endpoint. Valid under ANY ZbTelemetryOptions.Exporter value:
    /// AddZbTelemetry always wires the Prometheus exporter, and OTLP (ZbExporter.Otlp) is only an
    /// additive overlay — so /metrics serves scrape data even when Exporter = ZbExporter.Otlp.
    /// Call after app.UseRouting().
    public static IEndpointConventionBuilder MapZbMetrics(
        this IEndpointRouteBuilder endpoints);
}
```

---

## `ZB.MOM.WW.Telemetry.Serilog`

```csharp
namespace ZB.MOM.WW.Telemetry.Serilog;

/// Extension point for registering the Serilog application logger in DI on an IHostApplicationBuilder.
public static class ZbSerilogExtensions
{
    /// Registers the Serilog application logger in DI. Wires configuration-driven sinks
    /// (ReadFrom.Configuration reads Serilog:WriteTo sinks + Serilog:MinimumLevel) with
    /// fixed enrichers: SiteId + NodeRole from ZbTelemetryOptions; NodeHostname from
    /// Environment.MachineName (auto — not a caller-supplied option); TraceContextEnricher;
    /// and RedactionEnricher (applied only when ILogRedactor is registered).
    ///
    ///   MinimumLevel: AddZbSerilog reads "Serilog:MinimumLevel" from IConfiguration. Callers that
    ///             bind MinimumLevel from a different config key (e.g. ScadaBridge's
    ///             "ScadaBridge:Logging:MinimumLevel") apply that override themselves before or after
    ///             calling AddZbSerilog — this remains per-project and AddZbSerilog does not read it.
    ///
    /// IMPORTANT — no process-global state: AddZbSerilog does NOT set Log.Logger. It passes
    /// preserveStaticLogger: true to AddSerilog so the static logger is left untouched.
    /// This makes AddZbSerilog safe to call multiple times in one process (integration tests,
    /// multi-host apps) without hitting "The logger is already frozen".
    ///
    /// Apps that need a pre-Build() bootstrap logger (for startup exceptions before IConfiguration
    /// is available) should set Log.Logger themselves in Program.cs:
    ///   Log.Logger = new LoggerConfiguration().WriteTo.Console().CreateBootstrapLogger();
    /// That is an application-level decision — not done by this library.
    ///
    /// OTel log export is wired automatically: logs flow through the OTel pipeline with the same
    /// Resource as the metrics and traces (all three signals correlated in a backend).
    ///
    /// The configure delegate receives the same ZbTelemetryOptions used by AddZbTelemetry.
    /// Typically share a single options-population lambda across both calls.
    public static IHostApplicationBuilder AddZbSerilog(
        this IHostApplicationBuilder builder,
        Action<ZbTelemetryOptions>   configure);
}

/// Canonical Serilog property name constants for the identity enrichers.
/// Use these constants — not literal strings — when querying properties in sinks or tests.
public static class ZbLogEnricherNames
{
    /// Serilog property: physical or logical site identifier. Matches OTel Resource site.id.
    public const string SiteId       = "SiteId";

    /// Serilog property: node function (central, site, hub, standalone). Matches OTel node.role.
    public const string NodeRole     = "NodeRole";

    /// Serilog property: machine name (Environment.MachineName). Matches OTel host.name.
    public const string NodeHostname = "NodeHostname";
}

/// Stamps trace_id and span_id from Activity.Current onto every Serilog log event.
/// When Activity.Current is null (no active span — background services, startup, non-traced paths)
/// the enricher emits nothing; it does NOT inject empty strings or zero values.
/// This enables a log line to be clicked through to its originating trace in a backend.
public sealed class TraceContextEnricher : ILogEventEnricher
{
    public void Enrich(LogEvent logEvent, ILogEventPropertyFactory propertyFactory);
}

/// Seam for project-specific log-event redaction.
/// The shared library applies this via RedactionEnricher; each project provides its own
/// implementation that knows which fields (by property name) or which command payloads
/// must not leave the process in log events.
/// If no ILogRedactor is registered in DI, RedactionEnricher is a no-op.
public interface ILogRedactor
{
    /// Inspect and mutate properties in-place. Remove or replace any sensitive values.
    /// Called on every log event before it reaches any sink.
    void Redact(IDictionary<string, object?> properties);
}

/// Applies a registered ILogRedactor to every Serilog log event.
/// Registered automatically by AddZbSerilog. The enricher resolves ILogRedactor from DI
/// on first use; if none is registered it is permanently inert (no DI call per event).
public sealed class RedactionEnricher : ILogEventEnricher
{
    public RedactionEnricher(IServiceProvider serviceProvider);
    public void Enrich(LogEvent logEvent, ILogEventPropertyFactory propertyFactory);
}
```

---

## Consumer matrix

| Consumer | Packages | Notes |
|---|---|---|
| **MxGateway** | Both | MEL → Serilog migration: `GatewayLogScope`/`BeginScope` → `LogContext.PushProperty`; `GatewayLogRedactor` → `ILogRedactor` impl; `GatewayMetrics` stays, wired through `o.Meters`. **Done in this release.** |
| **OtOpcUa** | Both | Consolidate existing Serilog bootstrap; add `TraceContextEnricher` + `SiteId`/`NodeRole` enrichers; add Resource to existing OTel pipeline. Deferred to GAPS backlog. |
| **ScadaBridge** | Both | Add full OTel SDK (metrics + traces + export); consolidate `LoggerConfigurationFactory`; add `TraceContextEnricher`. Deferred to GAPS backlog. |

The net48 x86 mxaccessgw worker is excluded from both packages. Its `IWorkerLogger`
(stderr key=value format) is an out-of-process concern and remains bespoke.

---

## Open contract questions

1. **`IServiceCollection` overload completeness:** the `IHostApplicationBuilder`-based
   overload is the primary path (available in all three apps on .NET 10). The
   `IServiceCollection` overload is a fallback for unusual host configurations. Validate
   that both overloads wire OTel log export identically (same Resource, same enrichers).

2. **OTel log export channel:** `AddZbSerilog` uses `Serilog.Sinks.OpenTelemetry` to push
   logs into the OTel pipeline (sharing the Resource). Confirm the sink version is
   compatible with the OpenTelemetry SDK version pinned in `ZB.MOM.WW.Telemetry`
   (`Directory.Packages.props`).

3. **`RedactionEnricher` DI timing:** `RedactionEnricher` resolves `ILogRedactor` from
   `IServiceProvider` on first use (lazy, to avoid a circular-DI problem at host-build time).
   Validate that the service provider is fully built by the time the first post-startup log
   event fires. If MxGateway's `GatewayLogRedactor` has dependencies that are not yet
   available when the DI container is being composed, the lazy-resolve pattern protects it.
   (Note: the library no longer sets a Stage-1 bootstrap logger, so there is no Stage-1 vs.
   Stage-2 logger lifetime to reason about — only the single DI-registered application logger.)

4. **`SiteId` / `NodeRole` null handling:** `AddZbTelemetry` and `AddZbSerilog` silently
   omit null `SiteId`/`NodeRole` from the Resource and enricher set. Confirm this is the
   correct behavior for OtOpcUa, which may run in a single-site configuration where neither
   field is meaningful, versus ScadaBridge, where `SiteId` is essential for multi-cluster
   fleet visibility.

See [`../GAPS.md`](../GAPS.md) for the adoption order and effort/risk.