ServiceComposer.AspNetCore

OpenTelemetry

ServiceComposer emits OpenTelemetry traces using System.Diagnostics.ActivitySource, which is built into the .NET runtime. No additional NuGet packages are required in the application library itself.

Two independent activity sources are available — one per feature — so each can be opted into separately.

Feature	Activity source name
ViewModel Composition	`ServiceComposer.AspNetCore.ViewModelComposition`
Scatter/Gather	`ServiceComposer.AspNetCore.ScatterGather`

Setup

Add the OpenTelemetry.Extensions.Hosting NuGet package, then register the desired source(s) with the tracing pipeline.

ViewModel Composition only

builder.Services.AddOpenTelemetry()
    .WithTracing(b => b
        .AddSource("ServiceComposer.AspNetCore.ViewModelComposition"));

^{snippet source | anchor}

Scatter/Gather only

builder.Services.AddOpenTelemetry()
    .WithTracing(b => b
        .AddSource("ServiceComposer.AspNetCore.ScatterGather"));

^{snippet source | anchor}

Both features

AddSource supports * as a wildcard, so both sources can be registered in one call:

builder.Services.AddOpenTelemetry()
    .WithTracing(b => b
        .AddSource("ServiceComposer.AspNetCore.*"));

^{snippet source | anchor}

What gets traced

ViewModel Composition

Each ICompositionRequestsHandler execution produces a child span of the ASP.NET Core HTTP server span.

	Value
Operation name	`composition.handler`
Display name	Fully qualified handler type name
`composition.handler.type`	Fully qualified handler type name
`composition.handler.namespace`	Handler namespace

When a handler raises an event via context.RaiseEvent<TEvent>(), the event handling produces a child span of the raising handler’s span.

	Value
Operation name	`composition.event`
Display name	Fully qualified event type name
`composition.event.type`	Fully qualified event type name
`composition.event.namespace`	Event namespace

When a handler or event handler throws, the span sets ActivityStatusCode.Error and adds the following tags and an exception span event following the OTel exception conventions.

Tag	Value
`otel.status_code`	`"error"`
`otel.status_description`	Exception message

Scatter/Gather

Each IGatherer execution produces a child span of the ASP.NET Core HTTP server span.

	Value
Operation name	`scatter-gather.gatherer`
Display name	Gatherer key
`scatter_gather.gatherer.key`	The gatherer key

When a gatherer throws, the span sets ActivityStatusCode.Error with the same otel.status_code, otel.status_description tags and exception span event as described above.

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