OpenTelemetry Tracing Shim

.NET is different from other languages/runtimes that support OpenTelemetry. Tracing is implemented by the System.Diagnostics API, repurposing older constructs like ActivitySource and Activity to be OpenTelemetry-compliant under the covers.

OpenTelemetry for .NET also provides an API shim on top of the System.Diagnostics- based implementation. This shim is helpful if you’re working with other languages and OpenTelemetry in the same codebase, or if you prefer to use terminology consistent with the OpenTelemetry spec.

Initializing tracing

There are two main ways to initialize tracing, depending on whether you’re using a console app or something that’s ASP.NET Core-based.

Console app

To start tracing in a console app, you need to create a tracer provider.

First, ensure that you have the right packages:

dotnet add package OpenTelemetry
dotnet add package OpenTelemetry.Exporter.Console

And then use code like this at the beginning of your program, during any important startup operations.

using OpenTelemetry;
using OpenTelemetry.Trace;
using OpenTelemetry.Resources;

// ...

var serviceName = "MyServiceName";
var serviceVersion "1.0.0";

using var tracerProvider = Sdk.CreateTracerProviderBuilder()
    .AddSource(serviceName)
    .SetResourceBuilder(
        ResourceBuilder.CreateDefault()
            .AddService(serviceName: serviceName, serviceVersion: serviceVersion))
    .AddConsoleExporter()
    .Build();

//...

This is also where you can configure instrumentation libraries.

Note that this sample uses the Console Exporter. If you are exporting to another endpoint, you’ll have to use a different exporter.

ASP.NET Core

To start tracing in an ASP.NET Core-based app, use the OpenTelemetry extensions for ASP.NET Core setup.

First, ensure that you have the right packages:

dotnet add package OpenTelemetry --prerelease
dotnet add package OpenTelemetry.Instrumentation.AspNetCore --prerelease
dotnet add package OpenTelemetry.Extensions.Hosting --prerelease
dotnet add package OpenTelemetry.Exporter.Console --prerelease

And then configure it in your ASP.NET Core startup routine where you have access to an IServiceCollection.

using OpenTelemetry.Resources;
using OpenTelemetry.Trace;

// These can come from a config file, constants file, etc.
var serviceName = "MyCompany.MyProduct.MyService";
var serviceVersion = "1.0.0";

var builder = WebApplication.CreateBuilder(args);

// Configure important OpenTelemetry settings, the console exporter, and instrumentation library
builder.Services.AddOpenTelemetryTracing(tcb =>
{
    tcb
    .AddSource(serviceName)
    .SetResourceBuilder(
        ResourceBuilder.CreateDefault()
            .AddService(serviceName: serviceName, serviceVersion: serviceVersion))
    .AddAspNetCoreInstrumentation()
    .AddConsoleExporter();
});

In the preceding example, a Tracer corresponding to the service is injected during setup. This lets you get access to an instance in your endpoint mapping (or controllers if you’re using an older version of .NET).

It’s not required to inject a service-level tracer, nor does it improve performance either. You will need to decide where you’ll want your tracer instance to live, though.

This is also where you can configure instrumentation libraries.

Note that this sample uses the Console Exporter. If you are exporting to another endpoint, you’ll have to use a different exporter.

Setting up a tracer

Once tracing is initialized, you can configure a Tracer, which will be how you trace operations with Spans.

Typically, a Tracer is instantiated once per app/service that is being instrumented, so it’s a good idea to instantiate it once in a shared location. It is also typically named the same as the Service Name.

Injecting a tracer with ASP.NET Core

ASP.NET Core generally encourages injecting instances of long-lived objects like Tracers during setup.

using OpenTelemetry.Trace;

var builder = WebApplication.CreateBuilder(args);

// ...

builder.Services.AddSingleton(TracerProvider.Default.GetTracer(serviceName));

// ...

var app = builder.Build();

// ...

app.MapGet("/hello", (Tracer tracer) =>
{
    using var span = tracer.StartActiveSpan("hello-span");

    // do stuff
});

Acquiring a tracer from a TracerProvider

If you’re not using ASP.NET Core or would rather not inject an instance of a Tracer, create one from your instantialized TracerProvider:

// ...

var tracer = tracerProvider.GetTracer(serviceName);

// Assign it somewhere globally
 
//...

You’ll likely want to assign this Tracer instance to a variable in a central location so that you have access to it throughout your service.

You can instantiate as many Tracers as you’d like per service, although it’s generally sufficient to just have one defined per service.

Creating Spans

To create a span, give it a name and create it from your Tracer.

using var span = MyTracer.StartActiveSpan("SayHello");

// do work that 'span' will now track

Creating nested Spans

If you have a distinct sub-operation you’d like to track as a part of another one, you can create spans to represent the relationship.

public static void ParentOperation(Tracer tracer)
{
    using var parentSpan = tracer.StartActiveSpan("parent-span");

    // Do some work tracked by parentSpan

    ChildOperation(tracer);

    // Finish up work tracked by parentSpan again
}

public static void ChildOperation(Tracer tracer)
{
    using var childSpan = tracer.StartActiveSpan("child-span");

    // Track work in ChildOperation with childSpan
}

When you view spans in a trace visualization tool, child-span will be tracked as a nested operation under parent-span".

Nested Spans in the same scope

You may wish to create a parent-child relationsip in the same scope. Although possible, this is generally not recommended because you need to be careful to end any nested TelemetrySpan when you expect it to end.

public static void DoWork(Tracer tracer)
{
    using var parentSpan = tracer.StartActiveSpan("parent-span");

    // Do some work tracked by parentSpan

    using (var childSpan = tracer.StartActiveSpan("child-span"))
    {
        // Do some "child" work in the same function
    }

    // Finish up work tracked by parentSpan again
}

In the preceding example, childSpan is ended because the scope of the using block is explicitly defined, rather than scoped to DoWork itself like parentSpan.

Get the current Span

Sometimes it’s helpful to access whatever the current TelemetrySpan is at a point in time so you can enrich it with more information.

var span = Tracer.CurrentSpan;
// do cool stuff!

Note that using is not used in the prior example. Doing so will end current TelemetrySpan when it goes out of scope, which is unlikely to be desired behavior.

Add Attributes to a Span

Attributes let you attach key/value pairs to a TelemetrySpan so it carries more information about the current operation that it’s tracking.

using var span = tracer.StartActiveSpan("SayHello");

span.SetAttribute("operation.value", 1);
span.SetAttribute("operation.name", "Saying hello!");
span.SetAttribute("operation.other-stuff", new int[] { 1, 2, 3 });

Adding events

An event is a human-readable message on an TelemetrySpan that represents “something happening” during its lifetime. You can think of it like a primitive log.

using var span = tracer.StartActiveSpan("SayHello");

// ...

span.AddEvent("Doing something...");

// ...

span.AddEvent("Dit it!");

Events can also be created with a timestamp and a collection of attributes.

using var span = tracer.StartActiveSpan("SayHello");

// ...

span.AddEvent("event-message");
span.AddEvent("event-message2", DateTimeOffset.Now);

// ...

var attributeData = new Dictionary<string, object>
{
    {"foo", 1 },
    { "bar", "Hello, World!" },
    { "baz", new int[] { 1, 2, 3 } }
};

span.AddEvent("asdf", DateTimeOffset.Now, new(attributeData));

Adding links

A TelemetrySpan can be created with zero or more Links that are causally related.

// Get a context from somewhere, perhaps it's passed in as a parameter
var ctx = span.Context;

var links = new List<Link>
{
    new(ctx)
};

using var span = tracer.StartActiveSpan("another-span", links: links);

// do some work

Set span status

A status can be set on a span, typically used to specify that a span has not completed successfully - Status.Error. In rare scenarios, you could override the Error status with Ok, but don’t set Ok on successfully-completed spans.

The status can be set at any time before the span is finished:

using var span = tracer.StartActiveSpan("SayHello");

try
{
	// do something
}
catch (Exception ex)
{
    span.SetStatus(Status.Error, "Something bad happened!");
}

Record exceptions in spans

It can be a good idea to record exceptions when they happen. It’s recommended to do this in conjunction with setting span status.

using var span = tracer.StartActiveSpan("SayHello");

try
{
	// do something
}
catch (Exception ex)
{
    span.SetStatus(Status.Error, "Something bad happened!");
    span.RecordException(ex)
}

This will capture things like the current stack trace as attributes in the span.

Next steps

After you’ve setup manual instrumentation, you may want to use instrumentation libraries. Instrumentation libraries will instrument relevant libraries you’re using and generate data for things like inbound and outbound HTTP requests and more.

You’ll also want to configure an appropriate exporter to export your telemetry data to one or more telemetry backends.