# OpenTelemetry .NET API

[![NuGet](https://img.shields.io/nuget/v/OpenTelemetry.Api.svg)](https://www.nuget.org/packages/OpenTelemetry.Api)
[![NuGet](https://img.shields.io/nuget/dt/OpenTelemetry.Api.svg)](https://www.nuget.org/packages/OpenTelemetry.Api)

* [Installation](#installation)
* [Introduction](#introduction)
  * [Tracing API](#tracing-api)
  * [Logging API](#logging-api)
  * [Metrics API](#metrics-api)
  * [Baggage API](#baggage-api)
* [Introduction to OpenTelemetry .NET Tracing
  API](#introduction-to-opentelemetry-net-tracing-api)
* [Instrumenting a library/application with .NET Activity
  API](#instrumenting-a-libraryapplication-with-net-activity-api)
  * [Basic usage](#basic-usage)
  * [Activity creation options](#activity-creation-options)
  * [Adding Events](#adding-events)
  * [Setting Status](#setting-status)
* [Instrumenting a library/application with OpenTelemetry.API
  Shim](#instrumenting-using-opentelemetryapi-shim)
* [Troubleshooting](#troubleshooting)
* [References](#references)

## Installation

```shell
dotnet add package OpenTelemetry.Api
```

## Introduction

Application developers and library authors use OpenTelemetry API to instrument
their application/library. The API only surfaces necessary abstractions to
instrument an application/library. It does not address concerns like how
telemetry is exported to a specific telemetry backend, how to sample the
telemetry, etc. The API consists of [Tracing
API](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/trace/api.md),
[Logging
API](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/logs/overview.md),
[Metrics
API](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/api.md),
[Context and Propagation
API](https://github.com/open-telemetry/opentelemetry-specification/tree/main/specification/context),
and a set of [semantic
conventions](https://github.com/open-telemetry/opentelemetry-specification/tree/main/specification/trace/semantic_conventions).

### Tracing API

[Tracing
API](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/trace/api.md)
allows users to generate
[Spans](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/trace/api.md#span),
which represent a single operation within a trace. Spans can be nested to form a
trace tree. Each trace contains a root span, which typically describes the
entire operation and, optionally one or more child-spans for its
child-operations.

### Logging API

OpenTelemetry .NET does not introduce its own API for logging. Instead it
provides an integration with the well known [Microsoft.Extensions.Logging
API](https://docs.microsoft.com/dotnet/core/extensions/logging).

### Metrics API

[Metrics
API](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/api.md)
allows users to capture measurements about the execution of a computer program
at runtime. The Metrics API is designed to process raw measurements, generally
with the intent to produce continuous summaries of those measurements.

### Baggage API

[Baggage
API](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/baggage/api.md)
allows users to add context to metric, traces, and logs. Baggage can be
propagated out of proc using
[Propagators](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/context/api-propagators.md).
OpenTelemetry SDK ships a BaggagePropagator and enables it by default.

It is important to note that `Baggage` is not automatically attached to any
telemetry. User *can* explicitly read `Baggage` and use it to enrich metrics,
logs and traces. An example of doing this for traces is shown
[here](../../docs/trace/extending-the-sdk/README.md#processor).

```csharp
// Use GetBaggage to get all the key/value pairs present in Baggage
foreach (var item in Baggage.GetBaggage())
{
    Console.WriteLine(item.Key);
    Console.WriteLine(item.Value);
}

// Use SetBaggage method to add a key/value pair in Baggage
Baggage.SetBaggage("AppName", "MyApp");
Baggage.SetBaggage("Region", "West US");

// Use RemoveBaggage method to remove a key/value pair in Baggage
Baggage.RemoveBaggage("AppName");

// Use ClearBaggage method to remove all the key/value pairs in Baggage
Baggage.ClearBaggage();
```

The recommended way to add Baggage is to use the `Baggage.SetBaggage()` API.
OpenTelemetry users should not use the `Activity.AddBaggage` method.

## Introduction to OpenTelemetry .NET Tracing API

.NET runtime had `Activity` class for a long time, which was meant to be used
for tracing purposes and represents the equivalent of the OpenTelemetry
[Span](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/trace/api.md#span).
OpenTelemetry .NET is reusing the existing `Activity` and associated classes to
represent the OpenTelemetry `Span`. This means, users can instrument their
applications/libraries to emit OpenTelemetry compatible traces by using just the
.NET Runtime.

The `Activity` and associated classes are shipped as part of
`System.Diagnostics.DiagnosticSource` nuget package. Version 5.0.0 of this
package contains improvements to `Activity` class which makes it more closely
aligned with OpenTelemetry [API
specification](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/trace/api.md).

Even though `Activity` enables all the scenarios OpenTelemetry supports, users
who are already familiar with OpenTelemetry terminology may find it easy to
operate with that terminology. For instance, `StartSpan` may be preferred over
`StartActivity`. To help with this transition, the OpenTelemetry.API package has
[shim](#instrumenting-using-opentelemetryapi-shim) classes to wrap around the
.NET `Activity` classes.

The shim exist only in the API. OpenTelemetry SDK for .NET will be operating
entirely with `Activity` only. Irrespective of whether shim classes or
`Activity` is used for instrumentation, the end result would be same. i.e
Processors/Exporters see the same data.

The recommended way of instrumenting is by using the [.NET Activity
API](#instrumenting-a-libraryapplication-with-net-activity-api). Users are
required to just take dependency on the
[DiagnosticSource](https://www.nuget.org/packages/System.Diagnostics.DiagnosticSource/5.0.0).
Adding dependency to
[OpenTelemetry.API](https://www.nuget.org/packages/opentelemetry.api) is
required only for the following scenarios:

1. You want to use terminology matching OpenTelemetry spec (Span vs Activity).
   The [shim](#instrumenting-using-opentelemetryapi-shim) can be useful for such
   users. Refer to the [comparison of Activity API and OpenTelemetry Tracing
   API](https://github.com/open-telemetry/opentelemetry-dotnet/issues/947) if
   you want to compare the differences.

2. Your library performs communication with other libraries/components, and want
   to access
   [Propagators](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/context/api-propagators.md),
   to inject and extract context data. Some of the most common libraries
   requiring this include
   [HttpClient](../OpenTelemetry.Instrumentation.Http/README.md), [ASP.NET
   Core](../OpenTelemetry.Instrumentation.AspNetCore/README.md). This repo
   already provides instrumentation for these common libraries. If your library
   is not built on top of these, and want to leverage propagators, follow the
   [Context propagation](#context-propagation) section.

3. You want to leverage
   [Baggage](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/baggage/api.md)
   API.

## Instrumenting a library/application with .NET Activity API

### Basic usage

As mentioned in the introduction, the instrumentation API for OpenTelemetry .NET
is the .NET `Activity` API. Guidance for instrumenting using this API is
documented fully in the TBD(dotnet activity user guide link), but is described
here as well.

1. Install the `System.Diagnostics.DiagnosticSource` package version `5.0.1` or
   above to your application or library.

    ```xml
    <ItemGroup>
      <PackageReference
        Include="System.Diagnostics.DiagnosticSource"
        Version="5.0.1"
      />
    </ItemGroup>
    ```

2. Create an `ActivitySource`, providing the name and version of the
   library/application doing the instrumentation. `ActivitySource` instance is
   typically created once and is reused throughout the application/library.

    ```csharp
    static ActivitySource activitySource = new ActivitySource(
        "companyname.product.instrumentationlibrary",
        "semver1.0.0");
    ```

    The above requires import of the `System.Diagnostics` namespace.

3. Use the `ActivitySource` instance from above to create `Activity` instances,
   which represent a single operation within a trace. The parameter passed is
   the `DisplayName` of the activity.

    ```csharp
    var activity = activitySource.StartActivity("ActivityName");
    ```

    If there are no listeners interested in this activity, the activity above
    will be null. This happens when the final application does not enable
    OpenTelemetry (or other `ActivityListener`s), or when OpenTelemetry samplers
    chose not to sample this activity. Ensure that all subsequent calls using
    this activity are protected with a null check.

4. Populate activity with tags following the [OpenTelemetry semantic
   conventions](https://github.com/open-telemetry/opentelemetry-specification/tree/main/specification/trace/semantic_conventions).
   It is highly recommended to check `activity.IsAllDataRequested`, before
   populating any tags which are not readily available. `IsAllDataRequested` is
   the same as
   [Span.IsRecording](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/trace/api.md#isrecording)
   and will be false when samplers decide to not record the activity, and this
   can be used to avoid any expensive operation to retrieve tags.

    ```csharp
    activity?.SetTag("http.method", "GET");
    if (activity != null && activity.IsAllDataRequested == true)
    {
        activity.SetTag("http.url", "http://www.mywebsite.com");
    }
    ```

    The recommended way to [set span
    attributes](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/trace/api.md#set-attributes)
    in `Activity` class is by using `SetTag()`. OpenTelemetry users should not
    use other methods like `AddTag`, `SetCustomProperty` on `Activity`.

5. Perform application/library logic.

6. Stop the activity when done.

    ```csharp
    activity?.Stop();
    ```

    Alternately, as `Activity` implements `IDisposable`, it can be used with a
    `using` block, which ensures activity gets stopped upon disposal. This is
    shown below.

    ```csharp
    using (var activity = activitySource.StartActivity("ActivityName")
    {
        activity?.SetTag("http.method", "GET");
    } // Activity gets stopped automatically at end of this block during dispose.
    ```

The above showed the basic usage of instrumenting using `Activity`. The
following sections describes more features.

### Activity creation options

Basic usage example above showed how `StartActivity` method can be used to start
an `Activity`. The started activity will automatically becomes the `Current`
activity. It is important to note that the `StartActivity` returns `null`, if no
listeners are interested in the activity to be created. This happens when the
final application does not enable OpenTelemetry, or when OpenTelemetry samplers
chose not to sample this activity.

`StartActivity` has many overloads to control the activity creation.

1. `ActivityKind`

    `Activity` has a property called `ActivityKind` which represents
    OpenTelemetry
    [SpanKind](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/trace/api.md#spankind).
    The default value will be `Internal`. `StartActivity` allows passing the
    `ActivityKind` while starting an `Activity`.

    ```csharp
    var activity = activitySource.StartActivity("ActivityName", ActivityKind.Server);
    ```

2. Parent using `ActivityContext`

    `ActivityContext` represents the OpenTelemetry
    [SpanContext](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/trace/api.md#spancontext).
    While starting a new `Activity`, the currently active `Activity` is
    automatically taken as the parent of the new activity being created.
    `StartActivity` allows passing explicit `ActivityContext` to override this
    behavior.

    ```csharp
    var parentContext = new ActivityContext(
        ActivityTraceId.CreateFromString("0af7651916cd43dd8448eb211c80319c"),
        ActivitySpanId.CreateFromString("b7ad6b7169203331"),
        ActivityTraceFlags.None);

    var activity = activitySource.StartActivity(
        "ActivityName",
        ActivityKind.Server,
        parentContext);
    ```

    As `ActivityContext` follows the [W3C
    Trace-Context](https://w3c.github.io/trace-context), it is also possible to
    provide the parent context as a single string matching the `traceparent`
    header of the W3C Trace-Context. This is shown below.

    ```csharp
    var activity = activitySource.StartActivity(
        "ActivityName",
        ActivityKind.Server,
        "00-0af7651916cd43dd8448eb211c80319c-b7ad6b7169203331-01");
    ```

3. Initial Tags

   `Tags` in `Activity` represents the OpenTelemetry [Span
   Attributes](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/trace/api.md#set-attributes).
   Earlier sample showed the usage of `SetTag` method of `Activity` to add tags.
   Refer to the
   [specification](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/common/common.md#attribute-and-label-naming)
   for best practices on naming tags. It is also possible to provide an initial
   set of tags during activity creation, as shown below. It is recommended to
   provide all available `Tags` during activity creation itself, as
   [Samplers](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/trace/sdk.md#sampler)
   can only consider information present during activity creation time.

    ```csharp
    var initialTags = new ActivityTagsCollection();

    initialTags["com.mycompany.product.mytag1"] = "tagValue1";
    initialTags["com.mycompany.product.mytag2"] = "tagValue2";

    var activity = activitySource.StartActivity(
        "ActivityName",
        ActivityKind.Server,
        "00-0af7651916cd43dd8448eb211c80319c-b7ad6b7169203331-01",
        initialTags);
    ```

    The above requires import of the `System.Collections.Generic` namespace.

4. Activity Links

   Apart from the parent-child relation, activities can be linked using
   `ActivityLinks` which represent the OpenTelemetry
   [Links](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/overview.md#links-between-spans).
   The linked activities must be provided during the creation time, as shown
   below.

    ```csharp
    var activityLinks = new List<ActivityLink>();

    var linkedContext1 = new ActivityContext(
        ActivityTraceId.CreateFromString("0af7651916cd43dd8448eb211c80319c"),
        ActivitySpanId.CreateFromString("b7ad6b7169203331"),
        ActivityTraceFlags.None);

    var linkedContext2 = new ActivityContext(
        ActivityTraceId.CreateFromString("4bf92f3577b34da6a3ce929d0e0e4736"),
        ActivitySpanId.CreateFromString("00f067aa0ba902b7"),
        ActivityTraceFlags.Recorded);

    activityLinks.Add(new ActivityLink(linkedContext1));
    activityLinks.Add(new ActivityLink(linkedContext2));

    var activity = activitySource.StartActivity(
        "ActivityWithLinks",
        ActivityKind.Server,
        default(ActivityContext),
        initialTags,
        activityLinks);
    ```

    Note that `Activity` above is created with `default(ActivityContext)`
    parent, which makes it child of implicit `Activity.Current` or orphan if
    there is no `Current`.

### Adding Events

It is possible to [add
events](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/trace/api.md#add-events)
to `Activity` using the `AddEvent` method as shown below.

```csharp
activity?.AddEvent(new ActivityEvent("sample activity event."));
```

Apart from providing name, timestamp and attributes can be provided by using
corresponding overloads of `ActivityEvent`.

### Setting Status

OpenTelemetry defines a concept called
[Status](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/trace/api.md#set-status)
to be associated with `Activity`. Starting with [DiagnosticSource
6.0](https://www.nuget.org/packages/System.Diagnostics.DiagnosticSource/6.0.0),
`SetStatus` API on `Activity` can be used to set the status and description as
shown below:

```csharp
activity?.SetStatus(ActivityStatusCode.Ok);
activity?.SetStatus(ActivityStatusCode.Error, "Error Description");
```

All the official
[Exporters](../../docs/trace/extending-the-sdk/README.md#exporter) shipped from
this repo support the above shown mechanism of setting status.

#### Setting Status - DiagnosticSource version older than 6.0

Prior to [DiagnosticSource
6.0](https://www.nuget.org/packages/System.Diagnostics.DiagnosticSource/6.0.0)
there was no `Status` field in `Activity`, and hence `Status` is set to an
`Activity` using the following special tags:

`otel.status_code` is the `Tag` name used to store the `StatusCode`, and
`otel.status_description` is the `Tag` name used to store the optional
`Description`.

Example:

```csharp
activity?.SetTag("otel.status_code", "ERROR");
activity?.SetTag("otel.status_description", "error status description");
```

Values for the StatusCode tag must be one of the strings "UNSET", "OK", or
"ERROR", which correspond respectively to the enums `Unset`, `Ok`, and `Error`
from [`StatusCode`](./Trace/StatusCode.cs).

If using OpenTelemetry API [shim](#instrumenting-using-opentelemetryapi-shim),
then you can leverage the `SetStatus` extension method on `Activity` as well.

## Instrumenting using OpenTelemetry.API Shim

As mentioned in the introduction section, using OpenTelemetry.API Shim is only
recommended if you want to use OpenTelemetry terminology like Tracer, Span
instead of ActivitySource, Activity.

Follow [this](../../examples/Console/TestOTelShimWithConsoleExporter.cs) code
for example usage of this shim.

## Context propagation

[OpenTelemetry.API](https://www.nuget.org/packages/opentelemetry.api) must be
used to access [Propagators
API](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/context/api-propagators.md)
which defines how to extract and inject context across process boundaries. This
is typically required if you are not using any of the .NET communication
libraries which has instrumentations already available which does the
propagation (eg: Asp.Net Core or HttpClient). In such cases, context extraction
and propagation is the responsibility of the library itself. An example would be
a producer-consumer pattern using some queuing library like RabbitMQ. Follow the
[messaging example](../../examples/MicroserviceExample/README.md) for examples
on how to
[inject](../../examples/MicroserviceExample/Utils/Messaging/MessageSender.cs)
and
[extract](../../examples/MicroserviceExample/Utils/Messaging/MessageReceiver.cs)
context.

## Introduction to OpenTelemetry .NET Metrics API

Metrics in OpenTelemetry .NET are a somewhat unique implementation of the
OpenTelemetry project, as the Metrics API is incorporated directly into the .NET
runtime itself, as part of the
[`System.Diagnostics.DiagnosticSource`](https://www.nuget.org/packages/System.Diagnostics.DiagnosticSource/6.0.0)
package. This means, users can instrument their applications/libraries to emit
metrics by simply using the `System.Diagnostics.DiagnosticSource` package. This
package can be used in applications targeting any of the officially supported
versions of [.NET](https://dotnet.microsoft.com/download/dotnet) and [.NET
Framework](https://dotnet.microsoft.com/download/dotnet-framework) (an older
Windows-based .NET implementation).

## Instrumenting a library/application with .NET Metrics API

### Basic metric usage

1. Install the `System.Diagnostics.DiagnosticSource` package version `6.0.0` or
   above to your application or library.

    ```xml
    <ItemGroup>
      <PackageReference Include="System.Diagnostics.DiagnosticSource" Version="6.0.0" />
    </ItemGroup>
    ```

2. Create a `Meter`, providing the name and version of the library/application
   doing the instrumentation. The `Meter` instance is typically created once and
   is reused throughout the application/library.

    ```csharp
    static Meter meter = new Meter(
        "companyname.product.instrumentationlibrary",
        "semver1.0.0");
    ```

    The above requires import of the `System.Diagnostics.Metrics` namespace.

    **Note:** It is important to note that `Meter` instances are created by
    using its constructor, and *not* by calling a `GetMeter` method on the
    `MeterProvider`. This is an important distinction from the [OpenTelemetry
    specification](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/api.md#get-a-meter),
    where `Meter`s are obtained from `MeterProvider`.

3. Use the `Meter` instance from above to create instruments, which can be used
   to report measurements. Just like meter instances, the instrument instances
   are to be created once and reused throughout the application/library.

    ```csharp
    static Counter<long> MyFruitCounter = meter.CreateCounter<long>("MyFruitCounter");
    ```

4. Use the instruments to report measurements, along with the attributes.

    ```csharp
    MyFruitCounter.Add(1, new("name", "apple"), new("color", "red"));
    ```

The above showed the usage of a `Counter` instrument. The following sections
describes more kinds of instruments.

### Instrument types

// TODO - add all instruments.

## Troubleshooting

This component uses an
[EventSource](https://docs.microsoft.com/dotnet/api/system.diagnostics.tracing.eventsource)
with the name "OpenTelemetry-Api" for its internal logging. Please refer to [SDK
troubleshooting](../OpenTelemetry/README.md#troubleshooting) for instructions on
seeing these internal logs.

## References

* [OpenTelemetry Project](https://opentelemetry.io/)
