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Context

Status: Feature-freeze.

Table of Contents

Overview

A Context is a propagation mechanism which carries execution-scoped values across API boundaries and between logically associated execution units. Cross-cutting concerns access their data in-process using the same shared Context object.

A Context MUST be immutable, and its write operations MUST result in the creation of a new Context containing the original values and the specified values updated.

Languages are expected to use the single, widely used Context implementation if one exists for them. In the cases where an extremely clear, pre-existing option is not available, OpenTelemetry MUST provide its own Context implementation. Depending on the language, its usage may be either explicit or implicit.

Users writing instrumentation in languages that use Context implicitly are discouraged from using the Context API directly. In those cases, users will manipulate Context through cross-cutting concerns APIs instead, in order to perform operations such as setting trace or baggage entries for a specified Context.

A Context is expected to have the following operations, with their respective language differences:

Create a key

Keys are used to allow cross-cutting concerns to control access to their local state. They are unique such that other libraries which may use the same context cannot accidentally use the same key. It is recommended that concerns mediate data access via an API, rather than provide direct public access to their keys.

The API MUST accept the following parameter:

  • The key name. The key name exists for debugging purposes and does not uniquely identify the key. Multiple calls to CreateKey with the same name SHOULD NOT return the same value unless language constraints dictate otherwise. Different languages may impose different restrictions on the expected types, so this parameter remains an implementation detail.

The API MUST return an opaque object representing the newly created key.

Get value

Concerns can access their local state in the current execution state represented by a Context.

The API MUST accept the following parameters:

  • The Context.
  • The key.

The API MUST return the value in the Context for the specified key.

Set value

Concerns can record their local state in the current execution state represented by a Context.

The API MUST accept the following parameters:

  • The Context.
  • The key.
  • The value to be set.

The API MUST return a new Context containing the new value.

Optional Global operations

These operations are expected to only be implemented by languages using Context implicitly, and thus are optional. These operations SHOULD only be used to implement automatic scope switching and define higher level APIs by SDK components and OpenTelemetry instrumentation libraries.

Get current Context

The API MUST return the Context associated with the caller's current execution unit.

Attach Context

Associates a Context with the caller's current execution unit.

The API MUST accept the following parameters:

  • The Context.

The API MUST return a value that can be used as a Token to restore the previous Context.

Note that every call to this operation should result in a corresponding call to Detach Context.

Detach Context

Resets the Context associated with the caller's current execution unit to the value it had before attaching a specified Context.

This operation is intended to help making sure the correct Context is associated with the caller's current execution unit. Users can rely on it to identify a wrong call order, i.e. trying to detach a Context that is not the current instance. In this case the operation can emit a signal to warn users of the wrong call order, such as logging an error or returning an error value.

The API MUST accept the following parameters:

  • A Token that was returned by a previous call to attach a Context.

The API MAY return a value used to check whether the operation was successful or not.