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Baggage API

Status: Stable, Feature-freeze

Table of Contents

Overview

Baggage is a set of application-defined properties contextually associated with a distributed request or workflow execution (see also the W3C Baggage Specification). Baggage can be used, among other things, to annotate telemetry, adding contextual information to metrics, traces, and logs.

In OpenTelemetry Baggage is represented as a set of name/value pairs describing user-defined properties. Each name in Baggage MUST be associated with exactly one value. This is more restrictive than the W3C Baggage Specification, § 3.2.1.1 which allows duplicate entries for a given name.

Baggage names are any valid UTF-8 strings. Language API SHOULD NOT restrict which strings are used as baggage names. However, the specific Propagators that are used to transmit baggage entries across component boundaries may impose their own restrictions on baggage names. For example, the W3C Baggage specification restricts the baggage keys to strings that satisfy the token definition from RFC7230, Section 3.2.6. For maximum compatibility, alpha-numeric names are strongly recommended to be used as baggage names.

Baggage values are any valid UTF-8 strings. Language API MUST accept any valid UTF-8 string as baggage value in Set and return the same value from Get.

Language API MUST treat both baggage names and values as case sensitive. See also W3C Baggage Rationale.

Example:

baggage.Set('a', 'B% 💼');
baggage.Set('A', 'c');
baggage.Get('a'); // returns "B% 💼"
baggage.Get('A'); // returns "c"

The Baggage API consists of:

  • the Baggage as a logical container
  • functions to interact with the Baggage in a Context

The functions described here are one way to approach interacting with the Baggage via having struct/object that represents the entire Baggage content. Depending on language idioms, a language API MAY implement these functions by interacting with the baggage via the Context directly.

The Baggage API MUST be fully functional in the absence of an installed SDK. This is required in order to enable transparent cross-process Baggage propagation. If a Baggage propagator is installed into the API, it will work with or without an installed SDK.

The Baggage container MUST be immutable, so that the containing Context also remains immutable.

Operations

Get Value

To access the value for a name/value pair set by a prior event, the Baggage API MUST provide a function that takes the name as input, and returns a value associated with the given name, or null if the given name is not present.

REQUIRED parameters:

Name the name to return the value for.

Get All Values

Returns the name/value pairs in the Baggage. The order of name/value pairs MUST NOT be significant. Based on the language specifics, the returned value can be either an immutable collection or an iterator on the immutable collection of name/value pairs in the Baggage.

Set Value

To record the value for a name/value pair, the Baggage API MUST provide a function which takes a name, and a value as input. Returns a new Baggage that contains the new value. Depending on language idioms, a language API MAY implement these functions by using a Builder pattern and exposing a way to construct a Builder from a Baggage.

REQUIRED parameters:

Name The name for which to set the value, of type string.

Value The value to set, of type string.

OPTIONAL parameters:

Metadata Optional metadata associated with the name-value pair. This should be an opaque wrapper for a string with no semantic meaning. Left opaque to allow for future functionality.

Remove Value

To delete a name/value pair, the Baggage API MUST provide a function which takes a name as input. Returns a new Baggage which no longer contains the selected name. Depending on language idioms, a language API MAY implement these functions by using a Builder pattern and exposing a way to construct a Builder from a Baggage.

REQUIRED parameters:

Name the name to remove.

Context Interaction

This section defines all operations within the Baggage API that interact with the Context.

If an implementation of this API does not operate directly on the Context, it MUST provide the following functionality to interact with a Context instance:

  • Extract the Baggage from a Context instance
  • Insert the Baggage to a Context instance

The functionality listed above is necessary because API users SHOULD NOT have access to the Context Key used by the Baggage API implementation.

If the language has support for implicitly propagated Context (see here), the API SHOULD also provide the following functionality:

  • Get the currently active Baggage from the implicit context. This is equivalent to getting the implicit context, then extracting the Baggage from the context.
  • Set the currently active Baggage to the implicit context. This is equivalent to getting the implicit context, then inserting the Baggage to the context.

All the above functionalities operate solely on the context API, and they MAY be exposed as static methods on the baggage module, as static methods on a class inside the baggage module (it MAY be named BaggageUtilities), or on the Baggage class. This functionality SHOULD be fully implemented in the API when possible.

Clear Baggage in the Context

To avoid sending any name/value pairs to an untrusted process, the Baggage API MUST provide a way to remove all baggage entries from a context.

This functionality can be implemented by having the user set an empty Baggage object/struct into the context, or by providing an API that takes a Context as input, and returns a new Context with no Baggage associated.

Propagation

Baggage MAY be propagated across process boundaries or across any arbitrary boundaries (process, $OTHER_BOUNDARY1, $OTHER_BOUNDARY2, etc) for various reasons.

The API layer or an extension package MUST include the following Propagators:

See Propagators Distribution for how propagators are to be distributed.

Note: The W3C baggage specification does not currently assign semantic meaning to the optional metadata.

On extract, the propagator should store all metadata as a single metadata instance per entry. On inject, the propagator should append the metadata per the W3C specification format. Refer to the API Propagators Operation section for the additional requirements these operations need to follow.

Conflict Resolution

If a new name/value pair is added and its name is the same as an existing name, than the new pair MUST take precedence. The value is replaced with the added value (regardless if it is locally generated or received from a remote peer).