Propagators API

Status: Stable, Feature-Freeze

Overview

Cross-cutting concerns send their state to the next process using Propagators, which are defined as objects used to read and write context data to and from messages exchanged by the applications. Each concern creates a set of Propagators for every supported Propagator type.

Propagators leverage the Context to inject and extract data for each cross-cutting concern, such as traces and Baggage.

Propagation is usually implemented via a cooperation of library-specific request interceptors and Propagators, where the interceptors detect incoming and outgoing requests and use the Propagator’s extract and inject operations respectively.

The Propagators API is expected to be leveraged by users writing instrumentation libraries.

Propagator Types

A Propagator type defines the restrictions imposed by a specific transport and is bound to a data type, in order to propagate in-band context data across process boundaries.

The Propagators API currently defines one Propagator type:

• TextMapPropagator is a type that inject values into and extracts values from carriers as string key/value pairs.

A binary Propagator type will be added in the future (see #437).

Carrier

A carrier is the medium used by Propagators to read values from and write values to. Each specific Propagator type defines its expected carrier type, such as a string map or a byte array.

Carriers used at Inject are expected to be mutable.

Operations

Propagators MUST define Inject and Extract operations, in order to write values to and read values from carriers respectively. Each Propagator type MUST define the specific carrier type and MAY define additional parameters.

Inject

Injects the value into a carrier. For example, into the headers of an HTTP request.

Required arguments:

• A Context. The Propagator MUST retrieve the appropriate value from the Context first, such as SpanContext, Baggage or another cross-cutting concern context.
• The carrier that holds the propagation fields. For example, an outgoing message or HTTP request.

Extract

Extracts the value from an incoming request. For example, from the headers of an HTTP request.

If a value can not be parsed from the carrier, for a cross-cutting concern, the implementation MUST NOT throw an exception and MUST NOT store a new value in the Context, in order to preserve any previously existing valid value.

Required arguments:

• A Context.
• The carrier that holds the propagation fields. For example, an incoming message or HTTP request.

Returns a new Context derived from the Context passed as argument, containing the extracted value, which can be a SpanContext, Baggage or another cross-cutting concern context.

TextMap Propagator

TextMapPropagator performs the injection and extraction of a cross-cutting concern value as string key/values pairs into carriers that travel in-band across process boundaries.

The carrier of propagated data on both the client (injector) and server (extractor) side is usually an HTTP request.

In order to increase compatibility, the key/value pairs MUST only consist of US-ASCII characters that make up valid HTTP header fields as per RFC 7230.

Getter and Setter are optional helper components used for extraction and injection respectively, and are defined as separate objects from the carrier to avoid runtime allocations, by removing the need for additional interface-implementing-objects wrapping the carrier in order to access its contents.

Getter and Setter MUST be stateless and allowed to be saved as constants, in order to effectively avoid runtime allocations.

Fields

The predefined propagation fields. If your carrier is reused, you should delete the fields here before calling Inject.

Fields are defined as string keys identifying format-specific components in a carrier.

For example, if the carrier is a single-use or immutable request object, you don’t need to clear fields as they couldn’t have been set before. If it is a mutable, retryable object, successive calls should clear these fields first.

The use cases of this are:

• allow pre-allocation of fields, especially in systems like gRPC Metadata
• allow a single-pass over an iterator

Returns list of fields that will be used by the TextMapPropagator.

Observe that some Propagators may define, besides the returned values, additional fields with variable names. To get a full list of fields for a specific carrier object, use the Keys operation.

TextMap Inject

Injects the value into a carrier. The required arguments are the same as defined by the base Inject operation.

Optional arguments:

• A Setter to set a propagation key/value pair. Propagators MAY invoke it multiple times in order to set multiple pairs. This is an additional argument that languages are free to define to help inject data into the carrier.

Setter argument

Setter is an argument in Inject that sets values into given fields.

Setter allows a TextMapPropagator to set propagated fields into a carrier.

One of the ways to implement it is Setter class with Set method as described below.

Set

Replaces a propagated field with the given value.

Required arguments:

• the carrier holding the propagation fields. For example, an outgoing message or an HTTP request.
• the key of the field.
• the value of the field.

The implementation SHOULD preserve casing (e.g. it should not transform Content-Type to content-type) if the used protocol is case insensitive, otherwise it MUST preserve casing.

TextMap Extract

Extracts the value from an incoming request. The required arguments are the same as defined by the base Extract operation.

Optional arguments:

• A Getter invoked for each propagation key to get. This is an additional argument that languages are free to define to help extract data from the carrier.

Returns a new Context derived from the Context passed as argument.

Getter argument

Getter is an argument in Extract that get value from given field

Getter allows a TextMapPropagator to read propagated fields from a carrier.

One of the ways to implement it is Getter class with Get and Keys methods as described below. Languages may decide on alternative implementations and expose corresponding methods as delegates or other ways.

Keys

The Keys function MUST return the list of all the keys in the carrier.

Required arguments:

• The carrier of the propagation fields, such as an HTTP request.

The Keys function can be called by Propagators which are using variable key names in order to iterate over all the keys in the specified carrier.

For example, it can be used to detect all keys following the uberctx-{user-defined-key} pattern, as defined by the Jaeger Propagation Format.

Get

The Get function MUST return the first value of the given propagation key or return null if the key doesn’t exist.

Required arguments:

• the carrier of propagation fields, such as an HTTP request.
• the key of the field.

The Get function is responsible for handling case sensitivity. If the getter is intended to work with a HTTP request object, the getter MUST be case insensitive.

Injectors and Extractors as Separate Interfaces

Languages can choose to implement a Propagator type as a single object exposing Inject and Extract methods, or they can opt to divide the responsibilities further into individual Injectors and Extractors. A Propagator can be implemented by composing individual Injectors and Extractors.

Composite Propagator

Implementations MUST offer a facility to group multiple Propagators from different cross-cutting concerns in order to leverage them as a single entity.

A composite propagator can be built from a list of propagators, or a list of injectors and extractors. The resulting composite Propagator will invoke the Propagators, Injectors, or Extractors, in the order they were specified.

Each composite Propagator will implement a specific Propagator type, such as TextMapPropagator, as different Propagator types will likely operate on different data types.

There MUST be functions to accomplish the following operations.

• Create a composite propagator
• Extract from a composite propagator
• Inject into a composite propagator

Create a Composite Propagator

Required arguments:

• A list of Propagators or a list of Injectors and Extractors.

Returns a new composite Propagator with the specified Propagators.

Composite Extract

Required arguments:

• A Context.
• The carrier that holds propagation fields.

If the TextMapPropagator’s Extract implementation accepts the optional Getter argument, the following arguments are REQUIRED, otherwise they are OPTIONAL:

• The instance of Getter invoked for each propagation key to get.

Composite Inject

Required arguments:

• A Context.
• The carrier that holds propagation fields.

If the TextMapPropagator’s Inject implementation accepts the optional Setter argument, the following arguments are REQUIRED, otherwise they are OPTIONAL:

• The Setter to set a propagation key/value pair. Propagators MAY invoke it multiple times in order to set multiple pairs.

Global Propagators

The OpenTelemetry API MUST provide a way to obtain a propagator for each supported Propagator type. Instrumentation libraries SHOULD call propagators to extract and inject the context on all remote calls. Propagators, depending on the language, MAY be set up using various dependency injection techniques or available as global accessors.

Note: It is a discouraged practice, but certain instrumentation libraries might use proprietary context propagation protocols or be hardcoded to use a specific one. In such cases, instrumentation libraries MAY choose not to use the API-provided propagators and instead hardcode the context extraction and injection logic.

The OpenTelemetry API MUST use no-op propagators unless explicitly configured otherwise. Context propagation may be used for various telemetry signals - traces, metrics, logging and more. Therefore, context propagation MAY be enabled for any of them independently. For instance, a span exporter may be left unconfigured, although the trace context propagation was configured to enrich logs or metrics.

Platforms such as ASP.NET may pre-configure out-of-the-box propagators. If pre-configured, Propagators SHOULD default to a composite Propagator containing the W3C Trace Context Propagator and the Baggage Propagator specified in the Baggage API. These platforms MUST also allow pre-configured propagators to be disabled or overridden.

Get Global Propagator

This method MUST exist for each supported Propagator type.

Returns a global Propagator. This usually will be composite instance.

Set Global Propagator

This method MUST exist for each supported Propagator type.

Sets the global Propagator instance.

Required parameters:

• A Propagator. This usually will be a composite instance.

Propagators Distribution

The official list of propagators that MUST be maintained by the OpenTelemetry organization and MUST be distributed as OpenTelemetry extension packages:

• W3C TraceContext. MAY alternatively be distributed as part of the OpenTelemetry API.
• W3C Baggage. MAY alternatively be distributed as part of the OpenTelemetry API.
• B3.
• Jaeger.

This is a list of additional propagators that MAY be maintained and distributed as OpenTelemetry extension packages:

• OT Trace. Propagation format used by the OpenTracing Basic Tracers. It MUST NOT use OpenTracing in the resulting propagator name as it is not widely adopted format in the OpenTracing ecosystem.
• OpenCensus BinaryFormat. Propagation format used by OpenCensus, which describes how to format the span context into the binary format, and does not prescribe a key. It is commonly used with OpenCensus gRPC using the grpc-trace-bin propagation key.

Additional Propagators implementing vendor-specific protocols such as AWS X-Ray trace header protocol MUST NOT be maintained or distributed as part of the Core OpenTelemetry repositories.

B3 Requirements

B3 has both single and multi-header encodings. It also has semantics that do not map directly to OpenTelemetry such as a debug trace flag, and allowing spans from both sides of request to share the same id. To maximize compatibility between OpenTelemetry and Zipkin implementations, the following guidelines have been established for B3 context propagation.

B3 Extract

When extracting B3, propagators:

• MUST attempt to extract B3 encoded using single and multi-header formats. The single-header variant takes precedence over the multi-header version.
• MUST preserve a debug trace flag, if received, and propagate it with subsequent requests. Additionally, an OpenTelemetry implementation MUST set the sampled trace flag when the debug flag is set.
• MUST NOT reuse X-B3-SpanId as the id for the server-side span.

B3 Inject

When injecting B3, propagators:

• MUST default to injecting B3 using the single-header format
• MUST provide configuration to change the default injection format to B3 multi-header
• MUST NOT propagate X-B3-ParentSpanId as OpenTelemetry does not support reusing the same id for both sides of a request.

Fields

Fields MUST return the header names that correspond to the configured format, i.e., the headers used for the inject operation.

Configuration