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Propagators API

Table of Contents

Propagators API consists of two main formats:

  • BinaryFormat is used to serialize and deserialize a value into a binary representation.
  • HTTPTextFormat is used to inject and extract a value as text into carriers that travel in-band across process boundaries.

Deserializing must set IsRemote to true on the returned SpanContext.

Binary Format

BinaryFormat is a formatter to serialize and deserialize a value into a binary format.

BinaryFormat MUST expose the APIs that serializes values into bytes, and deserializes values from bytes.

ToBytes

Serializes the given value into the on-the-wire representation.

Required arguments:

  • the value to serialize, can be SpanContext or DistributedContext.

Returns the on-the-wire byte representation of the value.

FromBytes

Creates a value from the given on-the-wire encoded representation.

If the value could not be parsed, the underlying implementation SHOULD decide to return ether an empty value, an invalid value, or a valid value.

Required arguments:

  • on-the-wire byte representation of the value.

Returns a value deserialized from bytes.

HTTP Text Format

HTTPTextFormat is a formatter that injects and extracts a value as text into carriers that travel in-band across process boundaries.

Encoding is expected to conform to the HTTP Header Field semantics. Values are often encoded as RPC/HTTP request headers.

The carrier of propagated data on both the client (injector) and server (extractor) side is usually an http request. Propagation is usually implemented via library-specific request interceptors, where the client-side injects values and the server-side extracts them.

HTTPTextFormat MUST expose the APIs that injects values into carriers, and extracts values from carriers.

Fields

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

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 this formatter.

Inject

Injects the value downstream. For example, as http headers.

Required arguments:

  • the value to be injected, can be SpanContext or DistributedContext.
  • the carrier that holds propagation fields. For example, an outgoing message or http request.
  • the Setter invoked for each propagation key to add or remove.

Setter argument

Setter is an argument in Inject that puts value into given field.

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

Setter MUST be stateless and allowed to be saved as a constant to avoid runtime allocations. One of the ways to implement it is Setter class with Put method as described below.

Put

Replaces a propagated field with the given value.

Required arguments:

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

The implemenation 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.

Extract

Extracts the value from upstream. For example, as http headers.

If the value could not be parsed, the underlying implementation will decide to return an object representing either an empty value, an invalid value, or a valid value. Implementations MUST not return null.

Required arguments:

  • the carrier holds propagation fields. For example, an outgoing message or http request.
  • the instance of Getter invoked for each propagation key to get.

Returns the non-null extracted value.

Getter argument

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

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

Getter MUST be stateless and allowed to be saved as a constant to avoid runtime allocations. One of the ways to implement it is Getter class with Get method as described below.

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. To improve compatibility with other text-based protocols, text format implemenations MUST ensure to always use the canonical casing for their attributes. NOTE: Cannonical casing for HTTP headers is usually title case (e.g. Content-Type instead of content-type).