Context propagation requirements for messaging semantic conventions

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+# Context propagation requirements for messaging semantic conventions
+
+The [existing messaging semantic conventions for tracing](https://github.com/open-telemetry/opentelemetry-specification/blob/v1.11.0/specification/trace/semantic_conventions/messaging.md)
+implicitly impose certain requirements on context propagation mechanisms used.
+This document proposes a way to make these requirements explicit.
+
+This OTEP is based on [OTEP 0173](0173-messaging-semantic-conventions.md),
+which defines basic terms and describes messaging scenarios that should be
+supported by messaging semantic conventions.
+
+* [Terminology](#terminology)
+* [Motivation](#motivation)
+  - [Example](#example)
+* [Proposed addition to the messaging semantic conventions](#proposed-addition-to-the-messaging-semantic-conventions)
+  - [Context propagation](#context-propagation)
+  - [Requirements](#requirements)
+* [Future possibilities](#future-possibilities)
+  - [Transport context propagation](#transport-context-propagation)
+  - [Standards for context propagation](#standards-for-context-propagation)
+
+## Terminology
+
+For terms used in this document, refer to [OTEP 173](#0173-messaging-semantic-conventions.md#terminology).
+
+## Motivation
+
+The current [messaging semantic conventions for tracing](https://github.com/open-telemetry/opentelemetry-specification/blob/v1.11.0/specification/trace/semantic_conventions/messaging.md)
+provide a list of [examples](https://github.com/open-telemetry/opentelemetry-specification/blob/v1.11.0/specification/trace/semantic_conventions/messaging.md#examples).
+Those examples illustrate how producer and consumer spans can be correlated by
+parent/child relationships or links. All the examples assume that context
+information for a given message is propagated from the producer to the
+consumer.
+
+However, this is not a trivial assumption, and it is not easily accommodated by
+existing established context propagation mechanisms. Those mechanisms propagate
+context on a per-request basis, whereas the messaging semantic conventions
+assume that context is propagated on a per-message basis. This means, that
+although several requests might be involved in the processing of a single
+message (publishing a message, fetching a message, potentially multiple times by
+multiple consumers), it is assumed that all components have access to the same
+per-message context information that allows correlating all the stages of
+processing a message.
+
+To achieve this desired outcome, a context needs to be attached to a message,
+and intermediaries must not alter the context attached to the message. _This
+requirement should be documented, as it is an important factor in deciding how
+to propagate context for message scenarios and how to standardize context
+propagation for existing message protocols._
+
+The additions proposed in this document neither break nor invalidate any of
+the existing semantic conventions for messaging, but rather make an implicit
+requirement explicit.
+
+### Example
+
+Many intermediaries (message brokers) offer REST APIs for publishing and
+fetching messages. A producer can publish a message by sending an HTTP request
+to the intermediary and a consumer can pull a message by sending an HTTP request
+to the intermediary:
+
+```
+  +----------+
+  | Producer |
+  +----------+
+       |
+       | HTTP POST (publishing a message)
+       v
++--------------+
+| Intermediary |
++--------------+
+       ^
+       | HTTP GET (fetching a message)
+       |
+  +----------+
+  | Consumer |
+  +----------+
+```
+
+Existing semantic conventions suppose that the consumer can use context
+information from the producer trace to create links or parent/child
+relationships between consumer and producer traces. For this to be possible,
+context information from the producer needs to be propagated to the consumer.
+In the example outlined above, the consumers sends an HTTP GET request to the
+intermediary to fetch a message, the message is returned as part of the
+response. Via this HTTP request, context information can be propagated from the
+consumer to the intermediary, but not from the intermediary to the consumer.
+The consumer can obtain the necessary producer context information only if it
+is propagated as part of the message itself, independent of HTTP context
+propagation.
+
+For correlating producer and consumer traces without special intermediary
+instrumentation it is thus necessary to attach a producer context to the
+message so it can be extracted and used by the consumer, regardless of the
+contexts that are propagated on HTTP requests for publishing and fetching the
+message.
+
+Although OpenTelemetry semantic conventions cannot specify the exact mechanisms
+to achieve this for every intermediary and every protocol, this requirement
+must be clearly formulated, so that it can be implemented by protocols and
+instrumentations.
+
+## Proposed addition to the messaging semantic conventions
+
+### Context propagation
+
+A message may pass many different components and layers in one or more
+intermediaries when it is propagated from the producer to the consumer. It
+cannot be assumed, and in many cases, it is not even desired, that all those
+components and layers are instrumented and propagate context according to
+OpenTelemetry requirements.
+
+A _message creation context_ allows correlating the producer with the
+consumer(s) of a message, regardless of intermediary instrumentation. The
+message creation context is created by the producer and should be propagated to
+the consumer(s). It should not be altered by intermediaries. This context
+helps to model dependencies between producers and consumers, regardless of the
+underlying messaging transport mechanism and its instrumentation.
+
+Instrumentors are required to instrument producer and consumer applications
+so that context is attached to messages and extracted from messages in a
+coordinated way. Future versions of these conventions might recommend [context propagation according to certain industry standards](#standards-for-context-propagation).
+If the message creation context cannot be attached to the message and
+propagated, consumer traces cannot be directly correlated to producer traces.
+
+### Requirements
+
+A producer SHOULD attach a message creation context to each message. The message creation context
+SHOULD be attached in a way so that it is not possible to be changed by intermediaries.
+
+## Future possibilities
+
+### Transport context propagation
+
+A message creation context can be attached to a message, while different
+contexts are propagated with requests that publish and fetch a message. When
+coming up with conventions and guidance for intermediary instrumentation, it
+will be beneficial to clearly outline those two layers of context propagation
+and build conventions for intermediary instrumentation on top of this outline:
+
+1. The _message context layer_ allows correlating the producer with the
+   consumers of a message, regardless of intermediary instrumentation. The
+   creation context is created by the producer and must be propagated to the
+   consumers. It must not be altered by intermediaries.
+
+   This layer helps to model dependencies between producers and consumers,
+   regardless of the underlying messaging transport mechanism and its
+   instrumentation.
+2. An additional _transport context layer_ allows correlating the producer and
+   the consumer with an intermediary. It also allows correlating multiple
+   intermediaries among each other. The transport context can be changed by
+   intermediaries, according to intermediary instrumentations. Intermediaries that
+   are not instrumented might simply drop the transport context.
+
+   This layer helps to gain insights into details of the message transport.
+
+This would keep the existing correlation between producers and consumers intact
+while allowing intermediaries to use the transport context to correlate
+intermediary instrumentation with existing producer and consumer
+instrumentations.
+
+### Standards for context propagation
+
+Currently, instrumentation authors have to decide how to attach and extract
+context from messages to fulfil the [requirements for context propagation](#context-propagation).
+While preserving the freedom for instrumentation authors to choose how to
+propagate context, in the future these conventions should list recommended ways
+of how to propagate context using well-established messaging protocols.
+
+There are several work-in-progress efforts to standardize context propagation for different
+messaging protocols and scenarios:
+
+* [AMQP](https://w3c.github.io/trace-context-amqp/)
+* [MQTT](https://w3c.github.io/trace-context-mqtt/)
+* [CloudEvents via HTTP](https://github.com/cloudevents/spec/blob/v1.0.1/extensions/distributed-tracing.md)
+
+Once the standards reach a stable state, these semantic conventions will give a
+clear and stable recommendation for each protocol and scenario.