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Messaging systems

Definitions

Although messaging systems are not as standardized as, e.g., HTTP, it is assumed that the following definitions are applicable to most of them that have similar concepts at all (names borrowed mostly from JMS):

A message usually consists of headers (or properties, or meta information) and an optional body. It is sent by a single message producer to:

  • Physically: some message broker (which can be e.g., a single server, or a cluster, or a local process reached via IPC). The broker handles the actual routing, delivery, re-delivery, persistence, etc. In some messaging systems the broker may be identical or co-located with (some) message consumers.
  • Logically: some particular message destination.

Destinations

A destination is usually identified by some name unique within the messaging system instance, which might look like an URL or a simple one-word identifier. Two kinds of destinations are distinguished: topics and queues. A message that is sent (the send-operation is often called "publish" in this context) to a topic is broadcasted to all subscribers of the topic. A message submitted to a queue is processed by a message consumer (usually exactly once although some message systems support a more performant at-least-once mode for messages with idempotent processing).

Message consumption

The consumption of a message can happen in multiple steps. First, the lower-level receiving of a message at a consumer, and then the logical processing of the message. Often, the waiting for a message is not particularly interesting and hidden away in a framework that only invokes some handler function to process a message once one is received (in the same way that the listening on a TCP port for an incoming HTTP message is not particularly interesting). However, in a synchronous conversation, the wait time for a message is important.

Conversations

In some messaging systems, a message can receive a reply message that answers a particular other message that was sent earlier. All messages that are grouped together by such a reply-relationship are called a conversation. The grouping usually happens through some sort of "In-Reply-To:" meta information or an explicit conversation ID (sometimes called correlation ID). Sometimes a conversation can span multiple message destinations (e.g. initiated via a topic, continued on a temporary one-to-one queue).

Temporary destinations

Some messaging systems support the concept of temporary destination (often only temporary queues) that are established just for a particular set of communication partners (often one to one) or conversation. Often such destinations are unnamed or have an auto-generated name.

Conventions

Given these definitions, the remainder of this section describes the semantic conventions that shall be followed for Spans describing interactions with messaging systems.

Span name

The span name SHOULD be set to the message destination name and the operation being performed in the following format:

<destination name> <operation name>

The destination name SHOULD only be used for the span name if it is known to be of low cardinality (cf. general span name guidelines). This can be assumed if it is statically derived from application code or configuration. If the destination name is dynamic, such as a conversation ID or a value obtained from a Reply-To header, it SHOULD NOT be used for the span name. In these cases, an artificial destination name that best expresses the destination, or a generic, static fallback like "(temporary)" for temporary destinations SHOULD be used instead.

The values allowed for <operation name> are defined in the section Operation names below. If the format above is used, the operation name MUST match the messaging.operation attribute defined for message consumer spans below.

Examples:

  • shop.orders send
  • shop.orders receive
  • shop.orders process
  • print_jobs send
  • topic with spaces process
  • AuthenticationRequest-Conversations process
  • (temporary) send ((temporary) being a stable identifier for randomly generated, temporary destination names)

Span kind

A producer of a message should set the span kind to PRODUCER unless it synchronously waits for a response: then it should use CLIENT. The processor of the message should set the kind to CONSUMER, unless it always sends back a reply that is directed to the producer of the message (as opposed to e.g., a queue on which the producer happens to listen): then it should use SERVER.

Operation names

The following operations related to messages are defined for these semantic conventions:

Operation name Description
send A message is sent to a destination by a message producer/client.
receive A message is received from a destination by a message consumer/server.
process A message that was previously received from a destination is processed by a message consumer/server.

Messaging attributes

Attribute name Notes and examples Required?
messaging.system A string identifying the messaging system such as kafka, rabbitmq or activemq. Yes
messaging.destination The message destination name, e.g. MyQueue or MyTopic. This might be equal to the span name but is required nevertheless. Yes
messaging.destination_kind The kind of message destination: Either queue or topic. Yes, if either of them applies.
messaging.temp_destination A boolean that is true if the message destination is temporary. If temporary (assumed to be false if missing).
messaging.protocol The name of the transport protocol such as AMQP or MQTT. No
messaging.protocol_version The version of the transport protocol such as 0.9.1. No
messaging.url Connection string such as tibjmsnaming://localhost:7222 or https://queue.amazonaws.com/80398EXAMPLE/MyQueue. No
messaging.message_id A value used by the messaging system as an identifier for the message, represented as a string. No
messaging.conversation_id The conversation ID identifying the conversation to which the message belongs, represented as a string. Sometimes called "Correlation ID". No
messaging.message_payload_size_bytes The (uncompressed) size of the message payload in bytes. Also use this attribute if it is unknown whether the compressed or uncompressed payload size is reported. No
messaging.message_payload_compressed_size_bytes The compressed size of the message payload in bytes. No

Additionally at least one of net.peer.name or net.peer.ip from the network attributes is required and net.peer.port is recommended. Furthermore, it is strongly recommended to add the net.transport attribute and follow its guidelines, especially for in-process queueing systems (like Hangfire, for example). These attributes should be set to the broker to which the message is sent/from which it is received.

For message consumers, the following additional attributes may be set:

Attribute name Notes and examples Required?
messaging.operation A string identifying the kind of message consumption as defined in the Operation names section above. Only "receive" and "process" are used for this attribute. If the operation is "send", this attribute MUST NOT be set, since the operation can be inferred from the span kind in that case. No

The receive span is be used to track the time used for receiving the message(s), whereas the process span(s) track the time for processing the message(s). Note that one or multiple Spans with messaging.operation = process may often be the children of a Span with messaging.operation = receive. The distinction between receiving and processing of messages is not always of particular interest or sometimes hidden away in a framework (see the Message consumption section above) and therefore the attribute can be left out. For batch receiving and processing (see the Batch receiving and Batch processing examples below) in particular, the attribute SHOULD be set. Even though in that case one might think that the processing span's kind should be INTERNAL, that kind MUST NOT be used. Instead span kind should be set to either CONSUMER or SERVER according to the rules defined above.

Attributes specific to certain messaging systems

RabbitMQ

In RabbitMQ, the destination is defined by an exchange and a routing key. messaging.destination MUST be set to the name of the exchange. This will be an empty string if the default exchange is used. The routing key MUST be provided to the attribute messaging.rabbitmq.routing_key, unless it is empty.

Examples

Topic with multiple consumers

Given is a process P, that publishes a message to a topic T on messaging system MS, and two processes CA and CB, which both receive the message and process it.

Process P:  | Span Prod1 |
--
Process CA:              | Span CA1 |
--
Process CB:                 | Span CB1 |
Field or Attribute Span Prod1 Span CA1 Span CB1
Span name "T send" "T process" "T process"
Parent Span Prod1 Span Prod1
Links
SpanKind PRODUCER CONSUMER CONSUMER
Status Ok Ok Ok
net.peer.name "ms" "ms" "ms"
net.peer.port 1234 1234 1234
messaging.system "kafka" "kafka" "kafka"
messaging.destination "T" "T" "T"
messaging.destination_kind "topic" "topic" "topic"
messaging.operation "process" "process"
messaging.message_id "a1" "a1" "a1"

Batch receiving

Given is a process P, that sends two messages to a queue Q on messaging system MS, and a process C, which receives both of them in one batch (Span Recv1) and processes each message separately (Spans Proc1 and Proc2).

Since a span can only have one parent and the propagated trace and span IDs are not known when the receiving span is started, the receiving span will have no parent and the processing spans are correlated with the producing spans using links.

Process P: | Span Prod1 | Span Prod2 |
--
Process C:                      | Span Recv1 |
                                        | Span Proc1 |
                                               | Span Proc2 |
Field or Attribute Span Prod1 Span Prod2 Span Recv1 Span Proc1 Span Proc2
Span name "Q send" "Q send" "Q receive" "Q process" "Q process"
Parent Span Recv1 Span Recv1
Links Span Prod1 Span Prod2
SpanKind PRODUCER PRODUCER CONSUMER CONSUMER CONSUMER
Status Ok Ok Ok Ok Ok
net.peer.name "ms" "ms" "ms" "ms" "ms"
net.peer.port 1234 1234 1234 1234 1234
messaging.system "kafka" "kafka" "kafka" "kafka" "kafka"
messaging.destination "Q" "Q" "Q" "Q" "Q"
messaging.destination_kind "queue" "queue" "queue" "queue" "queue"
messaging.operation "receive" "process" "process"
messaging.message_id "a1" "a2" "a1" "a2"

Batch processing

Given is a process P, that sends two messages to a queue Q on messaging system MS, and a process C, which receives both of them separately (Span Recv1 and Recv2) and processes both messages in one batch (Span Proc1).

Since each span can only have one parent, C3 should not choose a random parent out of C1 and C2, but rather rely on the implicitly selected parent as defined by the tracing API spec. Similarly, only one value can be set as message_id, so C3 cannot report both a1 and a2 and therefore attribute is left out. Depending on the implementation, the producing spans might still be available in the meta data of the messages and should be added to C3 as links. The client library or application could also add the receiver span's span context to the data structure it returns for each message. In this case, C3 could also add links to the receiver spans C1 and C2.

The status of the batch processing span is selected by the application. Depending on the semantics of the operation. A span status Ok could, for example, be set only if all messages or if just at least one were properly processed.

Process P: | Span Prod1 | Span Prod2 |
--
Process C:                              | Span Recv1 | Span Recv2 |
                                                                   | Span Proc1 |
Field or Attribute Span Prod1 Span Prod2 Span Recv1 Span Recv2 Span Proc1
Span name "Q send" "Q send" "Q receive" "Q receive" "Q process"
Parent Span Prod1 Span Prod2
Links Span Prod1 + Prod2
SpanKind PRODUCER PRODUCER CONSUMER CONSUMER CONSUMER
Status Ok Ok Ok Ok Ok
net.peer.name "ms" "ms" "ms" "ms" "ms"
net.peer.port 1234 1234 1234 1234 1234
messaging.system "kafka" "kafka" "kafka" "kafka" "kafka"
messaging.destination "Q" "Q" "Q" "Q" "Q"
messaging.destination_kind "queue" "queue" "queue" "queue" "queue"
messaging.operation "receive" "receive" "process"
messaging.message_id "a1" "a2" "a1" "a2"