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.
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).
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.
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 a conversation can span multiple message destinations (e.g. initiated via a topic, continued on a temporary one-to-one queue).
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.
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 usually be set to the message destination name. The conversation ID should be used instead when it is expected to have lower cardinality. In particular, the conversation ID must be used if the message destination is unnamed or temporary unless multiple conversations can be combined to a logical destination of lower cardinality.
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.
| Attribute name | Notes and examples | Required? |
|---|---|---|
messaging.system |
A string identifying the messaging system vendor 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 |
A value 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 which part and kind of message consumption this span describes: either receive or process. (If the operation is send, this attribute must not be set: 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.
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.
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.
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 |
|---|---|---|---|
| Name | "T" |
"T" |
"T" |
| 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" |
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 |
|---|---|---|---|---|---|
| Name | "Q" |
"Q" |
"Q" |
"Q" |
"Q" |
| 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" |
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 |
|---|---|---|---|---|---|
| Name | "Q" |
"Q" |
"Q" |
"Q" |
"Q" |
| 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" |