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Messaging systems

Status: Experimental

Definitions

Message

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 is an envelope with a potentially empty payload. This envelope may offer the possibility to convey additional metadata, often in key/value form.

A message is sent by a 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 delivery, re-delivery, persistence, etc. In some messaging systems the broker may be identical or co-located with (some) message consumers. With Apache Kafka, the physical broker a message is written to depends on the number of partitions, and which broker is the leader of the partition the record is written to.
  • Logically: some particular message destination.

Messages can be delivered to 0, 1, or multiple consumers depending on the dispatching semantic of the protocol.

Producer

The "producer" is a specific instance, process or device that creates and publishes a message. "Publishing" is the process of sending a message or batch of messages to the intermediary or consumer.

Consumer

A "consumer" receives the message and acts upon it. It uses the context and data to execute some logic, which might lead to the occurrence of new events.

The consumer receives, processes, and settles a message. "Receiving" is the process of obtaining a message from the intermediary, "processing" is the process of acting on the information a message contains, "settling" is the process of notifying an intermediary that a message was processed successfully.

Intermediary

An "intermediary" receives a message to forward it to the next receiver, which might be another intermediary or a consumer.

Destinations and sources

A destination is usually uniquely identified by name within the messaging system instance. Examples of a destination name would be a URL or a simple one-word identifier. Sending messages to a destination is called "publish" in context of this specification.

A source represents an entity within messaging system messages are consumed from. Source and destination for specific message may be the same. However, if message is routed within one or multiple brokers, source and destination can be different.

Typical examples of destinations and sources include Kafka topics, RabbitMQ queues and topics.

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).

Conversations

In some messaging systems, a message can receive one or more reply messages 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 and anonymous 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 also unnamed (anonymous) or have an auto-generated name.

Conventions

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

Context propagation

A message may traverse many different components and layers in one or more intermediaries when it is propagated from the producer to the consumer(s). To be able to correlate consumer traces with producer traces using the existing context propagation mechanisms, all components must propagate context down the chain.

Messaging systems themselves may trace messages as the messages travels from producers to consumers. Such tracing would cover the transport layer but would not help in correlating producers with consumers. To be able to directly correlate producers with consumers, another context that is propagated with the message is required.

A message creation context allows correlating producers with consumers of a message and model the dependencies between them, regardless of the underlying messaging transport mechanism and its instrumentation.

The message creation context is created by the producer and should be propagated to the consumer(s). Consumer traces cannot be directly correlated with producer traces if the message creation context is not attached and propagated with the message.

A producer SHOULD attach a message creation context to each message. If possible, the message creation context SHOULD be attached in such a way that it cannot be changed by intermediaries.

This document does not specify the exact mechanisms on how the creation context is attached/extracted to/from messages. Future versions of these conventions will give clear recommendations, following industry standards including, but not limited to Trace Context: AMQP protocol and Trace Context: MQTT protocol once those standards reach a stable state.

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. Wherever possible, the real destination names after resolving logical or aliased names SHOULD be used. 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 "(anonymous)" for anonymous 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 publish
  • shop.orders receive
  • shop.orders process
  • print_jobs publish
  • topic with spaces process
  • AuthenticationRequest-Conversations process
  • (anonymous) publish ((anonymous) being a stable identifier for an unnamed destination)

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
publish 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 Type Description Examples Requirement Level
messaging.system string A string identifying the messaging system. kafka; rabbitmq; rocketmq; activemq; AmazonSQS Required
messaging.operation string A string identifying the kind of messaging operation as defined in the Operation names section above. [1] publish Required
messaging.batch.message_count int The number of messages sent, received, or processed in the scope of the batching operation. [2] 0; 1; 2 Conditionally Required: [3]
messaging.message.conversation_id string The conversation ID identifying the conversation to which the message belongs, represented as a string. Sometimes called "Correlation ID". MyConversationId Recommended: [4]
messaging.message.id string A value used by the messaging system as an identifier for the message, represented as a string. 452a7c7c7c7048c2f887f61572b18fc2 Recommended: [5]
messaging.message.payload_compressed_size_bytes int The compressed size of the message payload in bytes. 2048 Recommended: [6]
messaging.message.payload_size_bytes int 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. 2738 Recommended: [7]
net.peer.name string Logical remote hostname, see note below. [8] example.com Conditionally Required: If available.
net.protocol.name string Application layer protocol used. The value SHOULD be normalized to lowercase. amqp; mqtt Recommended
net.protocol.version string Version of the application layer protocol used. See note below. [9] 3.1.1 Recommended
net.sock.family string Protocol address family which is used for communication. inet6; bluetooth Conditionally Required: [10]
net.sock.peer.addr string Remote socket peer address: IPv4 or IPv6 for internet protocols, path for local communication, etc. 127.0.0.1; /tmp/mysql.sock Recommended
net.sock.peer.name string Remote socket peer name. proxy.example.com Recommended: [11]
net.sock.peer.port int Remote socket peer port. 16456 Recommended: [12]

[1]: If a custom value is used, it MUST be of low cardinality.

[2]: Instrumentations SHOULD NOT set messaging.batch.message_count on spans that operate with a single message. When a messaging client library supports both batch and single-message API for the same operation, instrumentations SHOULD use messaging.batch.message_count for batching APIs and SHOULD NOT use it for single-message APIs.

[3]: If the span describes an operation on a batch of messages.

[4]: Only if span represents operation on a single message.

[5]: Only for spans that represent an operation on a single message.

[6]: Only if span represents operation on a single message.

[7]: Only if span represents operation on a single message.

[8]: This should be the IP/hostname of the broker (or other network-level peer) this specific message is sent to/received from.

[9]: net.protocol.version refers to the version of the protocol used and might be different from the protocol client's version. If the HTTP client used has a version of 0.27.2, but sends HTTP version 1.1, this attribute should be set to 1.1.

[10]: If different than inet and if any of net.sock.peer.addr or net.sock.host.addr are set. Consumers of telemetry SHOULD accept both IPv4 and IPv6 formats for the address in net.sock.peer.addr if net.sock.family is not set. This is to support instrumentations that follow previous versions of this document.

[11]: If different than net.peer.name and if net.sock.peer.addr is set.

[12]: If defined for the address family and if different than net.peer.port and if net.sock.peer.addr is set.

messaging.operation has the following list of well-known values. If one of them applies, then the respective value MUST be used, otherwise a custom value MAY be used.

Value Description
publish publish
receive receive
process process

Additionally net.peer.port from the network attributes 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.

Attribute namespaces

  • messaging.message: Contains attributes that describe individual messages
  • messaging.destination: Contains attributes that describe the logical entity messages are published to. See Destinations and sources for more details
  • messaging.source: Contains attributes that describe the logical entity messages are received from
  • messaging.batch: Contains attributes that describe batch operations
  • messaging.consumer: Contains attributes that describe application instance that consumes a message. See consumer for more details

Communication with broker is described with general network attributes.

Messaging system-specific attributes MUST be defined in the corresponding messaging.{system} namespace as described in Attributes specific to certain messaging systems.

Producer attributes

The following additional attributes describe message producer operations.

Attribute Type Description Examples Requirement Level
messaging.destination.anonymous boolean A boolean that is true if the message destination is anonymous (could be unnamed or have auto-generated name). Conditionally Required: [1]
messaging.destination.name string The message destination name [2] MyQueue; MyTopic Conditionally Required: [3]
messaging.destination.template string Low cardinality representation of the messaging destination name [4] /customers/{customerId} Conditionally Required: [5]
messaging.destination.temporary boolean A boolean that is true if the message destination is temporary and might not exist anymore after messages are processed. Conditionally Required: [6]

[1]: If value is true. When missing, the value is assumed to be false.

[2]: Destination name SHOULD uniquely identify a specific queue, topic or other entity within the broker. If the broker does not have such notion, the destination name SHOULD uniquely identify the broker.

[3]: If one message is being published or if the value applies to all messages in the batch.

[4]: Destination names could be constructed from templates. An example would be a destination name involving a user name or product id. Although the destination name in this case is of high cardinality, the underlying template is of low cardinality and can be effectively used for grouping and aggregation.

[5]: If available. Instrumentations MUST NOT use messaging.destination.name as template unless low-cardinality of destination name is guaranteed.

[6]: If value is true. When missing, the value is assumed to be false.

Consumer attributes

The following additional attributes describe message consumer operations.

Note: Consumer spans can have attributes describing source and destination. Since messages could be routed by brokers, source and destination don't always match. If original destination information is available on the consumer, consumer instrumentations SHOULD populate corresponding messaging.destination attributes.

Attribute Type Description Examples Requirement Level
messaging.consumer.id string The identifier for the consumer receiving a message. For Kafka, set it to {messaging.kafka.consumer.group} - {messaging.kafka.client_id}, if both are present, or only messaging.kafka.consumer.group. For brokers, such as RabbitMQ and Artemis, set it to the client_id of the client consuming the message. mygroup - client-6 Recommended
messaging.destination.anonymous boolean A boolean that is true if the message destination is anonymous (could be unnamed or have auto-generated name). Recommended: If known on consumer
messaging.destination.name string The message destination name [1] MyQueue; MyTopic Recommended: If known on consumer
messaging.destination.temporary boolean A boolean that is true if the message destination is temporary and might not exist anymore after messages are processed. Recommended: If known on consumer
messaging.source.anonymous boolean A boolean that is true if the message source is anonymous (could be unnamed or have auto-generated name). Recommended: [2]
messaging.source.name string The message source name [3] MyQueue; MyTopic Conditionally Required: [4]
messaging.source.template string Low cardinality representation of the messaging source name [5] /customers/{customerId} Conditionally Required: [6]
messaging.source.temporary boolean A boolean that is true if the message source is temporary and might not exist anymore after messages are processed. Recommended: [7]

[1]: Destination name SHOULD uniquely identify a specific queue, topic or other entity within the broker. If the broker does not have such notion, the destination name SHOULD uniquely identify the broker.

[2]: When supported by messaging system and only if the source is anonymous. When missing, the value is assumed to be false.

[3]: Source name SHOULD uniquely identify a specific queue, topic, or other entity within the broker. If the broker does not have such notion, the source name SHOULD uniquely identify the broker.

[4]: If the value applies to all messages in the batch.

[5]: Source names could be constructed from templates. An example would be a source name involving a user name or product id. Although the source name in this case is of high cardinality, the underlying template is of low cardinality and can be effectively used for grouping and aggregation.

[6]: If available. Instrumentations MUST NOT use messaging.source.name as template unless low-cardinality of source name is guaranteed.

[7]: When supported by messaging system and only if the source is temporary. When missing, the value is assumed to be false.

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.

Per-message attributes

All messaging operations (publish, receive, process, or others not covered by this specification) can describe both single and/or batch of messages. Attributes in the messaging.message or messaging.{system}.message namespace describe individual messages. For single-message operations they SHOULD be set on corresponding span.

For batch operations, per-message attributes are usually different and cannot be set on the corresponding span. In such cases the attributes MAY be set on links. See Batch Receiving and Batch Processing for more information on correlation using links.

Some messaging systems (e.g., Kafka, Azure EventGrid) allow publishing a single batch of messages to different topics. In such cases, the attributes in messaging.destination and messaging.source MAY be set on links. Instrumentations MAY set source and destination attributes on the span if all messages in the batch share the same destination or source.

Attributes specific to certain messaging systems

All attributes that are specific for a messaging system SHOULD be populated in messaging.{system} namespace. Attributes that describe a message, a destination, a source, a consumer or a batch of messages SHOULD be populated under the corresponding namespace:

  • messaging.{system}.message: Describes attributes for individual messages
  • messaging.{system}.destination and messaging.{system}.source: Describe the destination and source a message (or a batch) are published to and received from respectively. The combination of attributes in these namespaces should uniquely identify the entity and include properties significant for this messaging system. For example, Kafka instrumentations should include partition identifier.
  • messaging.{system}.consumer: Describes message consumer properties
  • messaging.{system}.batch: Describes message batch properties

RabbitMQ

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

Attribute Type Description Examples Requirement Level
messaging.rabbitmq.destination.routing_key string RabbitMQ message routing key. myKey Conditionally Required: If not empty.

Apache Kafka

For Apache Kafka, the following additional attributes are defined:

Attribute Type Description Examples Requirement Level
messaging.kafka.message.key string Message keys in Kafka are used for grouping alike messages to ensure they're processed on the same partition. They differ from messaging.message.id in that they're not unique. If the key is null, the attribute MUST NOT be set. [1] myKey Recommended
messaging.kafka.consumer.group string Name of the Kafka Consumer Group that is handling the message. Only applies to consumers, not producers. my-group Recommended
messaging.kafka.client_id string Client Id for the Consumer or Producer that is handling the message. client-5 Recommended
messaging.kafka.destination.partition int Partition the message is sent to. 2 Recommended
messaging.kafka.source.partition int Partition the message is received from. 2 Recommended
messaging.kafka.message.offset int The offset of a record in the corresponding Kafka partition. 42 Recommended
messaging.kafka.message.tombstone boolean A boolean that is true if the message is a tombstone. Conditionally Required: [2]

[1]: If the key type is not string, it's string representation has to be supplied for the attribute. If the key has no unambiguous, canonical string form, don't include its value.

[2]: If value is true. When missing, the value is assumed to be false.

For Apache Kafka producers, peer.service SHOULD be set to the name of the broker or service the message will be sent to. The service.name of a Consumer's Resource SHOULD match the peer.service of the Producer, when the message is directly passed to another service. If an intermediary broker is present, service.name and peer.service will not be the same.

Apache RocketMQ

Specific attributes for Apache RocketMQ are defined below.

Attribute Type Description Examples Requirement Level
messaging.rocketmq.namespace string Namespace of RocketMQ resources, resources in different namespaces are individual. myNamespace Required
messaging.rocketmq.client_group string Name of the RocketMQ producer/consumer group that is handling the message. The client type is identified by the SpanKind. myConsumerGroup Required
messaging.rocketmq.client_id string The unique identifier for each client. myhost@8742@s8083jm Required
messaging.rocketmq.message.delivery_timestamp int The timestamp in milliseconds that the delay message is expected to be delivered to consumer. 1665987217045 Conditionally Required: [1]
messaging.rocketmq.message.delay_time_level int The delay time level for delay message, which determines the message delay time. 3 Conditionally Required: [2]
messaging.rocketmq.message.group string It is essential for FIFO message. Messages that belong to the same message group are always processed one by one within the same consumer group. myMessageGroup Conditionally Required: If the message type is FIFO.
messaging.rocketmq.message.type string Type of message. normal Recommended
messaging.rocketmq.message.tag string The secondary classifier of message besides topic. tagA Recommended
messaging.rocketmq.message.keys string[] Key(s) of message, another way to mark message besides message id. [keyA, keyB] Recommended
messaging.rocketmq.consumption_model string Model of message consumption. This only applies to consumer spans. clustering Recommended

[1]: If the message type is delay and delay time level is not specified.

[2]: If the message type is delay and delivery timestamp is not specified.

messaging.rocketmq.message.type MUST be one of the following:

Value Description
normal Normal message
fifo FIFO message
delay Delay message
transaction Transaction message

messaging.rocketmq.consumption_model MUST be one of the following:

Value Description
clustering Clustering consumption model
broadcasting Broadcasting consumption model

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 publish" "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 "rabbitmq" "rabbitmq" "rabbitmq"
messaging.destination.name "T"
messaging.source.name "T" "T"
messaging.operation "process" "process"
messaging.message.id "a1" "a1" "a1"

Apache Kafka with Quarkus or Spring Boot Example

Given is a process P, that publishes a message to a topic T1 on Apache Kafka. One process, CA, receives the message and publishes a new message to a topic T2 that is then received and processed by CB.

Frameworks such as Quarkus and Spring Boot separate processing of a received message from producing subsequent messages out. For this reason, receiving (Span Rcv1) is the parent of both processing (Span Proc1) and producing a new message (Span Prod2). The span representing message receiving (Span Rcv1) should not set messaging.operation to receive, as it does not only receive the message but also converts the input message to something suitable for the processing operation to consume and creates the output message from the result of processing.

Process P:  | Span Prod1 |
--
Process CA:              | Span Rcv1 |
                                | Span Proc1 |
                                  | Span Prod2 |
--
Process CB:                           | Span Rcv2 |
Field or Attribute Span Prod1 Span Rcv1 Span Proc1 Span Prod2 Span Rcv2
Span name "T1 publish" "T1 receive" "T1 process" "T2 publish" "T2 receive"
Parent Span Prod1 Span Rcv1 Span Rcv1 Span Prod2
Links
SpanKind PRODUCER CONSUMER CONSUMER PRODUCER CONSUMER
Status Ok Ok Ok Ok Ok
peer.service "myKafka" "myKafka"
service.name "myConsumer1" "myConsumer1" "myConsumer2"
messaging.system "kafka" "kafka" "kafka" "kafka" "kafka"
messaging.destination.name "T1"
messaging.source.name "T1" "T1" "T2" "T2"
messaging.operation "process" "receive"
messaging.kafka.message.key "myKey" "myKey" "myKey" "anotherKey" "anotherKey"
messaging.kafka.consumer.group "my-group" "my-group" "another-group"
messaging.kafka.client_id "5" "5" "5" "8"
messaging.kafka.partition "1" "1" "1" "3" "3"
messaging.kafka.message.offset "12" "12" "12" "32" "32"

Batch receiving

Given is a process P, that publishes 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 publish" "Q publish" "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 "rabbitmq" "rabbitmq" "rabbitmq" "rabbitmq" "rabbitmq"
messaging.destination.name "Q" "Q"
messaging.source.name "Q" "Q" "Q"
messaging.operation "receive" "process" "process"
messaging.message.id "a1" "a2" "a1" "a2"
messaging.batch.message_count 2

Batch processing

Given is a process P, that publishes two messages to a queue Q on messaging system MS, and a process C, which receives them separately in two different operations (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. 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 SpanContext 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 publish" "Q publish" "Q receive" "Q receive" "Q process"
Parent Span Prod1 Span Prod2
Links [Span Prod1, Span Prod2 ]
Link attributes Span Prod1: messaging.message.id: "a1"
Span Prod2: messaging.message.id: "a2"
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 "rabbitmq" "rabbitmq" "rabbitmq" "rabbitmq" "rabbitmq"
messaging.destination.name "Q" "Q"
messaging.source.name "Q" "Q" "Q"
messaging.operation "receive" "receive" "process"
messaging.message.id "a1" "a2" "a1" "a2"
messaging.batch.message_count 1 1 2