Message events are events which reference a message; they are used to wait until a proper message is received.
A process can have one or more message start events (besides other types of start events). Each of the message events must have a unique message name.
When a process is deployed, it creates a message subscription for each message start event. Message subscriptions of the previous version of the process (based on the BPMN process id) are closed.
When the message subscription is created, a message can be correlated to the start event if the message name matches. On correlating the message, a new process instance is created and the corresponding message start event is activated.
Messages are not correlated if they were published before the process was deployed or if a new version of the process is deployed without a proper start event.
correlationKey of a published message can be used to control the process instance creation. If an instance of this process is active (independently from its version) and it was triggered by a message with the same
correlationKey, the message is not correlated and no new instance is created.
When the active process instance is completed or terminated and a message with the same
correlationKey and a matching message name is buffered (i.e. TTL > 0), this message is correlated and a new instance of the latest version of the process is created.
correlationKey of a message is empty, it creates a new process instance and does not check if an instance is already active.
When an intermediate message catch event is entered, a corresponding message subscription is created. The process instance stops at this point and waits until the message is correlated. When a message is correlated, the catch event is completed and the process instance continues.
An alternative to intermediate message catch events are receive tasks, which behaves the same but can be used together with boundary events.
An activity can have one or more message boundary events. Each of the message events must have a unique message name.
When the activity is entered, it creates a corresponding message subscription for each boundary message event. If a non-interrupting boundary event is triggered, the activity is not terminated and multiple messages can be correlated.
A process can contain intermediate message throw events or message end events to model the publication of a message to an external system, for example, to a Kafka topic.
At the moment, intermediate message throw events and message end events behave exactly like service tasks or send tasks , and have the same job-related properties (e.g. job type, custom headers, etc.). The message throw events and the tasks are based on jobs and job workers. The differences between the message throw events and the tasks are the visual representation and the semantics for the model. Read more about the job properties here.
When a process instance enters a message throw event, it creates a corresponding job and waits for its completion. A job worker should request jobs of this job type and process them. When the job is complete, the process instance continues or completes if it is a message end event.
Message throw events are not processed by Zeebe itself (i.e. to correlate a message to a message catch event). Instead, it creates jobs with the defined job type. To process them, provide a job worker.
A message can be referenced by one or more message events. It must define the name of the message (e.g.
Money collected) and the
correlationKey expression (e.g.
= orderId). If the message is only referenced by message start events, the
correlationKey is not required.
Usually, the name of the message is defined as a static value (e.g.
order canceled), but it can also be defined as expression (e.g.
= "order " + awaitingAction). If the expression belongs to a message start event of the process, it is evaluated on deploying the process. Otherwise, it is evaluated on activating the message event. The evaluation must result in a
correlationKey is an expression that usually accesses a variable of the process instance that holds the correlation key of the message. The expression is evaluated on activating the message event and must result either in a
string or in a
To correlate a message to the message event, the message is published with the defined name (e.g.
Money collected) and the value of the
correlationKey expression. For example, if the process instance has a variable
orderId with value
"order-123", the message must be published with the correlation key
By default, all message variables are merged into the process instance. This behavior can be customized by defining an output mapping at the message catch event.
A message start event with message definition:
<bpmn:message id="Message_0z0aft4" name="order-placed" /> <bpmn:startEvent id="order-placed" name="Order placed"> <bpmn:messageEventDefinition messageRef="Message_0z0aft4" /></bpmn:startEvent>
An intermediate message catch event with message definition:
<bpmn:message id="Message_1iz5qtq" name="money-collected"> <bpmn:extensionElements> <zeebe:subscription correlationKey="= orderId" /> </bpmn:extensionElements></bpmn:message> <bpmn:intermediateCatchEvent id="money-collected" name="Money collected" > <bpmn:messageEventDefinition messageRef="Message_1iz5qtq" /></bpmn:intermediateCatchEvent>
A boundary message event:
<bpmn:boundaryEvent id="order-canceled" name="Order Canceled" attachedToRef="collect-money"> <bpmn:messageEventDefinition messageRef="Message_1iz5qtq" /></bpmn:boundaryEvent>