Events in BPMN can be thrown (i.e. sent), or caught (i.e. received),
respectively referred to as throw or catch events,
message throw event,
timer catch event.
Additionally, a distinction is made between start, intermediate, and end events:
- Start events (catch events, as they can only react to something) are used to denote the beginning of a process or sub-process.
- End events (throw events, as they indicate something has happened) are used to denote the end of a particular sequence flow.
- Intermediate events can be used to indicate that something has happened (i.e. intermediate throw events), or to wait and react to certain events (i.e. intermediate catch events).
Intermediate catch events can be inserted into your process in two different contexts: normal flow, or attached to an activity, and are called boundary events.
In normal flow, an intermediate throw event will execute its event (e.g. send a message) once the token has reached it, and once done the token will continue to all outgoing sequence flows.
An intermediate catch event, however, will stop the token, and wait until the event it is waiting for happens, at which execution will resume, and the token will move on.
Boundary events provide a way to model what should happen if an event occurs while an activity is currently active. For example, if a process is waiting on a user task to happen which is taking too long, an intermediate timer catch event can be attached to the task, with an outgoing sequence flow to notification task, allowing the modeler to automate sending a reminder email to the user.
A boundary event must be an intermediate catch event, and can be either interrupting or non-interrupting. Interrupting in this case means that once triggered, before taking any outgoing sequence flow, the activity the event is attached to will be terminated. This allows modeling timeouts where we want to prune certain execution paths if something happens, e.g. the process takes too long.
At the moment, Zeebe only supports catching events, and only the following: