The send task allows you to send a message to a receive task in another process, and the receive task allows you to receive a message from a send task in another process. The send task is similar to the throw message event; however, you cannot use the send task to invoke a process that starts with a message start event. There are no send and receive tasks for signals, only for messages. Send and receive tasks also allow you to attach boundary events (which will be discussed in Chapter 4, Handling Exceptions) to them. This is an important difference.
You can use the receive task to start a process, however, in this case, you must set the Create Instance property and there must be a single start node of type "none" immediately before the receive task.
The following diagram shows three processes that use the methods we have discussed to communicate with each other. The dotted arrows indicate where throw and catch message events are used by Process3 to invoke Process1, and by Process1 to return some data to Process3 when it is finished. The red arrows indicate where send and receive message tasks are used by Process1 to invoke Process2, and by Process2 to return some data to Process1 when it is finished.
Let us consider what happens when an instance of Process3
is executed:
Process3starts.Process3throws a message event to startProcess1.Right away,
Process3goes on toActivity.At the same time (more or less,)
Process1starts.Process1sends a message to startProcess2.Right away,
Process1goes on toDo something.At the same time (more or less),
Process2starts.Process2goes on toDo something else.While all of this is going on, when
Process3finished doingActivity, it went on toCatchEventand paused there waiting for a response back fromProcess1.Similarly, when
Process1finishedDo something, it went on toReceiveTaskand paused there waiting for a response back fromProcess2.When
Process2finishedDo something else, it sent a response (in this case by sending a message) back toProcess1.Process1wakes up upon receiving a response (message) fromProcess2and then sends its response (by throwing a message event) back toProcess3.Process3wakes up upon receiving a response (catching a message event) fromProcess1and then moves on to its end.