Message Tasks vs. Message Events¶
In Business Process Model and Notation (BPMN), Message Tasks and Message Events facilitate communication between different participants in a process. Although both involve message exchanges, they serve different purposes and are used in different contexts.
This guide explains when to use a Message Task versus a Message Event, along with examples and best practices.
1. Message Task¶
A Message Task represents a task where sending or receiving a message is the primary activity within a process.
When to Use a Message Task¶
Performing an Activity: The task itself involves sending or receiving a message.
Example: An employee sends a request to another department.
Synchronous Communication: The process waits for a response before proceeding.
Example: A service request is sent, and the process pauses until a reply is received.
Part of a Sequence: The message exchange is an essential part of the workflow.
Example: An order confirmation message must be sent before an item is shipped.
Example: Message Task Usage¶
A customer service representative sends a refund request to the finance department and waits for approval.
BPMN Representation:
A Message Task labeled “Send Refund Request” is part of the process sequence.
The process cannot continue until the request is sent and/or a response is received.
Key Takeaway: Use a Message Task when a specific task directly involves sending or receiving a message as part of process execution.
2. Message Event¶
A Message Event represents the occurrence of a message in a process but does not represent a specific task. It affects the process flow based on the message received or sent.
When to Use a Message Event¶
Process Interruption: The message interrupts the flow or triggers a reaction.
Example: A customer cancels an order, stopping the shipment process.
Boundary Events: The message is attached to an activity to indicate that the process should react when the message is received.
Example: A process is waiting for approval via a message; if rejected, it follows a different path.
Intermediate Events: The message is sent or received within a process without being an explicit task.
Example: A notification is sent to a supplier when an order is updated.
Start Events: A Message Start Event starts a process in response to receiving a message from an external source.
Example: A service request is received, triggering the support ticket process.
Example: Message Event Usage¶
A customer cancels an order before it is processed.
BPMN Representation:
A Message Start Event triggers the “Cancel Order” process upon receiving a cancellation request.
A Message Intermediate Event indicates that an update notification is sent to the supplier.
A Message Boundary Event attached to “Prepare Shipment” allows cancellation before shipment processing starts.
Key Takeaway: Use a Message Event when you need to indicate that a message occurrence influences the process flow but is not an activity in itself.
3. Summary: Message Task vs. Message Event¶
Feature |
Message Task |
Message Event |
|---|---|---|
Purpose |
Represents a task where sending or receiving a message is an explicit action. |
Represents the occurrence of a message that affects process flow. |
Usage |
When the process explicitly involves sending or receiving a message as an activity. |
When a message triggers, interrupts, or signals process changes. |
Flow Behavior |
Appears as a task in the process sequence and must be executed. |
Can be a Start, Intermediate, or Boundary Event, influencing process flow. |
Example |
A task where an employee sends a request to another department. |
A message received that cancels an ongoing process. |
Use Message Tasks when the act of sending/receiving a message is part of an activity.
Use Message Events when a message affects the process flow but is not a discrete task.
Understanding the distinction between Message Tasks and Message Events ensures accurate BPMN modeling of message-based interactions.