Message Flow vs Sequence Flow in BPMN: Talking Between Pools
When two organisations talk, BPMN uses message flows, not sequence flows. Draw a customer pool and an online shop pool in the free bpmn.io modeler, add message events, and connect the pools with dashed message flows.
Step by step
Two kinds of arrow
- Message flow. A solid arrow and a dashed arrow mean very different things in BPMN. Here's a customer and our online shop talking to each other. Let's draw it.
- New diagram. Start a new diagram in bpmn.io and name the start event basket ready.
Customer pool
- Customer pool. Drag a pool from the palette. It takes in the start event. Make it wider, and name it customer. Add the customer's task from the start event. Place order.
- Wait for a message. Then the customer waits for an answer. Add the double circle from the task's menu. That's an intermediate event. Name it confirmation in. Use the wrench to make it a message intermediate catch event. The white envelope means: wait here until a message arrives.
- Customer pool. Finish the customer's side with an end event. Order done.
Shop pool
- Shop pool. The shop is a separate organisation, so it gets its own pool. Drag a second pool below, make it the same size, and name it online shop. The shop's process starts when the order arrives. Drag a start event into the shop pool. Order received. Make it a message start event with the wrench. The process starts because a message came in. Add the shop's task from the start event's menu. Confirm order. And end the shop's side. Order confirmed.
Message flows
- Message flow. Now connect the pools. Drag the arrow tool from place order onto the shop's start event. Between pools, the line is dashed, with a small circle and an open arrow. That's a message flow. And the answer goes back the same way, from confirm order up to the waiting event.
- The finished model. Click away. Solid sequence flows stay inside each pool. Dashed message flows go between them.
Recap
- The rule. So, a sequence flow never leaves its pool, a message flow only goes between pools, and message events show where a process waits for or starts from a message. Subscribe for more BPMN tips for business analysts.
