Message Diagrams

I’ve developed a new diagram that I think it much better at communicating how the messages in a system flow between components. I’ve used sequence diagrams for this in the past, but I’ve always found a number of problems with them.

• They have a complex syntax that many people, especially non-technical people, don’t understand.
• They take time to write.
• If there are more that a couple of participants, then they get very large.
• If you need to make a small change, you often have to re-write the entire diagram - not whiteboard friendly.

This very simple flow produces a large diagram:

What’s wrong with this?

• The end of the flow, and the outputs, is far away from the start and the inputs.
• If I want to add a participant early in the diagram, the whole thing changes.
• I need to know and understand the alt syntax.
• The line between the Website and the Audit Log is really long.

I’ve set out to create a new type of diagram to replace sequence diagrams:

I decided to focus on the process rather than the participant, use a small number of familiar symbols (anything else, such as conditional or loops are done using notes), and keep the syntax terse.

The rules are basic:

• Arrows show the flow of messages; they are always clockwise
• There are only actors, processes, and databases.
• A process is not a service, it can appear more than once (this maybe confusing the first time you see this)
• Everything else is a note

I’d love to get your feedback - please add your comments below.