Common Mistakes with Sequence Diagrams

Every diagram shall be readable. A common rule says that you can’t include everything on a single diagram. The same applies to the sequence diagrams as well. We use those diagrams to represent the messages that are exchanged between the systems. Don’t make any of the common mistakes.

• Too many steps are included. Displaying too many messages on the diagram makes it unreadable. You might get lost and need extra time to find the message that you are looking for. Anyway all messages cannot be implemented at once. The best would be to split the scenario into its main steps and create a sequence diagram for each step. It will contain less messages and probably less lifelines.
• Too many scenarios are represented. It is hard to represent if-then-else logic on the sequence diagram, especially when you have many cases. Of course you can use the “Alt” frame to represent alternatives. When the alternatives contain many messages, the diagram again will become complicated and unreadable. Make sure that you represent each major case on a separate sequence diagram. Use the “Alt” frame only for small difference in the flow.
• The user is missing. The actor has its own dedicated lifeline. This is the user. The user doesn’t send messages to the system in most cases. However the actions, that the user does, trigger messages between the systems. Adding the user to the sequence diagram will improve the readability a lot.