Sequence diagramming that's fit for purpose

Sequence diagramming in practice - test yourself

Continuing with our sample internet bookstore application, the following diagram shows an excerpt from a sequence diagram for the "Create New Book" use case. This use case is intended for bookstore staff, so that they can add new book titles to their online catalog. There are a couple of problems with this diagram excerpt - one of which is repeated many times in our diagram. See if you can find them both. Hint: the errors relate back to items one and seven in the top-10 list.

Hidden problems: flawed sequence diagram

Our next illustration highlights the parts of the sequence diagram that have gone wrong. The main issue is that the sequence diagram is being used as a flowchart, instead of for its primary purpose in life: to allocate behavior to classes.

Flowcharting on sequence diagrams isn't necessarily an evil thing in and of itself, and it is almost certainly better than not doing the sequence diagram at all. But we consider it to be (at best) a weak usage of a sequence diagram because it doesn't leverage the ability to assign operations to classes while drawing message arrows.

Since, in our opinion, this activity is pretty much the fundamental place where "real" object oriented design happens, we've flagged it as an error.

The second issue is that there’s no validation performed on the incoming form data - and therefore no error handling code for rejecting bad data. Either the validation steps were left out of the use case or the designer didn't draw the sequence diagram directly from the use case text.

Problems exposed in the sequence diagram