Sequence diagram

A sequence diagram shows which object calls which, and in what order. It takes one flow, the same kind an activity diagram walks, and asks a different question about it. Not what happens, but who does it and who they ask.

It has two dimensions, and that is most of the notation. Objects run across the top. Time runs down the page.

Reading one

• A box across the top is one object, not a class. catalog, not Catalog, although most people write the class name and nobody objects.
• The dashed line below it is its lifeline. Time passes downwards along it.