UML for the Software Developer, Part 6: Interaction Diagrams: "UML for the Software Developer, Part 6: Interaction Diagrams
Interaction or sequence diagrams are the critical link between the static class diagrams and how the classes behave. A well-defined sequence diagram will find missing classes, elucidate the sequence of events for class interaction, and define the methods needed for each class.
by Mark Goetsch July 22, 2005
Sequence diagrams are used to map out class interactions. A sequence diagram is made up of objects, each with its own object lifeline. Interactions between the objects occur by messages that are sent from one object to the next. By convention, the messages are sent from left to right across the flow of a diagram, with the message returns being sent back right to left. There are situations where two objects are communicating back and forth or in sequence diagrams with multiple sets of actions where the messages will not follow the convention but those should be minimized as much as possible. Objects can create and destroy other objects; they can communicate synchronously and asynchronously; and act upon themselves."