What are sequence diagrams in UML?

Answer 1

Sequence diagrams are used to model the logic of usage scenarios. A usage scenario is exactly what its name indicates -- the description of a potential way your system is used. The logic of a usage scenario may be part of a use case, perhaps an alternate course. It may also be one entire pass through a use case, such as the logic described by the basic course of action or a portion of the basic course of action, plus one or more alternate scenarios. The logic of a usage scenario may also be a pass through the logic contained in several use cases. For example, a student enrolls in the university, and then immediately enrolls in three seminars. Sequence diagrams model the flow of logic within your system in a visual manner, enabling you to both document and validate your logic, and are commonly used for both analysis and design purposes.

Sequence diagram is the most common kind of Interaction Diagram in UML 2. It describes Message interchange between a number of Lifelines(interacting objects). Messages sent from one lifetime to another could be: synchronous calls, asynchronous calls, return messages, signals, create and delete messages.

Sequence diagram shows how lifelines interact with each other as a sequence of messages and signals sent, received back, or lost. This sequence could have loops (iterations), conditional messages, parallel messages, etc.