Lesson 10 - UML - Sequence Diagram

In the previous lesson, UML - Package Diagram and Deployment Diagram, we discussed UML Package diagrams and Deployment diagrams. In today's UML tutorial, we're going to describe Sequence diagrams in detail. The Sequence diagram captures a sequential interaction between a group of objects over time. It belongs to the UML interaction diagrams. Unlike the Activity diagram which described the flow of use cases, Sequence diagrams describe chronological communication between different classes or objects over time. This is a more detailed and implementation-dependent diagram, where not only the steps of the activity but also the interactions between the objects are crucial. These objects were completely omitted in the Activity diagram. Sometimes you can see a definition that the Sequence diagram describes the calls of operations or messages. Besides the description of behavior inside an application, we can also use the diagram e.g. to describe how a program cooperates with an API server.

Symbols

Let's take a look at the symbols, the elements of the diagram:

Lifeline

The lifeline indicates how the time of the interaction participants flows. Each object has its own lifeline and their order is not important. It's basically a timeline. A participant, besides the objects, is often also the user, represented by a

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