Sequence Diagrams

Models the interaction of a solution. Is made using a class and use case diagrams.

Example: Lending Books

Simplified

This example shows the interaction of the Librarian actor with the system. In this example only one book is lent to a customer.

Multiple books

In the next diagram a loop is introduced, this allows the diagram to show that a process repeats.

Execution of a use case

Sequence diagrams describe how a use case is executed.

Elements of a sequence diagram:

• Actor
• Control Object
• Data Objects
• Various types of messages
• Control structures

Heuristic: Two parts of a sequence diagram

1. Step-by-step description of the interaction between the user and the system. (between the user and the control object, mediated by the user interface)
2. Actions to create/read/update/delete objects in the system.

Elements of a Sequence Diagram

A sequence diagram describes interactions between objects

• Actor object
• Control object
• lifeline: vertical dotted line
• activation: period during which the object is active (vertical bar)

Interaction operators

Interaction operators are control structures. denoted by a box with the operator at the top left.

A Interaction operator can have an interaction constraint which indicates when a part of the box is executed.

• a constraint is denoted in brackets: [condition]

Interaction operator	Use
loop	Repeat until the interaction constraint for the loop is no longer true
alt	Alternative behaviors, depending on condition(s)
opt	Will only execute if the interaction constraint is true
ref	Refers to an interaction fragment described in a different diagram

Alternative Behavior (if/else)

Using the alt operator we can define alternative behavior: And using the ref operator we can make reference another diagram that describes this part in more detail like a zoom in.

Example