Sequence Diagrams
Sequence Diagrams are about modeling how different parts of a system interact over time to accomplish a specific task.
Where Class Diagrams are showing the structure of a system, sequence diagrams are how we begin showing dynamic behaviour, how objects communicate to make something happen. They help us visualize the flow of messages between objects, methods, or systems in a particular use case.
A Sequence Diagram represents a scenario as a series of interactions arranged vertically over time. Each object or component involved is shown as a lifeline, and the messages they send to each other are shown as horizontal arrows.
Think of it as a timeline that shows who talks to whom, in what order, and what the result is.
Elements
A Sequence Diagram is made up of a few elements:
Actors: Represent external users or systems that start the interaction (e.g., a User).
Objects (Lifelines): Represent parts of the system that participate in the interaction (e.g., LibrarySystem, Book, Database).
Messages: Arrows showing communication between lifelines. These can represent method calls, signals, or data requests.
Return Messages: Dotted arrows showing responses or results returned from an operation.
Usage
Sequence Diagrams are most useful when you need to:
- Clarify the flow of a specific use case
- Identify which classes or components handle which responsibilities
- Communicate how data and control move through the system
- Validate that your design matches real-world behaviour.
Like most of the diagrams we've spoken about so far, they are very useful in early design discussions, where having a simple visual aid can be helpful in revealing overly complicated interactions before you've committed to building them.
To create a Sequence Diagram, you should start with a clear scenario. Identify the main actors and components, and then map out the messages that are required to occur, and the order in which they occur.
- Identify participants: who's involved in the interaction?
- List the key steps: what happens from start to finish?
- Draw lifelines for each participant.
- Add messages between lifelines to show communication flow.
- Include activations and returns to show when and how each participant responds.