A sequence diagram shows how parts of a system interact over time, as a series of requests and responses between components. It comes from UML, the Unified Modelling Language.
Why it matters: Mapping flows helps testers and developers share an understanding of a system's states and interactions. That shared view helps decide which tests belong at which level, preventing duplicated or invalid tests.
In practice: Testers sketch sequence diagrams with developers when discussing a feature, either on a whiteboard or with a diagramming tool. Walking through each request and response often reveals missing error handling, unclear responsibilities and untested paths.
Examples:
- Sketching the requests between a web app, its backend and a third-party service before agreeing a test strategy.
- Spotting a missing error response by walking through a sequence diagram.
Common pitfalls:
- Drawing diagrams once and never updating them as the system changes.
- Diagramming only the happy path, and missing the error flows.
What's the difference between a sequence diagram and a flowchart? A flowchart shows the steps and decisions in a process. A sequence diagram shows the messages passed between components over time.
