Sequence Diagrams

Definition

A Sequence Diagram models the chronological order of interactions between actors, systems, or components — showing who communicates with whom, in what sequence, and what messages or data are exchanged at each step. They are especially valuable for clarifying integration dependencies, event-driven flows, and complex multi-step interactions where timing and order affect behavior.

Inputs

• Identified actors, systems, and components involved in the scenario
• A defined scenario to model (typically the happy path first)
• Input from architects, developers, and business SMEs on interaction design
• Process models or use cases that reference the interaction being detailed

Outputs

• A sequence diagram showing actors, lifelines, messages, and interaction order
• Documented alternate flows and failure scenarios
• Clarified integration points, dependencies, and handoff timing
• Inputs to API design, system integration requirements, and test case design

When to Use

• System integrations and API or service orchestration where sequence matters
• Multi-step workflows with dependencies between systems or actors
• Complex event-driven processes where timing and ordering affect correctness
• Identifying and documenting failure paths and exception handling at integration points

When Not to Use

• Simple, single-system interactions with no meaningful sequence complexity
• When a higher-level process model is sufficient and the added detail of sequencing is not needed
• Very early discovery before actors and systems have been identified