**A tool for displaying how interactions between participants occur - very useful for software & process development.** Sequence diagrams depict a sequence of activities, arranged sequentially along the vertical access, between two or more participants (which may be people, systems, or objects). The lines flowing between the participants carry text describing that portion of the interaction, this is often called ‘messaging’ in this context. The primary value proposition for these types of diagrams is in relating the constraints in _sequencing_ of these messages. **Lifelines** are the boxes along the vertical axis that shows when participants are engaged in the sequencing process. I’ve found Sequence Diagrams very helpful when designing simple systems. # Example From my actual use of this... ```mermaid sequenceDiagram participant dbHost participant apiHost actor client Note over client: Search for Entries client-->>apiHost: query string apiHost-->apiHost: parse request alt is invalid apiHost-->client: bad query else is valid apiHost-->>dbHost: xlated query dbHost-->dbHost: fetch results dbHost-->>apiHost: raw data apiHost-->apiHost: perform shaping apiHost-->>client: results end ``` **** ## Source ## Related - [[UML]] / [[SysML]] - [[Diagram Types (index)]]