Sequence diagram
Definition
A sequence diagram is a UML interaction diagram that shows how a set of objects, services, or actors exchange messages in chronological order, with time running top to bottom.
Sequence diagrams are part of the UML specification and are the go-to tool for documenting and designing system interactions. Each participant gets a vertical 'lifeline,' and messages between them are drawn as horizontal arrows at the appropriate vertical position. Activation bars on a lifeline show when that participant is actively processing.
Engineers reach for sequence diagrams when they need to reason about API calls, authentication flows, distributed system behavior, or any scenario where the order of operations matters. They're also useful during design reviews for catching race conditions and missing error paths before code is written.
In whiteboard sessions, sequence diagrams often get sketched informally — vertical columns with actor names, horizontal arrows with labels. The formal UML notation (synchronous vs. asynchronous arrows, return dotted lines, combined fragments) may or may not appear. BoardSnap AI reads the columns, arrows, and labels to capture the essence of the interaction, even if the whiteboard sketch is looser than strict UML.
Examples
- OAuth login flow: User → App → Auth server → Token endpoint → App → User
- Microservice call chain: API gateway → Auth service → Product service → Database → Product service → API gateway
- WebSocket handshake: Client → Server (HTTP upgrade) → Server → Client (101 switching protocols)
- Checkout flow: Browser → Cart service → Inventory service → Payment service → Order service
Snap a sequence diagram. Ship its actions.
BoardSnap turns any whiteboard — including this one — into a summary and action plan.