OpenAPI to Sequence Diagram

Demo: document one checkout endpoint instead of the whole API

A sequence diagram is most useful when it explains one workflow. For OpenAPI, that usually means selecting a small group of related paths instead of the entire specification.

• Keep POST /checkout and the most relevant success and error responses.
• Use operationId or summary fields to create readable message labels.
• Link to the full OpenAPI spec from the document instead of copying everything into the diagram.

paths:
  /checkout:
    post:
      operationId: createCheckout
      summary: Create checkout session
      responses:
        "201": { description: Checkout created }
        "402": { description: Payment required }

This focused input creates a cleaner sequence diagram than a full API dump.

API review: turn a spec endpoint into a request story

OpenAPI describes endpoints, but reviewers often need to understand the request story: who calls the endpoint, which auth step happens first, which services are involved, and what error paths matter.

• Start from the path, method, auth scheme, request body, and main response codes.
• Add backend services as inferred participants only when the contract names them clearly.
• Use the sequence output to discuss behavior, not as a replacement for the OpenAPI spec.

POST /orders
Authorization: Bearer <token>
201 Created -> orderId
409 Conflict -> duplicate cart or inventory lock

Troubleshooting: compare contract diagrams with runtime evidence

OpenAPI sequence diagrams are strongest when paired with runtime evidence. Use the contract diagram to show intended behavior, then compare HAR, Postman, or trace data when an incident suggests the real call order is different.

• Use OpenAPI for expected request and response paths.
• Use HAR or Postman collections for observed request order.
• Use the mismatch as the discussion point in backend review or incident follow-up.

Expected: Browser -> API -> Payment provider -> API -> Browser
Observed: Browser -> API -> API retry -> Payment provider timeout -> 503