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API / Swagger Interview questions

Explain the execution flow of generating server stubs from an OpenAPI document?

Server stub generation mirrors client SDK generation in mechanism — parsing the spec, mapping schemas to types, applying language/framework-specific templates — but the output is routing and controller scaffolding rather than an API-calling client.

flowchart TD A[OpenAPI document] --> B[Parse paths, operations, and schemas] B --> C[Map each operation to a controller method signature] C --> D[Generate routing configuration matching paths/methods] D --> E[Generate request/response model classes] E --> F[Generate method stubs with TODO business logic] F --> G[Developer implements actual business logic inside stubs]

For each operation, the generator creates a method signature with correctly typed parameters (matching the spec's parameter and request body schemas) already wired into the target framework's routing mechanism — a Spring @RestController method with the right @RequestMapping, for instance — but leaves the method body as a stub, usually returning a placeholder or throwing a "not implemented" exception, since business logic is something the spec inherently can't describe.

This is most valuable in a contract-first workflow: the generated stub guarantees the implementation's routes, parameter types, and response shapes stay consistent with the agreed contract from the moment a developer starts filling in logic, rather than the developer manually wiring routing and hoping it matches the spec that other teams are already building against.

What is typically left for a developer to fill in when generating server stubs?
Why is stub generation especially valuable in a contract-first workflow?

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