Walk through what happens to a request in FastAPI, from the ASGI server to the response.

Summary

The ASGI server calls the app; Starlette's outer ServerErrorMiddleware wraps your middleware (the last added runs first), then ExceptionMiddleware applies your exception handlers; the router matches the path; FastAPI resolves dependencies (once per request by default) and validates parameters and the body with Pydantic (422 on failure); the endpoint runs (async def on the event loop, def in a thread pool); the return value is serialised through the response model; dependency exit code and background tasks run afterwards.

Longer Answer

Knowing the layers explains common surprises: an HTTPException raised in middleware becomes a 500 because it's outside the exception layer; validation errors never reach your endpoint; a blocking call in an async def endpoint stalls every request on the event loop. Exactly when dependencies with yield run their exit code relative to sending the response has changed between FastAPI versions, so check the documentation for your version.

Key Points

• ServerErrorMiddleware → your middleware → ExceptionMiddleware → router.
• Dependencies resolved and cached per request.
• Pydantic validation, 422 on failure.
• async def on the loop, def in a thread pool.

Deep dive: Request Lifecycle course: FastAPI and Starlette: Middleware, Exceptions and Dependencies

What interviewers look for

The layer order and at least one consequence of it.