FastAPI Internals - How does it work?
summary
FastAPI proves to be a thin layer over Starlette and Pydantic wired through ASGI servers like Uvicorn, with concrete surprises: response_start timing differs between servers (Uvicorn sends headers early, Hypercorn after body), routing is first-route-wins so order matters, dependency injection resolves nested calls with a per-request cache, sync dependencies run in a thread pool limited by default thread pool 40, input validation returns 422 for input 500 for output, and WebSockets are delegated to Starlette.
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chapters (11)
- 0:00Introduction and setup
- 1:10FastAPI overview and basics
- 2:36ASGI and server interaction
- 7:12Middleware stack in FastAPI
- 10:42Routing in Starlet
- 13:58Dependency injection flow
- 17:55Async vs sync dependencies
- 19:00Input/output validation
- 19:42When to use async or sync
- 21:09Closing remarks and demo
- 21:34Q&A session
