FastAPI Internals — Marcelo Trylesinski

FastAPI is built on two main dependencies: Pydantic for data validation and Starlet for web-related functionality

The ASGI (Asynchronous Server Gateway Interface) specification defines how FastAPI communicates between server and application through three parameters:

• Scope: Contains connection data and request information
• Receive: Callable for getting data from server
• Send: Callable for sending data back to server

Middleware processing occurs before routing in FastAPI:

• Server error middleware handles uncaught exceptions
• Exception middleware manages custom exception handlers
• Custom middlewares can be added but won’t have access to endpoint information

Route matching follows a “first match wins” principle:

• When multiple routes could match a request, the first registered route takes precedence
• This applies across multiple routers with same prefixes

Dependencies in FastAPI:

• Results are cached based on function hash
• Can be async or sync
• For non-CPU intensive tasks, async is more efficient
• CPU-bound tasks should use sync with thread pool

WebSocket handling:

• Must explicitly accept WebSocket handshake
• Need to properly close connections to prevent memory leaks
• Based primarily on Starlet implementation

Server behavior variations:

• Different servers (Uvicorn, Hypercorn) may handle response timing differently
• Some servers send headers immediately upon status code
• Can lead to situations where 200 status is sent before errors occur