What is the relationship between Uvicorn and FastAPI?

FastAPI and Uvicorn are complementary tools at different layers of the stack. FastAPI is the ASGI application framework; Uvicorn is the ASGI server that runs it. FastAPI generates the HTTP routing, request validation, and response serialisation; Uvicorn handles the network I/O, event loop, and connection lifecycle.

# main.py - FastAPI application
from fastapi import FastAPI

app = FastAPI()

@app.get("/")
async def root():
    return {"message": "Hello World"}

# Uvicorn runs the FastAPI app:
# uvicorn main:app --reload

# FastAPI is itself an ASGI-compliant application:
# It implements: async def __call__(self, scope, receive, send)
# Uvicorn calls this callable for every incoming request

# Development:
# uvicorn main:app --reload --host 127.0.0.1 --port 8000

# Production:
# gunicorn main:app -w 4 -k uvicorn_worker.UvicornWorker

Installation: pip install fastapi uvicorn[standard] gives you both. FastAPI's documentation recommends Uvicorn as the server, and Uvicorn's documentation uses FastAPI in examples - they are tightly coupled in the ecosystem though architecturally independent (any ASGI framework can run on Uvicorn).

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