What is the ASGI lifespan protocol and how does Uvicorn support it?

The ASGI lifespan protocol provides startup and shutdown events for an ASGI application, allowing it to initialise resources (database connections, caches, background tasks) before serving requests and clean them up on shutdown.

# FastAPI lifespan example (modern approach)
from contextlib import asynccontextmanager
from fastapi import FastAPI

@asynccontextmanager
async def lifespan(app: FastAPI):
    # Startup: runs before the server begins accepting requests
    print("Starting up...")
    app.state.db = await connect_to_database()
    yield  # server is running here
    # Shutdown: runs after the server stops accepting requests
    print("Shutting down...")
    await app.state.db.close()

app = FastAPI(lifespan=lifespan)

# Pure ASGI lifespan implementation
async def app(scope, receive, send):
    if scope["type"] == "lifespan":
        while True:
            message = await receive()
            if message["type"] == "lifespan.startup":
                # do startup work
                await send({"type": "lifespan.startup.complete"})
            elif message["type"] == "lifespan.shutdown":
                # do cleanup
                await send({"type": "lifespan.shutdown.complete"})
                return
    # ... HTTP handling

The lifespan protocol is especially important for managing async resources like database connection pools (asyncpg, motor), which must be created inside an async context and properly closed to avoid connection leaks.

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