How do you run startup and shutdown logic in FastAPI using lifespan?

FastAPI supports a lifespan context manager (introduced in FastAPI 0.93, replaces the deprecated on_event decorators) for running code at application startup and shutdown — e.g. creating DB connection pools, loading ML models, or connecting to message brokers.

from contextlib import asynccontextmanager
from fastapi import FastAPI
import asyncpg

# Module-level storage for shared resources
db_pool: asyncpg.Pool | None = None

@asynccontextmanager
async def lifespan(app: FastAPI):
    # --- STARTUP: runs before the app starts serving requests ---
    global db_pool
    db_pool = await asyncpg.create_pool(
        "postgresql://user:pass@localhost/db",
        min_size=5,
        max_size=20,
    )
    print("Database pool created")

    yield  # app runs here

    # --- SHUTDOWN: runs after last request is processed ---
    await db_pool.close()
    print("Database pool closed")

app = FastAPI(lifespan=lifespan)

@app.get("/users")
async def list_users():
    async with db_pool.acquire() as conn:
        rows = await conn.fetch("SELECT id, username FROM users")
    return [dict(r) for r in rows]

Common startup tasks: create DB connection pools, initialise Redis clients, load ML models into memory, connect to message brokers (Kafka, RabbitMQ).

Common shutdown tasks: close DB pools, flush metrics, close broker connections, gracefully drain request queues.

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