Lifespan context manager in FastAPI - Performance & Optimization

Performance: Lifespan context manager

MEDIUM IMPACT

This affects the startup and shutdown phases of a FastAPI app, impacting initial load time and resource cleanup.

Managing app startup and shutdown tasks

FastAPI

from fastapi import FastAPI
from contextlib import asynccontextmanager

@asynccontextmanager
async def lifespan(app: FastAPI):
    # Async startup tasks
    await some_async_setup()
    yield
    # Async cleanup tasks
    await some_async_cleanup()

app = FastAPI(lifespan=lifespan)

Uses async context manager to run startup and cleanup without blocking event loop, improving startup speed.

📈 Performance GainNon-blocking startup, reduces LCP delay, smoother app readiness

Managing app startup and shutdown tasks

FastAPI

from fastapi import FastAPI

app = FastAPI()

@app.on_event("startup")
async def startup_event():
    # Blocking long task
    import time
    time.sleep(5)  # blocks event loop

@app.on_event("shutdown")
async def shutdown_event():
    # Cleanup without async
    pass

Blocking calls in startup delay app readiness and block event loop, causing slow initial response.

📉 Performance CostBlocks rendering for 5 seconds, delaying LCP significantly

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Blocking startup with sync sleep	N/A	N/A	Blocks LCP for seconds	[X] Bad
Async lifespan context manager	N/A	N/A	Non-blocking startup, fast LCP	[OK] Good

Rendering Pipeline

The lifespan context manager runs before the app starts serving requests, affecting the critical rendering path by controlling when the app becomes ready.

→Startup

→Resource Initialization

→Shutdown

⚠️ BottleneckStartup blocking tasks that delay app readiness

Core Web Vital Affected

LCP

This affects the startup and shutdown phases of a FastAPI app, impacting initial load time and resource cleanup.

Optimization Tips

1Avoid blocking synchronous calls in lifespan startup and shutdown.

2Use async context managers to run startup tasks without blocking.

3Keep startup tasks minimal to improve app readiness and LCP.

Performance Quiz - 3 Questions

What is the main performance benefit of using an async lifespan context manager in FastAPI?

A It prevents blocking the event loop during startup

B It reduces the size of the app bundle

C It improves CSS rendering speed

D It eliminates the need for database connections

DevTools: Network and Performance

How to check: Open DevTools, go to Network tab, reload app and observe time until first response; then use Performance tab to record startup timeline.

What to look for: Look for long blocking times before first response and delayed LCP indicating slow startup.

Practice

1. What is the main purpose of the lifespan context manager in a FastAPI application?

easy

A. To manage user authentication and authorization

B. To handle HTTP requests and responses

C. To define API routes and endpoints

D. To run setup code when the app starts and cleanup code when it stops

Solution

1. Step 1: Understand the role of lifespan context manager

   The lifespan context manager is designed to run code at the start and end of the FastAPI app lifecycle.

2. Step 2: Identify its main use

   It is used to set up resources like database connections when the app starts and clean them up when the app stops.

3. Final Answer:

   To run setup code when the app starts and cleanup code when it stops -> Option D

4. Quick Check:

   Lifespan manages startup and shutdown code = A [OK]

Hint: Lifespan runs code at app start and stop [OK]

Common Mistakes:

• Confusing lifespan with route handling
• Thinking lifespan manages HTTP requests
• Assuming lifespan handles user sessions

2. Which of the following is the correct way to define a lifespan context manager in FastAPI?

easy

A. async def lifespan(app): yield

B. def lifespan(): return app

C. async def lifespan(): return app

D. def lifespan(app): yield

Solution

1. Step 1: Recall lifespan signature

   The lifespan function must be async and accept the app parameter to manage startup and shutdown.

2. Step 2: Confirm use of yield

   Using yield inside the async function allows running code before and after the yield for startup and shutdown.

3. Final Answer:

   async def lifespan(app): yield -> Option A

4. Quick Check:

   Async + app param + yield = A [OK]

Hint: Lifespan is async with app param and uses yield [OK]

Common Mistakes:

• Forgetting async keyword
• Missing app parameter
• Using return instead of yield

3. Given this FastAPI lifespan code snippet, what will be printed when the app starts and stops?

async def lifespan(app):
    print('Starting app')
    yield
    print('Stopping app')

medium

A. Only 'Starting app' is printed

B. 'Starting app' prints on start, 'Stopping app' prints on shutdown

C. Only 'Stopping app' is printed

D. Neither message is printed

Solution

1. Step 1: Understand yield in lifespan

   The code before yield runs at startup, and code after yield runs at shutdown.

2. Step 2: Match prints to lifecycle events

   So 'Starting app' prints when app starts, and 'Stopping app' prints when app stops.

3. Final Answer:

   'Starting app' prints on start, 'Stopping app' prints on shutdown -> Option B

4. Quick Check:

   Code before yield = start, after yield = stop [OK]

Hint: Code before yield runs on start, after yield on stop [OK]

Common Mistakes:

• Thinking both prints run immediately
• Assuming yield blocks all prints
• Confusing start and stop timing

4. What is wrong with this lifespan context manager code?

async def lifespan(app):
    print('Starting')
    return
    print('Stopping')

medium

A. The app parameter is not used, causing runtime error

B. Missing async keyword causes syntax error

C. Using return instead of yield prevents shutdown code from running

D. Print statements are not allowed in lifespan functions

Solution

1. Step 1: Identify use of return instead of yield

   The lifespan function must use yield to separate startup and shutdown code.

2. Step 2: Understand effect of return

   Using return exits the function immediately, so shutdown code after it never runs.

3. Final Answer:

   Using return instead of yield prevents shutdown code from running -> Option C

4. Quick Check:

   Return exits early; yield separates start/stop [OK]

Hint: Use yield, not return, to run shutdown code [OK]

Common Mistakes:

• Confusing return and yield in async functions
• Ignoring that shutdown code runs after yield
• Assuming print statements cause errors

5. You want to open a database connection when your FastAPI app starts and close it when the app stops using the lifespan context manager. Which code correctly implements this?

hard

A. async def lifespan(app):
db = await connect_db()
app.state.db = db
yield
await db.close()

B. async def lifespan(app):
db = await connect_db()
yield
app.state.db = db
await db.close()

C. def lifespan(app):
db = connect_db()
app.state.db = db
yield
db.close()

D. async def lifespan():
db = await connect_db()
app.state.db = db
yield
await db.close()

Solution

1. Step 1: Confirm async function with app parameter

   The lifespan function must be async and accept the app parameter to store the db connection.

2. Step 2: Check order of operations

   Connect to the database before yield, store it on app.state, then close it after yield.

3. Step 3: Verify correct use of await and yield

   Await connect_db and db.close, yield separates startup and shutdown code.

4. Final Answer:

   async def lifespan(app): db = await connect_db() app.state.db = db yield await db.close() -> Option A

5. Quick Check:

   Async + app param + yield + await connect/close = D [OK]

Hint: Connect before yield, close after, store in app.state [OK]

Common Mistakes:

• Placing yield before storing db connection
• Missing async or await keywords
• Not passing app parameter to lifespan