Startup and shutdown events in FastAPI

Introduction

Startup and shutdown events let your app run code when it starts or stops. This helps prepare or clean up resources.

Connect to a database when the app starts

Load configuration or cache data before handling requests

Close database connections or files when the app stops

Log messages to track app start and stop times

Syntax

FastAPI

from fastapi import FastAPI

app = FastAPI()

@app.on_event("startup")
async def startup_event():
    # code to run on startup

@app.on_event("shutdown")
async def shutdown_event():
    # code to run on shutdown

Use @app.on_event("startup") to run code when the app starts.

Use @app.on_event("shutdown") to run code when the app stops.

Examples

This prints a message when the app starts.

FastAPI

from fastapi import FastAPI

app = FastAPI()

@app.on_event("startup")
async def startup_event():
    print("App is starting")

This prints a message when the app stops.

FastAPI

from fastapi import FastAPI

app = FastAPI()

@app.on_event("shutdown")
async def shutdown_event():
    print("App is stopping")

This simulates connecting and closing a database on startup and shutdown.

FastAPI

from fastapi import FastAPI

app = FastAPI()

@app.on_event("startup")
async def connect_db():
    print("Connecting to database...")

@app.on_event("shutdown")
async def close_db():
    print("Closing database connection...")

Sample Program

This FastAPI app prints messages when it starts and stops. It also has a simple route that returns a greeting.

FastAPI

from fastapi import FastAPI
import uvicorn

app = FastAPI()

@app.on_event("startup")
async def startup_event():
    print("Starting up: preparing resources")

@app.on_event("shutdown")
async def shutdown_event():
    print("Shutting down: cleaning up resources")

@app.get("/")
async def read_root():
    return {"message": "Hello, FastAPI!"}

if __name__ == "__main__":
    uvicorn.run(app, host="127.0.0.1", port=8000)

Important Notes

Startup and shutdown functions should be async for best performance.

These events run once when the app starts or stops, not on every request.

Use these events to manage resources like database connections or caches.

Summary

Startup and shutdown events run code when the app starts or stops.

Use @app.on_event("startup") and @app.on_event("shutdown") decorators.

They help prepare and clean up resources for your app.

Practice

1. What is the main purpose of using @app.on_event("startup") in a FastAPI application?

easy

A. To define API routes for the application.

B. To handle HTTP requests from clients.

C. To shut down the server immediately.

D. To run code when the application starts, like initializing resources.

Solution

1. Step 1: Understand the startup event role

   The @app.on_event("startup") decorator marks a function to run when the app starts, useful for setup tasks.

2. Step 2: Differentiate from other app parts

   Handling requests or shutting down are different concerns; startup is specifically for initialization.

3. Final Answer:

   To run code when the application starts, like initializing resources. -> Option D

4. Quick Check:

   Startup event = run code at app start [OK]

Hint: Startup event runs code once when app launches [OK]

Common Mistakes:

• Confusing startup with request handling
• Thinking startup runs multiple times per request
• Mixing startup with shutdown event

2. Which of the following is the correct syntax to define a shutdown event handler in FastAPI?

easy

A. @app.on_event("start")
def cleanup():
print("Cleaning up")

B. @app.shutdown_event
def cleanup():
print("Cleaning up")

C. @app.on_event("shutdown")
def cleanup():
print("Cleaning up")

D. @app.event("shutdown")
def cleanup():
print("Cleaning up")

Solution

1. Step 1: Recall correct decorator for shutdown

   The correct decorator is @app.on_event("shutdown") to run code when the app stops.

2. Step 2: Check syntax correctness

   Options A, B, and D use incorrect decorator names or syntax.

3. Final Answer:

   @app.on_event("shutdown")\ndef cleanup():\n print("Cleaning up") -> Option C

4. Quick Check:

   Shutdown event decorator = @app.on_event("shutdown") [OK]

Hint: Shutdown event uses @app.on_event("shutdown") exactly [OK]

Common Mistakes:

• Using wrong event name like "start" instead of "shutdown"
• Missing @app.on_event decorator
• Using non-existent decorators like @app.shutdown_event

3. Consider this FastAPI code snippet:

from fastapi import FastAPI
app = FastAPI()

@app.on_event("startup")
async def startup_event():
    print("Starting app")

@app.on_event("shutdown")
async def shutdown_event():
    print("Stopping app")

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

What will be printed when the server starts and then stops?

medium

A. Stopping app (on start), Starting app (on shutdown)

B. Starting app (on start), Stopping app (on shutdown)

C. Only Starting app when server starts

D. No output printed at all

Solution

1. Step 1: Identify startup and shutdown prints

   The startup_event prints "Starting app" when the app starts, and shutdown_event prints "Stopping app" when the app stops.

2. Step 2: Understand event timing

   These events run once each: startup at launch, shutdown at server stop.

3. Final Answer:

   Starting app (on start), Stopping app (on shutdown) -> Option B

4. Quick Check:

   Startup prints "Starting app", shutdown prints "Stopping app" [OK]

Hint: Startup prints on launch, shutdown prints on stop [OK]

Common Mistakes:

• Swapping startup and shutdown outputs
• Expecting prints on every request
• Thinking no output occurs without explicit call

4. This FastAPI code is intended to print a message on shutdown but does not work:

from fastapi import FastAPI
app = FastAPI()

def shutdown_event():
    print("App is stopping")

app.on_event("shutdown", shutdown_event)

What is the problem?

medium

A. The decorator syntax is incorrect; should use @app.on_event("shutdown") above the function.

B. FastAPI does not support shutdown events.

C. The function name must be shutdown_handler, not shutdown_event.

D. The function must be async to work as an event handler.

Solution

1. Step 1: Check decorator usage

   The code uses app.on_event("shutdown", shutdown_event) which incorrectly passes two arguments to on_event; it expects only the event name and returns a decorator to apply to a function.

2. Step 2: Identify common mistake

   FastAPI expects the decorator syntax @app.on_event("shutdown") placed above the function definition for clarity and proper registration.

3. Final Answer:

   The decorator syntax is incorrect; should use @app.on_event("shutdown") above the function. -> Option A

4. Quick Check:

   Use @app.on_event("shutdown") decorator syntax [OK]

Hint: Always use @app.on_event("shutdown") decorator syntax [OK]

Common Mistakes:

• Assuming function must be async (it can be sync)
• Using wrong function names
• Believing FastAPI lacks shutdown support

5. You want to open a database connection when your FastAPI app starts and close it when it stops. Which code correctly uses startup and shutdown events to do this?

from fastapi import FastAPI
app = FastAPI()
db = None

# Option A
@app.on_event("startup")
async def connect_db():
    global db
    db = "Connected"

@app.on_event("shutdown")
async def close_db():
    global db
    db = None

# Option B
@app.on_event("startup")
def connect_db():
    db = "Connected"

@app.on_event("shutdown")
def close_db():
    db = None

# Option C
@app.on_event("startup")
async def connect_db():
    db = "Connected"

@app.on_event("shutdown")
async def close_db():
    db = None

# Option D
@app.on_event("startup")
async def connect_db():
    global db
    db = None

@app.on_event("shutdown")
async def close_db():
    global db
    db = "Connected"

hard

A. Option A correctly manages the global db connection on startup and shutdown.

B. Option B correctly manages db without global keyword.

C. Option C correctly manages db asynchronously without global keyword.

D. Option D reverses connection and disconnection logic.

Solution

1. Step 1: Understand global variable usage

   Since db is defined outside functions, to modify it inside functions, global db is needed.

2. Step 2: Check startup and shutdown logic

   The correct version uses global db in both async event handlers, setting db = "Connected" on startup and db = None on shutdown. Versions without global db create local variables. One version reverses the connection and disconnection logic.

3. Final Answer:

   Option A correctly manages the global db connection on startup and shutdown. -> Option A

4. Quick Check:

   Use global to modify external variables in event handlers [OK]

Hint: Use global keyword to modify external variables inside event functions [OK]

Common Mistakes:

• Forgetting global keyword causes local variable shadowing
• Reversing startup and shutdown logic
• Assuming async is required for all event handlers