Mastering the Art of Graceful Shutdown in FastAPI with Ctrl+C
When developing FastAPI applications, it's common to use threading to handle multiple tasks concurrently. But what happens when you need to stop your application abruptly? The standard practice of hitting Ctrl+C can sometimes leave your threads running in the background, leading to unexpected behavior or resource leaks. This article will guide you through the process of gracefully shutting down FastAPI applications, ensuring all threads are terminated correctly when using Ctrl+C.
The Problem:
Let's imagine you have a FastAPI application running with multiple background threads using the asyncio library:
from fastapi import FastAPI
import asyncio
app = FastAPI()
@app.on_event("startup")
async def startup_event():
async def background_task():
while True:
print("Background task running...")
await asyncio.sleep(1)
asyncio.create_task(background_task())
@app.get("/")
async def root():
return {"message": "Hello World"}
if __name__ == "__main__":
import uvicorn
uvicorn.run(app, host="127.0.0.1", port=8000)
In this example, the background_task function runs indefinitely, printing a message every second. If you press Ctrl+C while the server is running, the main FastAPI process will terminate, but the background task will continue to run indefinitely.
The Solution: Graceful Shutdown with Signals
To handle Ctrl+C gracefully and ensure all threads are terminated, we can utilize the signal module in Python. This allows us to define a custom handler for the SIGINT signal, which is triggered when Ctrl+C is pressed.
import asyncio
import signal
from fastapi import FastAPI
import uvicorn
app = FastAPI()
@app.on_event("startup")
async def startup_event():
async def background_task():
while True:
print("Background task running...")
await asyncio.sleep(1)
asyncio.create_task(background_task())
async def shutdown_event():
print("Shutting down...")
# Add logic to gracefully stop your background tasks here
# e.g., set a flag to signal tasks to stop
@app.on_event("shutdown")
async def shutdown_event():
await shutdown_event()
def signal_handler(sig, frame):
print('Ctrl+C pressed, shutting down...')
asyncio.run(shutdown_event())
exit(0)
signal.signal(signal.SIGINT, signal_handler)
if __name__ == "__main__":
uvicorn.run(app, host="127.0.0.1", port=8000)
In this improved version:
- We define the
shutdown_eventfunction, which will be executed when the application receives theSIGINTsignal. - We create a
signal_handlerfunction that gracefully exits the application whenSIGINTis received. This function also runs theshutdown_eventcoroutine, allowing you to implement any custom logic to stop your background tasks. - We register the
signal_handlerwith thesignal.signalfunction for theSIGINTsignal.
Now, when you press Ctrl+C, the signal_handler will be called, triggering the shutdown_event and allowing you to implement logic to gracefully terminate your background tasks.
Important Considerations:
- Thread Safety: Ensure that any shared resources or data structures accessed by your background tasks are thread-safe.
- Timeout: Consider adding a timeout mechanism to your shutdown logic to prevent long-running tasks from blocking the shutdown process.
- Clean Up: In your
shutdown_eventfunction, close any connections, release resources, and perform any necessary cleanup tasks.
Conclusion:
By utilizing the signal module and implementing a graceful shutdown mechanism in your FastAPI application, you can ensure proper termination of all threads and avoid unexpected behavior when using Ctrl+C. Remember to tailor the shutdown logic to the specific needs of your application and its background tasks.
Additional Resources:
By following these guidelines and incorporating best practices, you can create robust and reliable FastAPI applications that handle shutdown events gracefully, ensuring smooth and controlled termination of all processes.
