Fire and Forget: Background Tasks

Sometimes, an API request triggers an action that takes time to complete, such as sending a confirmation email, processing a video file, or crunching heavy data. If you make the user wait for these tasks to finish, your API will feel slow and unresponsive.

FastAPI provides a simple mechanism to solve this: Background Tasks. This allows you to return a response to the user immediately while the server continues to process the heavy logic in the background.

Using the BackgroundTasks Class

To use this feature, simply declare a parameter of type BackgroundTasks in your path operation function. You do not need to install extra libraries like Celery for simple use cases; this is built right into FastAPI (via Starlette).

Scheduling Operations

Instead of calling your heavy function directly, you use the .add_task() method on the background_tasks object. You pass the function reference and any arguments it needs.

Example: Sending a Notification

Let’s simulate sending an email notification. We define a standard Python function for the logic, and then schedule it in our endpoint.

from fastapi import BackgroundTasks, FastAPI

app = FastAPI()

# This is our heavy task
def write_notification(email: str, message=""):
    with open("log.txt", mode="w") as email_file:
        content = f"notification for {email}: {message}"
        email_file.write(content)

@app.post("/send-notification/{email}")
def send_notification(email: str, background_tasks: BackgroundTasks):
    # Schedule the task
    background_tasks.add_task(write_notification, email, message="some notification")

    # Return the response immediately
    return {"message": "Notification sent in the background"}

How It Works

When the client calls /send-notification/user@example.com:

The send_notification function executes.
The task is added to the queue.
FastAPI sends the JSON response {"message": "Notification sent in the background"} to the client.
After the response is sent, the write_notification function runs on the server.

This “Fire and Forget” pattern ensures your API remains snappy and provides a better user experience.