Fast Background Tasks

Running tasks in background in FastAPI

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1. What are Background Tasks?

• Background tasks let you run logic after returning a response to the client.
• Useful when you don’t want the client to wait for heavy operations.

👉 Example use cases:

• Sending emails after signup
• Logging / analytics
• Data processing (e.g., resizing images)
• Notifications

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2. Why Background Tasks?

• ✅ Keep APIs fast → respond immediately.
• ✅ Offload non-critical work.
• ✅ Built-in → no need for Celery/Redis for small tasks.

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3. Basic Example

from fastapi import FastAPI, BackgroundTasks

app = FastAPI()

def write_log(message: str):
    with open("log.txt", "a") as f:
        f.write(message + "\n")

@app.post("/log/")
def log(background_tasks: BackgroundTasks, msg: str):
    background_tasks.add_task(write_log, msg)
    return {"status": "Message logged in background"}

✔️ Flow:

1. Client calls /log/
2. Response returned immediately ✅
3. write_log() runs in background

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4. Advanced Example — Sending Email

import time
from fastapi import FastAPI, BackgroundTasks

app = FastAPI()

def send_email(email: str, subject: str, body: str):
    time.sleep(5)  # Simulate email sending delay
    print(f"Sent email to {email}: {subject}")

@app.post("/send-mail/")
def send_mail(background_tasks: BackgroundTasks, email: str):
    background_tasks.add_task(send_email, email, "Welcome!", "Thanks for joining 🎉")
    return {"status": f"Email to {email} is being sent in background"}

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5. Combining with Depends

Background tasks can also be injected along with dependencies:

from fastapi import Depends

def get_current_user(token: str = "test123"):
    if token != "test123":
        raise Exception("Invalid Token")
    return {"username": "admin"}

@app.post("/notify/")
def notify_user(background_tasks: BackgroundTasks, user: dict = Depends(get_current_user)):
    background_tasks.add_task(write_log, f"Notification sent to {user['username']}")
    return {"status": "Notification scheduled"}

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6. Key Benefits in FastAPI

• ✅ Integrated, no extra setup
• ✅ Perfect for lightweight tasks
• ✅ Runs in the same process (not a queue system)
• ✅ Great for logging, caching, notifications, async jobs

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7. How FastAPI Uses Background Tasks Under the Hood

Request cycle with Background Tasks:

Client Request
     ↓
FastAPI Router
     ↓
Run Endpoint
     ↓
Return Response (immediately)
     ↓
Background Task executes (after response sent)

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⚠️ Note:

• For heavy tasks (long-running, CPU-intensive), you should use Celery, RQ, Dramatiq with a message broker (Redis, RabbitMQ).
• BackgroundTasks are best for small async jobs.