FastAPI Background Tasks Demo

This is a simple FastAPI server to work through blocking and non-blocking background tasks. It simulates a blocking upload (e.g., to S3) and shows how FastAPI’s BackgroundTasks returns a response immediately while work continues in the background.

Tech Stack

• FastAPI (standard extras)
• Python 3.13+

How it works

• upload_to_s3() simulates a blocking operation using time.sleep(5).
• GET /process-files enqueues upload_to_s3 as a background task via BackgroundTasks.
• The endpoint returns immediately, while the blocking function runs in the background.
• Logs illustrate the timing:
  • App: “Before process files” → “After process files”
  • Background task: “Before sleep” → “After sleep”

To experiment with non-blocking behavior, compare time.sleep(5) with an async approach (e.g., await asyncio.sleep(5)) and adapt the task accordingly.

Running the app

Using uv (recommended)

1. Install dependencies:
   • uv sync
2. Start the dev server:
   • uv run fastapi dev main.py
3. Open the interactive docs:
   • http://127.0.0.1:8000/docs

Using pip/venv

1. Create and activate a virtual environment:
   • python -m venv .venv
   • source .venv/bin/activate
2. Install dependencies:
   • pip install "fastapi[standard]>=0.116.1"
3. Start the server:
   • uvicorn main:app --reload
4. Open the interactive docs:
   • http://127.0.0.1:8000/docs

Endpoint

• GET /process-files
  • Queues a background task that simulates a blocking upload with time.sleep(5).
  • Returns: {"ok": true} immediately.

Example:

• curl http://127.0.0.1:8000/process-files

Notes

• The demo code lives in main.py, with upload_to_s3() as the blocking task and the /process-files route scheduling it in the background.
• Switch time.sleep(5) to await asyncio.sleep(5) and make the task async if you want to visualize non-blocking behavior end-to-end.