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Module 05 / Project 05 — Async Web Server

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— This project — — Flashcards — —

Focus

  • Async FastAPI endpoints
  • Background tasks with BackgroundTasks
  • Streaming responses with StreamingResponse
  • Running async operations inside endpoints

Why this project exists

FastAPI is async by default. This project brings together everything from this module — async functions, concurrent operations, queues — into a real web server. You will see how async makes web servers handle many requests without blocking.

Run

cd projects/modules/05-async-python/05-async-web-server
pip install -r ../requirements.txt
python app.py
# Visit http://127.0.0.1:8000/docs to see the API

Expected output

INFO:     Uvicorn running on http://127.0.0.1:8000
INFO:     Application startup complete.

Then visit /docs in your browser to interact with the API.

Alter it

  1. Add an endpoint that fetches data from 3 external APIs concurrently and returns combined results.
  2. Add a WebSocket endpoint that sends progress updates to the client.
  3. Add a /stats endpoint that returns how many background tasks have completed.

Break it

  1. Use time.sleep(10) in an endpoint instead of asyncio.sleep(). Try hitting another endpoint while it sleeps.
  2. Remove async from an endpoint that uses await. What error do you get?
  3. Start a background task that raises an exception. What happens to the response?

Fix it

  1. Replace time.sleep() with asyncio.sleep() so other requests are not blocked.
  2. Add error handling in background tasks so failures are logged, not lost.
  3. Add a timeout to the slow endpoint using asyncio.wait_for().

Explain it

  1. Why are FastAPI endpoints async def by default?
  2. What happens when you define an endpoint with def (no async) in FastAPI?
  3. How do background tasks differ from running something in the endpoint itself?
  4. What would you use for truly long-running tasks that outlive a request?

Mastery check

You can move on when you can: - write async FastAPI endpoints, - use BackgroundTasks for deferred work, - explain why blocking calls hurt async servers, - stream data to clients with StreamingResponse.

Next

Go back to Module index or continue to Module 06 — Databases & ORM.