Changing def to async def let one blocking call freeze an entire FastAPI app

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Deep dive into async Python pitfall with FastAPI, highly actionable for backend engineers.

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Why It Matters

This matters because it exposes a critical performance pitfall in FastAPI that can undermine developer experience and architecture decisions in async Python services, especially relevant for platform engineers building scalable APIs.

Key Takeaways

Always use async HTTP clients (like httpx.AsyncClient) in async def endpoints and monitor thread pool saturation for def endpoints to avoid concurrency cliffs.

Changing def to async def let one blocking call freeze an entire FastAPI app

Summary

Switching a FastAPI endpoint from `def` to `async def` without replacing blocking calls like `requests.get` with an async client (e.g., `httpx.AsyncClient`) can freeze the entire server, as the blocking call occupies the event loop. Benchmarks show a 20,000-fold slowdown (from 1.6ms to 34s) for an unrelated endpoint, and the anyio thread pool (default 40 threads) only protects `def` endpoints up to 40 concurrent requests, after which performance degrades sharply.

Author

Remdore