Why Does FastAPI Run API Calls Serially Instead of in Parallel?

FastAPI Runs API Calls in Serial Instead of Parallel Fashion

Q: Why do FastAPI API calls run serially instead of in parallel?

A: According to FastAPI's documentation, when using def instead of async def to define a path operation function, it is run in an external thread pool that is then awaited, instead of called directly. This is done to prevent blocking the server.

Additional Considerations for Parallel Execution:

async def vs. def endpoints:

• async def: Functions run directly in the event loop; will be processed concurrently, as long as they await for non-blocking I/O-bound operations.
• def: Functions run in a separate thread from an external thread pool; will be processed serially, unless run outside the event loop.

Using Blocking Operations:

If an async def endpoint contains a blocking operation and does not await its completion, it will block the event loop and requests will be processed serially.

Solutions:

• Define the endpoint with normal def instead of async def (if there are no awaitable operations).
• Use FastAPI's run_in_threadpool() function to execute blocking tasks in a separate thread.
• Use asyncio's loop.run_in_executor() or asyncio.to_thread() to run blocking tasks in a separate executor or thread.

Other Optimization Strategies:

• Use more workers to take advantage of multi-core CPUs.
• Consider using Celery or AsyncIOScheduler for heavy background computation.

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