Why is it not recommended to use ThreadPoolExecutor in FastAPI endpoints?

Potential Pitfalls of Using ThreadPoolExecutor in FastAPI Endpoints

Using concurrent.futures.ThreadPoolExecutor in FastAPI endpoints raises concerns about thread management and potential system resource exhaustion. Here are the key considerations:

Thread Proliferation and Resource Starvation

ThreadPoolExecutor manages a pool of threads. Each endpoint call can potentially create new threads, leading to excessive thread proliferation. This can strain system resources, especially when multiple requests occur simultaneously.

Improved Approach with HTTPX

To mitigate these risks, it's recommended to use the HTTPX library instead. HTTPX provides an asynchronous client that efficiently handles multiple requests without creating new threads.

HTTPX Configuration

The HTTPX client can be configured to control the number of connections and keep-alive connections, allowing you to tailor the behavior to your application's needs.

Async Support in FastAPI

FastAPI natively supports asynchronous operations using the async keyword. This allows you to perform HTTP requests asynchronously, without blocking the event loop.

Async Functions and HTTPX

To use HTTPX asynchronously in a FastAPI endpoint, define an async function that makes the HTTP requests using the AsyncClient instance.

Managing HTTPX Client

You can manage the HTTPX client's lifetime using a lifespan hook in FastAPI. This ensures that the client is initialized at startup and closed at shutdown to handle resource cleanup properly.

Streaming Responses

To avoid reading the entire response body into memory, consider using streaming responses in HTTPX and FastAPI's StreamingResponse class.

Example Code

Here's an example of a FastAPI endpoint that uses HTTPX and optimizes thread management:

from fastapi import FastAPI, Request
from contextlib import asynccontextmanager
import httpx
import asyncio

async def lifespan(app: FastAPI):
    # HTTPX client settings
    limits = httpx.Limits(max_keepalive_connections=5, max_connections=10)
    timeout = httpx.Timeout(5.0, read=15.0)

    # Initialize the HTTPX client
    async with httpx.AsyncClient(limits=limits, timeout=timeout) as client:
        yield {'client': client}

app = FastAPI(lifespan=lifespan)

@asynccontextmanager
async def send(client):
    req = client.build_request('GET', URL)
    yield await client.send(req, stream=True)

@app.get('/')
async def main(request: Request):
    client = request.state.client

    # Make HTTPX requests in a loop
    responses = [await send(client) for _ in range(5)]
    # Use a streaming response to return the first 50 chars of each response
    return StreamingResponse(iter_response(responses))

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