Is having a concurrent.futures.ThreadPoolExecutor call dangerous in a FastAPI endpoint?

You should rather use the HTTPX library, which provides an async API. As described in this answer , you spawn a Client and reuse it every time you need it. To make asynchronous requests with HTTPX, you’ll need an AsyncClient.

You could control the connection pool size as well, using the limits keyword argument on the Client, which takes an instance of httpx.Limits. For example:

limits = httpx.Limits(max_keepalive_connections=5, max_connections=10)
client = httpx.AsyncClient(limits=limits)

You can adjust the above per your needs. As per the documentation on Pool limit configuration:

• max_keepalive_connections, number of allowable keep-alive connections, or None to always allow. (Defaults 20)
• max_connections, maximum number of allowable connections, or None for no limits. (Default 100)
• keepalive_expiry, time limit on idle keep-alive connections in seconds, or None for no limits. (Default 5)

If you would like to adjust the timeout as well, you can use the timeout paramter to set timeout on an individual request, or on a Client/AsyncClient instance, which results in the given timeout being used as the default for requests made with this client (see the implementation of Timeout class as well). You can specify the timeout behavior in a fine grained detail; for example, setting the read timeout parameter will specify the maximum duration to wait for a chunk of data to be received (i.e., a chunk of the response body). If HTTPX is unable to receive data within this time frame, a ReadTimeout exception is raised. If set to None instead of some positive numerical value, there will be no timeout on read. The default is 5 seconds timeout on all operations.

You can use await client.aclose() to explicitly close the AsyncClient when you are done with it (this could be done inside a shutdown event handler, for instance).

To run multiple asynchronous operations—as you need to request five different URLs, when your API endpoint is called—you can use the awaitable asyncio.gather(). It will execute the async operations and return a list of results in the same order the awaitables (tasks) were passed to that function.

Working Example:

from fastapi import FastAPI
import httpx
import asyncio

URLS = ['https://www.foxnews.com/',
        'https://edition.cnn.com/',
        'https://www.nbcnews.com/',
        'https://www.bbc.co.uk/',
        'https://www.reuters.com/']
        
limits = httpx.Limits(max_keepalive_connections=5, max_connections=10)
timeout = httpx.Timeout(5.0, read=15.0) # 15s timeout on read. 5s timeout elsewhere.
client = httpx.AsyncClient(limits=limits, timeout=timeout)
app = FastAPI()

@app.on_event('shutdown')
async def shutdown_event():
    await client.aclose()

async def send(url, client):
    return await client.get(url)

@app.get("https://stackoverflow.com/")
async def main():
    tasks = [send(url, client) for url in URLS]
    responses = await asyncio.gather(*tasks)
    # for demo purposes, return only the first 50 chars of each response
    return [r.text[:50] for r in responses]

If you would like to avoid reading the entire response body into RAM, you could use Streaming responses, as described in this answer and demonstrated below:

from fastapi import FastAPI
from fastapi.responses import StreamingResponse
import httpx
import asyncio

URLS = ['https://www.foxnews.com/',
        'https://edition.cnn.com/',
        'https://www.nbcnews.com/',
        'https://www.bbc.co.uk/',
        'https://www.reuters.com/']
        
limits = httpx.Limits(max_keepalive_connections=5, max_connections=10)
timeout = httpx.Timeout(5.0, read=15.0) # 15s timeout on read. 5s timeout elsewhere.
client = httpx.AsyncClient(limits=limits, timeout=timeout)
app = FastAPI()

@app.on_event('shutdown')
async def shutdown_event():
    await client.aclose()

async def send(url, client):
    req = client.build_request('GET', url)
    return await client.send(req, stream=True)

async def iter_content(responses):
     for r in responses:
        async for chunk in r.aiter_text():
            # for demo purposes, return only the first 50 chars of each response
            yield chunk[:50]
            yield '\n'
            break
        await r.aclose()
        
@app.get("https://stackoverflow.com/")
async def main():
    tasks = [send(url, client) for url in URLS]
    responses = await asyncio.gather(*tasks)
    return StreamingResponse(iter_content(responses), media_type="text/event-stream")