aiohttp set number of requests per second

Question

I'm writing an API in Flask with 1000+ requests to get data and I'd like to limit the number of requests per second. I tried with:

conn = aiohttp.TCPConnector(limit_per_host=20)

and 

conn = aiohttp.TCPConnector(limit=20)

But is seems doesn't work

My code looks like this:

import logging
import asyncio
import aiohttp

logging.basicConfig(filename="logfilename.log", level=logging.INFO, format='%(asctime)s %(levelname)s:%(message)s')


async def fetch(session, url):
    async with session.get(url, headers=headers) as response:
        if response.status == 200:
            data = await response.json()
            json = data['args']
        return json

async def fetch_all(urls, loop):
    conn = aiohttp.TCPConnector(limit=20)
    async with aiohttp.ClientSession(connector=conn, loop=loop) as session:
        results = await asyncio.gather(*[fetch(session, url) for url in urls], return_exceptions=True)
        return results

async def main():
    loop = asyncio.new_event_loop()
    url_list = []
    args = ['a', 'b', 'c', +1000 others]
    urls = url_list
    for i in args:
        base_url = 'http://httpbin.org/anything?key=%s' % i
        url_list.append(base_url)

    htmls = loop.run_until_complete(fetch_all(urls, loop))
    for j in htmls:
        key = j['key']
        # save to database
        logging.info(' %s was added', key)

If I run code, within 1s I send over than 200 requests. Is there any way to limit requests?

Answer 1

The code above works as expected (apart from a small error regarding headers being undefined).

Tested on my machine the httpbin URL responds in around 100ms which means that with a concurrency of 20 it will serve around 200 requests in 1 second (which is what you're seeing as well):

100 ms per request means 10 requests are completed in a second
10 requests per second with a concurrency of 20 means 200 requests in one second

The limit option ( aiohttp.TCPConnector ) limits the number of concurrent requests and does not have any time dimension.

To see the limit in action try with more values like 10 , 20 , 50 :

# time to complete 1000 requests with different keys
aiohttp.TCPConnector(limit=10): 12.58 seconds 
aiohttp.TCPConnector(limit=20): 6.57 seconds
aiohttp.TCPConnector(limit=50): 3.1 seconds

If you want to use a requests per second limit send batch of requests (20 for example) and use asyncio.sleep(1.0) to pause for a second, then send the next batch and so on.