My little idea of how to run multiple web crawler instances simultaneously with big input file.

aiohttp_crawler.py

# -*- coding: utf-8 -*-
"""To run multiple aiohttp instances concurrently.

code snippets I found online all create a list tasks to run,
but my example file abc.csv is too big, and contains 10 million rows,
which cannot fit into limited memory on a AWS ec2.micro instance,
so I use a global generator instead, to feed task dynamically.
requires python version 3.6.4 for f string to work.
"""

import os
import csv
import requests
from random import choice
from bs4 import BeautifulSoup as bsp
import asyncio
import aiohttp
import async_timeout


user_agents = [
    'Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US) AppleWebKit/525.19 (KHTML, like Gecko) Chrome/1.0.154.53 Safari/525.19',
    'Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US) AppleWebKit/525.19 (KHTML, like Gecko) Chrome/1.0.154.36 Safari/525.19',
    'Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US) AppleWebKit/534.10 (KHTML, like Gecko) Chrome/7.0.540.0 Safari/534.10',
    'Mozilla/5.0 (Windows; U; Windows NT 5.2; en-US) AppleWebKit/534.4 (KHTML, like Gecko) Chrome/6.0.481.0 Safari/534.4',
    'Mozilla/5.0 (Macintosh; U; Intel Mac OS X; en-US) AppleWebKit/533.4 (KHTML, like Gecko) Chrome/5.0.375.86 Safari/533.4',
    'Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US) AppleWebKit/532.2 (KHTML, like Gecko) Chrome/4.0.223.3 Safari/532.2',
    'Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US) AppleWebKit/532.0 (KHTML, like Gecko) Chrome/4.0.201.1 Safari/532.0',
    'Mozilla/5.0 (Windows; U; Windows NT 5.2; en-US) AppleWebKit/532.0 (KHTML, like Gecko) Chrome/3.0.195.27 Safari/532.0',
    'Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US) AppleWebKit/530.5 (KHTML, like Gecko) Chrome/2.0.173.1 Safari/530.5',
    'Mozilla/5.0 (Windows; U; Windows NT 5.2; en-US) AppleWebKit/534.10 (KHTML, like Gecko) Chrome/8.0.558.0 Safari/534.10',
    'Mozilla/5.0 (X11; U; Linux x86_64; en-US) AppleWebKit/540.0 (KHTML,like Gecko) Chrome/9.1.0.0 Safari/540.0',
    'Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US) AppleWebKit/534.14 (KHTML, like Gecko) Chrome/9.0.600.0 Safari/534.14',
    'Mozilla/5.0 (X11; U; Windows NT 6; en-US) AppleWebKit/534.12 (KHTML, like Gecko) Chrome/9.0.587.0 Safari/534.12',
    'Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US) AppleWebKit/534.13 (KHTML, like Gecko) Chrome/9.0.597.0 Safari/534.13',
    'Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US) AppleWebKit/534.16 (KHTML, like Gecko) Chrome/10.0.648.11 Safari/534.16',
    'Mozilla/5.0 (Windows; U; Windows NT 6.0; en-US) AppleWebKit/534.20 (KHTML, like Gecko) Chrome/11.0.672.2 Safari/534.20',
    'Mozilla/5.0 (Windows NT 6.0) AppleWebKit/535.1 (KHTML, like Gecko) Chrome/14.0.792.0 Safari/535.1',
    'Mozilla/5.0 (Windows NT 5.1) AppleWebKit/535.2 (KHTML, like Gecko) Chrome/15.0.872.0 Safari/535.2',
    'Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/535.7 (KHTML, like Gecko) Chrome/16.0.912.36 Safari/535.7',
    'Mozilla/5.0 (Windows NT 6.0; WOW64) AppleWebKit/535.11 (KHTML, like Gecko) Chrome/17.0.963.66 Safari/535.11',
    'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_6_8) AppleWebKit/535.19 (KHTML, like Gecko) Chrome/18.0.1025.45 Safari/535.19',
    'Mozilla/5.0 (Windows NT 6.2; WOW64) AppleWebKit/535.24 (KHTML, like Gecko) Chrome/19.0.1055.1 Safari/535.24',
    'Mozilla/5.0 (Windows NT 6.2) AppleWebKit/536.6 (KHTML, like Gecko) Chrome/20.0.1090.0 Safari/536.6',
    'Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.1 (KHTML, like Gecko) Chrome/22.0.1207.1 Safari/537.1',
    'Mozilla/5.0 (Windows NT 6.2; WOW64) AppleWebKit/537.15 (KHTML, like Gecko) Chrome/24.0.1295.0 Safari/537.15',
    'Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/27.0.1453.93 Safari/537.36',
    'Mozilla/5.0 (Windows NT 6.2) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/28.0.1467.0 Safari/537.36',
    'Mozilla/5.0 (Windows NT 6.3; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/30.0.1599.101 Safari/537.36',
    'Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/31.0.1623.0 Safari/537.36',
    'Mozilla/5.0 (Windows NT 6.2; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/34.0.1847.116 Safari/537.36',
    'Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/37.0.2062.103 Safari/537.36',
    'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_2) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/40.0.2214.38 Safari/537.36',
    'Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/46.0.2490.71 Safari/537.36',
    'Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/51.0.2704.103 Safari/537.36',
    'Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/62.0.3202.62 Safari/537.36',
    'Mozilla/5.0 (Macintosh; U; PPC Mac OS X; fi-fi) AppleWebKit/420+ (KHTML, like Gecko) Safari/419.3',
    'Mozilla/5.0 (Macintosh; U; PPC Mac OS X; de-de) AppleWebKit/125.2 (KHTML, like Gecko) Safari/125.7',
    'Mozilla/5.0 (Macintosh; U; PPC Mac OS X; en-us) AppleWebKit/312.8 (KHTML, like Gecko) Safari/312.6',
    'Mozilla/5.0 (Windows; U; Windows NT 5.1; cs-CZ) AppleWebKit/523.15 (KHTML, like Gecko) Version/3.0 Safari/523.15',
    'Mozilla/5.0 (Windows; U; Windows NT 6.0; en-US) AppleWebKit/528.16 (KHTML, like Gecko) Version/4.0 Safari/528.16',
    'Mozilla/5.0 (Macintosh; U; PPC Mac OS X 10_5_6; it-it) AppleWebKit/528.16 (KHTML, like Gecko) Version/4.0 Safari/528.16',
    'Mozilla/5.0 (Windows; U; Windows NT 6.1; zh-HK) AppleWebKit/533.18.1 (KHTML, like Gecko) Version/5.0.2 Safari/533.18.5',
    'Mozilla/5.0 (Windows; U; Windows NT 6.1; sv-SE) AppleWebKit/533.19.4 (KHTML, like Gecko) Version/5.0.3 Safari/533.19.4',
    'Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US) AppleWebKit/533.20.25 (KHTML, like Gecko) Version/5.0.4 Safari/533.20.27',
    'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_7_3) AppleWebKit/534.55.3 (KHTML, like Gecko) Version/5.1.3 Safari/534.53.10',
    'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_8_2) AppleWebKit/536.26.17 (KHTML, like Gecko) Version/6.0.2 Safari/536.26.17',
    'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_8_5) AppleWebKit/537.75.14 (KHTML, like Gecko) Version/6.1.3 Safari/537.75.14',
    'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_0) AppleWebKit/600.3.10 (KHTML, like Gecko) Version/8.0.3 Safari/600.3.10',
    'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11) AppleWebKit/601.1.39 (KHTML, like Gecko) Version/9.0 Safari/601.1.39',
    'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13) AppleWebKit/603.1.13 (KHTML, like Gecko) Version/10.1 Safari/603.1.13',
    'Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.5; en-US; rv:1.9.1b3) Gecko/20090305 Firefox/3.1b3 GTB5',
    'Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.5; ko; rv:1.9.1b2) Gecko/20081201 Firefox/3.1b2',
    'Mozilla/5.0 (X11; U; SunOS sun4u; en-US; rv:1.9b5) Gecko/2008032620 Firefox/3.0b5',
    'Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.8.1.12) Gecko/20080214 Firefox/2.0.0.12',
    'Mozilla/5.0 (Windows; U; Windows NT 5.1; cs; rv:1.9.0.8) Gecko/2009032609 Firefox/3.0.8',
    'Mozilla/5.0 (X11; U; OpenBSD i386; en-US; rv:1.8.0.5) Gecko/20060819 Firefox/1.5.0.5',
    'Mozilla/5.0 (Windows; U; Windows NT 5.0; es-ES; rv:1.8.0.3) Gecko/20060426 Firefox/1.5.0.3',
    'Mozilla/5.0 (Windows; U; WinNT4.0; en-US; rv:1.7.9) Gecko/20050711 Firefox/1.0.5',
    'Mozilla/5.0 (Windows; Windows NT 6.1; rv:2.0b2) Gecko/20100720 Firefox/4.0b2',
    'Mozilla/5.0 (X11; Linux x86_64; rv:2.0b4) Gecko/20100818 Firefox/4.0b4',
    'Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.2) Gecko/20100308 Ubuntu/10.04 (lucid) Firefox/3.6 GTB7.1',
    'Mozilla/5.0 (Windows NT 6.1; WOW64; rv:2.0b7) Gecko/20101111 Firefox/4.0b7',
    'Mozilla/5.0 (Windows NT 6.1; WOW64; rv:2.0b8pre) Gecko/20101114 Firefox/4.0b8pre',
    'Mozilla/5.0 (X11; Linux x86_64; rv:2.0b9pre) Gecko/20110111 Firefox/4.0b9pre',
    'Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:2.0b9pre) Gecko/20101228 Firefox/4.0b9pre',
    'Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:2.2a1pre) Gecko/20110324 Firefox/4.2a1pre',
    'Mozilla/5.0 (X11; U; Linux amd64; rv:5.0) Gecko/20100101 Firefox/5.0 (Debian)',
    'Mozilla/5.0 (Windows NT 6.1; WOW64; rv:6.0a2) Gecko/20110613 Firefox/6.0a2',
    'Mozilla/5.0 (X11; Linux i686 on x86_64; rv:12.0) Gecko/20100101 Firefox/12.0',
    'Mozilla/5.0 (Windows NT 6.1; rv:15.0) Gecko/20120716 Firefox/15.0a2',
    'Mozilla/5.0 (X11; Ubuntu; Linux armv7l; rv:17.0) Gecko/20100101 Firefox/17.0',
    'Mozilla/5.0 (Windows NT 6.1; rv:21.0) Gecko/20130328 Firefox/21.0',
    'Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:22.0) Gecko/20130328 Firefox/22.0',
    'Mozilla/5.0 (Windows NT 5.1; rv:25.0) Gecko/20100101 Firefox/25.0',
    'Mozilla/5.0 (Macintosh; Intel Mac OS X 10.8; rv:25.0) Gecko/20100101 Firefox/25.0',
    'Mozilla/5.0 (Windows NT 6.1; rv:28.0) Gecko/20100101 Firefox/28.0',
    'Mozilla/5.0 (X11; Linux i686; rv:30.0) Gecko/20100101 Firefox/30.0',
    'Mozilla/5.0 (Windows NT 5.1; rv:31.0) Gecko/20100101 Firefox/31.0',
    'Mozilla/5.0 (Windows NT 6.1; WOW64; rv:33.0) Gecko/20100101 Firefox/33.0',
    'Mozilla/5.0 (Windows NT 10.0; WOW64; rv:40.0) Gecko/20100101 Firefox/40.0',
]

headers = {
    'Accept': 'text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8',
    'Accept-Encoding': 'gzip, deflate',
    'Accept-Language': 'en-US,en;q=0.9,zh-CN;q=0.8,zh;q=0.7',
    'Cache-Control': 'max-age=0',
    'Connection': 'keep-alive',
    'Upgrade-Insecure-Requests': '1',
    'User-Agent': '',
}

input_file = 'abc.csv'
output_file = open('123.csv', 'a')
writer = csv.writer(output)

co_num = 10

def parse(uid, html):
    try:
        soup = bsp(html, 'lxml')
        lorem = soup.select_one('div#lorem')
        writer.writerow([uid, lorem])
    except Exception as e:
        print(e)

def get_uid_url():
    with open(input_file, 'r') as urls:
        reader = csv.reader(urls)
        base_url = 'http://www.example.com/'
        
        for row in reader:
            uid = row[0]
            whole_url = base_url + row[1]
            yield uid, whole_url

uid_urls = get_uid_url()

async def fetch(number, session):
    print(f'task number: {number}')
    user_agent = choice(user_agents)
    headers['User-Agent'] = user_agent

    while True:
        try:
            uid, url = next(uid_urls)
        except:
            return
        else:
            with async_timeout.timeout(10):
                async with session.get(url, headers=headers) as response:
                    html = await response.text()
                    parse(uid, html)

def main():
    with aiohttp.ClientSession() as session:
        tasks = [fetch(i, session) for i in range(co_num)]

        loop = asyncio.get_event_loop()
        loop.run_until_complete(asyncio.wait(tasks))
        loop.close()
        output_file.close()


if __name__ == '__main__':
    main()