How to run tasks concurrently in asyncio?
Your crawl
method is waiting after each individual task; you should change it to this:
async def crawl(self):
tasks = []
while self.active_fetches < self.MAX_ACTIVE_FETCHES:
if self.urlq:
url = self.urlq.pop()
tasks.append(asyncio.create_task(self.fetch(url)))
await asyncio.gather(*tasks)
EDIT: Here's a cleaner version with comments that fetches and processes all at the same time, while preserving the basic ability to put a cap on the maximum number of fetchers.
import asyncio
class Crawler:
def __init__(self, urls, max_workers=2):
self.urls = urls
# create a queue that only allows a maximum of two items
self.fetching = asyncio.Queue()
self.max_workers = max_workers
async def crawl(self):
# DON'T await here; start consuming things out of the queue, and
# meanwhile execution of this function continues. We'll start two
# coroutines for fetching and two coroutines for processing.
all_the_coros = asyncio.gather(
*[self._worker(i) for i in range(self.max_workers)])
# place all URLs on the queue
for url in self.urls:
await self.fetching.put(url)
# now put a bunch of `None`'s in the queue as signals to the workers
# that there are no more items in the queue.
for _ in range(self.max_workers):
await self.fetching.put(None)
# now make sure everything is done
await all_the_coros
async def _worker(self, i):
while True:
url = await self.fetching.get()
if url is None:
# this coroutine is done; simply return to exit
return
print(f'Fetch worker {i} is fetching a URL: {url}')
page = await self.fetch(url)
self.process(page)
async def fetch(self, url):
print("Fetching URL: " + url);
await asyncio.sleep(2)
return f"the contents of {url}"
def process(self, page):
print("processed page: " + page)
# main loop
c = Crawler(['http://www.google.com', 'http://www.yahoo.com',
'http://www.cnn.com', 'http://www.gamespot.com',
'http://www.facebook.com', 'http://www.evergreen.edu'])
asyncio.run(c.crawl())
You can make htmlq
an asyncio.Queue()
, and change htmlq.append
to htmlq.push
. Then your main
can be async, like this:
async def main():
c = Crawler()
asyncio.create_task(c.crawl())
while True:
page = await c.htmlq.get()
if page is None:
break
c.process(page)
Your top-level code boils down to a call to asyncio.run(main())
.
Once you are done with crawling, crawl()
can enqueue None
to notify the main coroutine that the work is done.