Lock example - graceful shutdown

Graceful shutdown, an example use case for Lock.

poll continuously fetches http://tornadoweb.org, and after 5 seconds, shutdown stops the IOLoop. We want any request that poll has begun to complete before the loop stops, so poll acquires the lock before starting each HTTP request and releases it when the request completes. shutdown also acquires the lock before stopping the IOLoop.

(Inspired by a post to the Tornado mailing list.)

import datetime
from tornado import ioloop, gen, httpclient
import toro

lock = toro.Lock()
loop = ioloop.IOLoop.current()


@gen.coroutine
def poll():
    client = httpclient.AsyncHTTPClient()
    while True:
        with (yield lock.acquire()):
            print 'Starting request'
            response = yield client.fetch('http://www.tornadoweb.org/')
            print response.code

        # Wait a tenth of a second before next request
        yield gen.Task(loop.add_timeout, datetime.timedelta(seconds=0.1))


@gen.coroutine
def shutdown():
    # Get the lock: this ensures poll() isn't in a request when we stop the
    # loop
    print 'shutdown() is acquiring the lock'
    yield lock.acquire()
    loop.stop()
    print 'Loop stopped.'


if __name__ == '__main__':
    # Start polling
    poll()

    # Arrange to shutdown cleanly 5 seconds from now
    loop.add_timeout(datetime.timedelta(seconds=5), shutdown)
    loop.start()