Event example - a caching proxy serverΒΆ

An oversimplified caching HTTP proxy - start it, and configure your browser to use localhost:8888 as the proxy server. It doesn’t do cookies or redirects, nor does it obey cache-control headers.

The point is to demonstrate Event. Imagine a client requests a page, and while the proxy is downloading the page from the external site, a second client requests the same page. Since the page is not yet in cache, an inefficient proxy would launch a second external request.

This proxy instead places an Event in the cache, and the second client request waits for the event to be set, thus requiring only a single external request.

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

class CacheEntry(object):
    def __init__(self):
        self.event = toro.Event()
        self.type = self.body = None

cache = {}

class ProxyHandler(web.RequestHandler):
    def get(self):
        path = self.request.path
        entry = cache.get(path)
        if entry:
            # Block until the event is set, unless it's set already
            yield entry.event.wait()
            print path
            cache[path] = entry = CacheEntry()

            # Actually fetch the page
            response = yield httpclient.AsyncHTTPClient().fetch(path)
            entry.type = response.headers.get('Content-Type', 'text/html')
            entry.body = response.body

        self.set_header('Content-Type', entry.type)

if __name__ == '__main__':
    print 'Listening on port 8888'
    print 'Configure your web browser to use localhost:8888 as an HTTP Proxy.'
    print 'Try visiting some web pages and hitting "refresh".'
    web.Application([('.*', ProxyHandler)], debug=True).listen(8888)