txconnpool 0.1.1

Example Description

Assume that we’ve got a web application, which performs some expensive computations, and then caches them in a memcached server. The simple way to achieve this in Twisted is to create a ClientCreator for the MemCacheProtocol and whenever we need to communicate with the server, we can simply use that.

This works for low volumes of queries, but let’s say that now we start hitting memcached a lot–several times per web request, of which we are receiving many per second. Very quickly, the connection overhead can become a problem.

Instead of creating a new connection for every query, it would be much better to maintain a pool of open connections, and simply reuse those open connections; queuing up any queries if all of the connections are in use. With txconnpool, setting this up can be quite easy.

Example Implementation

First we need to create a few classes of boilerplate, to transform a MemCacheProtocol into a PooledMemcachedProtocol, and then create a pool:

from twisted.protocols.memcache import MemCacheProtocol

from txconnpool.pool import PooledClientFactory, Pool

class PooledMemCacheProtocol(MemCacheProtocol):
    """
    A MemCacheProtocol that will notify a connectionPool that it is ready
    to accept requests.
    """
    factory = None

    def connectionMade(self):
        """
        Notify our factory that we're ready to accept connections.
        """
        MemCacheProtocol.connectionMade(self)

        self.factory.connectionPool.clientFree(self)

        if self.factory.deferred is not None:
            self.factory.deferred.callback(self)
            self.factory.deferred = None

class MemCacheClientFactory(PooledClientFactory):
    protocol = PooledMemCacheProtocol

class MemCachePool(Pool):
    clientFactory = MemCacheClientFactory

    def get(self, *args, **kwargs):
        return self.performRequest('get', *args, **kwargs)

    def set(self, *args, **kwargs):
        return self.performRequest('set', *args, **kwargs)

    def delete(self, *args, **kwargs):
        return self.performRequest('delete', *args, **kwargs)

    def add(self, *args, **kwargs):
        return self.performRequest('add', *args, **kwargs)

Now, with this having been created, we can go ahead and use it:

from twisted.internet.address import IPv4Address

addr = IPv4Address('TCP', '127.0.0.1', 11211)
mc_pool = MemCachePool(addr, maxClients=20)

d = mc_pool.get('cached-data')

def gotCachedData(data):
    flags, value = data
    if value:
        print 'Yay, we got a cache hit'
    else:
        print 'Boo, it was a cache miss'

d.addCallback(gotCachedData)