txconnpool 0.1

txconnpool
==========

A generalized connection pooling library for Twisted.


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.

..sourcecode:: python

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.

@ivar factory: A L{MemCacheClientFactory} instance.
"""
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:

.. sourcecode:: python

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)


.. _memcached: http://memcached.org/
.. _ClientCreator: http://twistedmatrix.com/documents/current/api/twisted.internet.protocol.ClientCreator.html
.. _MemCacheProtocol: http://twistedmatrix.com/documents/current/api/twisted.protocols.memcache.MemCacheProtocol.html