Python-twisted client connection failover

Question

I am writing a tcp proxy with Twisted framework and need a simple client failover. If proxy can not connect to one backend, then connect to next one in the list. I used reactor.connectTCP(host, port, factory) for proxy till I came to this task, but it does not spit out error if it can not connect. How can I catch, that it can not connect and try other host or should I use some other connection method?

Answer 1

You can use a deferred to do that

class MyClientFactory(ClientFactory):

    protocol = ClientProtocol

    def __init__(self, request):
        self.request = request
        self.deferred = defer.Deferred()

    def handleReply(self, command, reply):
        # Handle the reply
        self.deferred.callback(0)

    def clientConnectionFailed(self, connector, reason):
        self.deferred.errback(reason)

def send(_, host, port, msg):
    factory = MyClientFactory(msg)
    reactor.connectTCP(host, port, factory)
    return factory.deferred

d = Deferred()
d.addErrback(send, host1, port1, msg1)
d.addErrback(send, host2, port2, msg2)
# ...
d.addBoth(lambda _: print "finished")

This will trigger the next errback if the first one fails, otherwise goto the print function.