Question

I want to use twisted (and StarPy which is a protocol implementation for asterisk ami) to connect to an asterisk server. The application initiates a outgoing fax there. I found some hints on my problem, but I cannot find out how to handle this correctly.

The first fax is sent out correctly.

Problem is, if I call twisted for the
second time, the application keeps
hanging in main loop.

I know I may NOT do this like here:

from starpy import manager
from twisted.internet import reactor

def main():
    f = manager.AMIFactory(cUser, cPass)
    print "Login"
    df = f.login(cServer, cPort)

    def onLogin(protocol):
        print "Logoff again"
        df = protocol.logoff()

        def onLogoff( result ):
            print "Logoff erfolgt"
            reactor.stop()

        return df.addCallbacks( onLogoff, onLogoff )

    def onFailure( reason ):
        print "Login failed"
        print reason.getTraceback()

    df.addCallbacks( onLogin, onFailure )
    return df

if __name__ == "__main__":
    reactor.callWhenRunning( main )
    reactor.run(installSignalHandlers=0)
    print "runned the first time"

    reactor.callWhenRunning( main )
    reactor.run(installSignalHandlers=0)
    print "will never reach this point"

I simplified the code – it does nothing than login + logoff again. It will never return from the second reactor.run() call.

How is this done correctly? I’m stuck here – thanks in advance.

Best Regards,
Florian.

Answer 1

Thanks for your answers, I haven’t implemented a solution right now but I know how I could do that now…

Here a short summary of things I learned.

First, in short – the problems I had with twisted:

1. I didn’t understand the asynchronous basics of twisted. I used something like that in gui-frameworks but didn’t see the benefit for a long time.
2. Second, I tried to think of an synchronous call of the event loop multiple times. This was necessary in my mind because I may not use more than one outgoing fax line at a time. Because the event loop of twisted is not restartable this is no option. As I could read in the docs “deferToThread” could help me here, but I think it’s not the best solution.

In my concept I solved these problems:

• Use of defer.DeferredSemaphore() which allows me to limit concurrency to 1.
  I found an example here: http://oubiwann.blogspot.com/2008/06/async-batching-with-twisted-walkthrough.html
• calling twisted anynchronously and let the Defereds do the work (send a fax)
• Writing the fax status to the database and sending the receipt mail is called from the defereds

I needed a lot of re-thinking but as soon as you get it it looks really easy.

Thanks to iny and Jean-Paul Calderone for your help.