Question

I have an existing program that has its own main loop, and does computations based on input it receives – let’s say from the user, to make it simple. I want to now do the computations remotely instead of locally, and I decided to implement the RPCs in Twisted.

Ideally I just want to change one of my functions, say doComputation(), to make a call to twisted to perform the RPC, get the results, and return. The rest of the program should stay the same. How can I accomplish this, though? Twisted hijacks the main loop when I call reactor.run(). I also read that you don’t really have threads in twisted, that all the tasks run in sequence, so it seems I can’t just create a LoopingCall and run my main loop that way.

Answer 1

You have a couple of different options, depending on what sort of main loop your existing program has.

If it’s a mainloop from a GUI library, Twisted may already have support for it. In that case, you can just go ahead and use it.

You could also write your own reactor. There isn’t a lot of great documentation for this, but you can look at the way that qtreactor implements a reactor plugin externally to Twisted.

You can also write a minimal reactor using threadedselectreactor. The documentation for this is also sparse, but the wxpython reactor is implemented using it. Personally I wouldn’t recommend this approach as it is difficult to test and may result in confusing race conditions, but it does have the advantage of letting you leverage almost all of Twisted’s default networking code with only a thin layer of wrapping.

If you are really sure that you don’t want your doComputation to be asynchronous, and you want your program to block while waiting for Twisted to answer, do the following:

• start Twisted in another thread before your main loop starts up, with something like twistedThread = Thread(target=reactor.run); twistedThread.start()
• instantiate an object to do your RPC communication (let’s say, RPCDoer) in your own main loop’s thread, so that you have a reference to it. Make sure to actually kick off its Twisted logic with reactor.callFromThread so you don’t need to wrap all of its Twisted API calls.
• Implement RPCDoer.doRPC to return a Deferred, using only Twisted API calls (i.e. don’t call into your existing application code, so you don’t need to worry about thread safety for your application objects; pass doRPC all the information that it needs as arguments).

• You can now implement doComputation like this:

  def doComputation(self):
      rpcResult = blockingCallFromThread(reactor, self.myRPCDoer.doRPC)
      return self.computeSomethingFrom(rpcResult)
  

• Remember to call reactor.callFromThread(reactor.stop); twistedThread.join() from your main-loop’s shutdown procedure, otherwise you may see some confusing tracebacks or log messages on exit.

Finally, one option that you should really consider, especially in the long term: dump your existing main loop, and figure out a way to just use Twisted’s. In my experience this is the right answer for 9 out of 10 askers of questions like this. I’m not saying that this is always the way to go – there are plenty of cases where you really need to keep your own main loop, or where it’s just way too much effort to get rid of the existing loop. But, maintaining your own loop is work too. Keep in mind that the Twisted loop has been extensively tested by millions of users and used in a huge variety of environments. If your loop is also extremely mature, that may not be a big deal, but if you’re writing a small, new program, the difference in reliability may be significant.