Hi Andrew, > > Hi Doug: > > >This state machine initializes a system, issues an asynchronous > command > >that takes time to complete (say a motor move command) and then waits > >for that command to be done before exiting. In the context of a > >framework that is calling this state machine, the WAITCMD1 is executed > >repeatedly (polling) while the asynchronous command completes. A > system > >can be constructed with lots of little state machines like this and be > >made to look like it is doing many things at once even though the > >execution of the program is single threaded. > > >I understand (pretty much) the Twisted framework is like this and > >implmenting event handlers like connectionMade(), etc., is a state > >machine, but I'm wondering how to implement what I've outlined above > in > >one of Twisted's state event handlers, or inside something like > >callLater(). For example, let's say I want to use Twisted to run a > long > >running daemon process that has an XMLRPC interface. That XMLRPC > >interface is an interface to a state machine inside Twisted that > allows > >the caller to change state, or start new state machines, or get the > >status of a running state machine. In that case I'm thinking I want a > >state machine the runs continuously in the Twisted loop, structured > like > >the example above; co-operatively yielding back to Twisted, but > running > >non-stop. Something like callLater(1, stateMachine), but non-stop, > >without even the 1 second call loop. > > If I understand your correctly, I don't think you need to implement a > state machine to simulate concurrency with Twisted - Twisted does a lot > of that for you. You can think of a Twisted application as a state > machine - the callback being the state and the completion of the > operation and the calling of the callback is the transition. These > callbacks at runtime act like a thread of execution. > > def Initialize(...): > # do something > deferred = someFunctionThatReturnsADeferred() > deferred.addCallback(State2) > > def State2(...): > # do something > deferred = someFunctionThatReturnsADeferred() > deferred.addCallback(State3) > > def State3(someData): > # do something > if someData == 'State4': > deferred = someFunctionThatReturnsADeferred() > func = State4 > elif someData == 'State5': > deferred = someOtherFunctionThatReturnsADeferred() > func = State5 > ... > deferred.addCallback(func) > > if __name__ = "__main__": > initialize() > reactor.run() > > As for the Twisted loop. Well, you don't really see the Twisted loop > since that is hidden in the reactor. Also you should distinguish > between writing a new protocol and using an existing one. > > In the case of XMLRPC, creating the server isn't the problem. > > http://twistedmatrix.com/projects/web/documentation/howto/xmlrpc.html > > Once a XMLRPC server is created, Twisted will take responsibility for > creating new instances (or threads if you want to see it that way). If > you still need a state machine, then the only hiccup I can see is > sharing state machine (if you really need one) between XMLRPC method > invocations. > > Cheers, > Andrew > [Doug Farrell] Thank you very much for your detailed response, I'm kinda getting it and am going to try out a prototype to make sure I do. As you mention, Twisted does take care of a lot of the issues dealing with concurrency. Having written a few threaded applications, I didn't really want to get into starting my state machine in a thread and then having to deal with all the cross thread data safety issues.
One thing I'm still a little confused by in your reply is how you're getting the deferred instance. Is there a particular reason your example does this: deferred = someOtherFunctionThatReturnsADeferred() rather than this: deferred = defer.deferred() I just want to understand if there a reason the deferred is being created in the other function. Again, thanks for your help and response! Doug _______________________________________________ Twisted-Python mailing list Twisted-Python@twistedmatrix.com http://twistedmatrix.com/cgi-bin/mailman/listinfo/twisted-python
