[Twisted-Python] xmlrpc deferred

[Jp Calderone]

On Fri, 27 May 2005 22:28:06 +0300, Catalin Constantin <catalin at dazoot.ro> wrote:
>Hi there,
>
>I have the following xmlrpc method:
>
>class FeederResource(xmlrpc.XMLRPC):
>        def __init__(self):
>                xmlrpc.XMLRPC.__init__(self)
>                self.feeder=Feeder()
>
>        def xmlrpc_getList(self, id):
>                return self.feeder.get_urls(id)
>
>The thing is that the self.feeder.get_urls takes too long to execute
>and while the request is running all the others are blocked.
>I want that while it computes the result the other XML RPC methods to
>be available.

The only answer here is to make get_urls() take less time.

What is it doing?  Is it blocking on network I/O?  Querying a database?  Prompting for user input?   _It_ should be creating and returned a Deferred (and later calling it back with a result), most likely, since it is the long-running operation.

>
>I wanted to use deferrals but i found no viable example.
>
>Eg what i've tried to do:
>        def xmlrpc_getList(self, id):
>                log.debug("getList is here for id %s" % id)
>                d = defer.Deferred()
>                d.addCallback(self.feeder.get_urls)
>                return d

Deferred don't make things asynchronous, cooperative, or non-blocking.  They only make dealing with callbacks more convenient.  If you add a blocking function as the callback to a Deferred, it will block the reactor just as effectively as if you called it yourself (bacause all that happens inside the Deferred is that the function gets called).

>
>My method feeder.get_urls is never called !
>

In the above code, nothing ever "fires" the Deferred - calls .callback() on it - so, never having a result, it never bothers to invoke any of its callbacks.  Deferred just hook results up to callbacks.

Jp