A Twisted Design Decision

koranthala koranthala at gmail.com
Wed Jan 28 17:05:13 CET 2009


On Jan 28, 8:36 pm, Jean-Paul Calderone <exar... at divmod.com> wrote:
> On Wed, 28 Jan 2009 06:30:32 -0800 (PST), koranthala <koranth... at gmail.com> wrote:
> >On Jan 28, 7:10 pm, Jean-Paul Calderone <exar... at divmod.com> wrote:
> >> On Wed, 28 Jan 2009 02:02:57 -0800 (PST), koranthala <koranth... at gmail.com> wrote:
> >> >On Jan 27, 9:27 pm, koranthala <koranth... at gmail.com> wrote:
> >> >> On Jan 27, 6:57 pm, Jean-Paul Calderone <exar... at divmod.com> wrote:
> >> > [snip]
>
> >> >> Thank you Jean-Paul.
> >> >> My code is more complex than what I have mentioned. When I mentioned
> >> >> msg.send, the msg object actually gets the data from DB etc to send.
> >> >> And there are many other items being done.
> >> >> I will try to see whether I can change the code to incorporate what
> >> >> you mentioned.
>
> >> >> I rewrote most of my code after learning just raw deferreds - I had
> >> >> planned to study inlineCallbacks - but then it slipped my mind  - now
> >> >> it has come to bit me. :-(
>
> >> >Hi,
> >> >  I tried to update the code as per the suggestion, but to no avail.
> >> >  My system uses Python2.4.3 (cannot move to 2.5) - so I tried to
> >> >rewrite with deferredGenerators - since I thought inlineCallbacks are
> >> >similar to deferredGenerators.
>
> >> >  But I cannot seem to rewrite it in a format where the functional
> >> >encapsulation is not broken.
> >> >  i.e. as I mentioned in the first example - I have to pass SELF to
> >> >child objects for them to modify it.
>
> >> Why?  You don't do this in the original version of your code.  Why do
> >> it after switching to Twisted (particularly since you seem to want *not*
> >> to)?
>
> >> Jean-Paul
>
> >Without that, I am unable to increment success and failure counters
> >which are part of the message handler object.
> >In the original version, if send fails the return value of
> >protocol.send is propogated back to msg.send and to msg handler.send.
> >But in twisted, it is not so. So, I have to pass in SELF through to
> >increment success and failure counter.
> >Is it possible otherwise in twisted?
>
> Why isn't the return value of protocol.send propagated back to msg.send?
> It sounds like it should be.
>
> Jean-Paul

Thank you very much again Jean-Paul for helping me out.
I am unable to understand how I will be able to propogate the return
value of protocol.send to msg.send.
Maybe I am being foolish - but my understanding is as follows.

In a non-reactor pattern scenario:
msg_handler.send_message calls msg.send which inturn calls
protocol.send.
So, the reply to protocol.send actually goes up the stack till
msg_handler.send_message wherein I can increment/decrement success/
failure counter.

In reactor pattern:
msg_handler.send_message calls msg.send which call protocol.send which
causes a deferred to be created.
Now, when the deferred finishes its work, reactor calls the callback
associated - but the original context (stack etc) is lost.
Now, the only mechanism of interaction is via the parameters passed in
the callback.
This means that msg_handler has to pass in its object to msg.send
which inturn has to send either msg_handler or self to protocol.send
so that it is stored in the parameter to the callback.
When callback is hit, I use this parameter to call methods in each
object.

This is what I was trying to say in my first mail that - Twisted,
being twisted in its behavior is causing quite a lot of
confusion in design decisions - because now I have to break functional
encapsulation - by asking lower layer objects to handler upper layer
objects behaviors.

As I said earlier, maybe I am being completely stupid -  there might
be a very easy and obvious solution. But I cannot seem to get it at
all.




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