[Twisted-Python] Bug in Finger Tutorial?

[Dorian Raymer]

On Thu, Sep 29, 2011 at 2:44 PM, David Kao <a.libran at gmail.com> wrote:

> I'll take care of the first question.
>
> From The Evolution of Finger: making a finger library,
>
> h = internet.TCPServer(79, IFingerFactory(f))
>
> notice IFingerFactory? If you read the earlier parts of the tutorial,
> you'll see stuff like components.registerAdapter. It's just magic to
> create a FingerFactoryFromService, giving its constructor f, the
> "Finger Service". What happens here is that "f", the Finger Service,
> is different from the service.Service within the Twisted Framework.
>

The FingerService inherits service.Service and implements the
startServiceevent, so it *is* a service that can be run by the service
framework; it
just isn't one of the twisted.application.internet services, like
internet.TCPServer.
Moreover, it needs to have startService called, somehow... and what I'm
saying is that in the full example (provided in
 twisted/doc/core/howto/tutorial/listings/finger/finger/finger.py) it is not
called, and if you actually try using it, it doesn't work.


You might as well call "f", the Finger Service, "Finger Shared Data".
>
> What really happens here is
>
> h is the service that has parent s, MultiService. s is then returned
> and started by the application.
>
> h contains a factory that gets run.
>
> The factory knows "f", the Finger Service, which has nothing to do
> with twisted's mechanism. It's more of a demo for the component based
> framework. f is a way to share data and methods and states across
> multiple factories.
>
> I am also very new to twisted, so I **hope** this understanding of
> mine is correct ...
>
> I strongly recommend you read through the entire tutorial. They build
> very incrementally, so most of the time the code is repeated so you
> can just skim through it if you had started from the beginning.
>

I've read it many times over the years ;) It's surprisingly complete as an
example...you just don't know that in the beginning.

The topic area of this question is something I'm trying to squeeze out of
this tutorial series; I'm encountering conceptual questions beyond the
mechanics of how the framework works (which the tutorial does a great job of
explaining) like:
should I ever make a tree of dependent services?
What if I do something in a startService method that I want to make sure
works before other services get their startService called?
If I create a bunch of business services (that just provide pure
functionality) that I then adapt for deployment in different t.a.internet
services, should I put the business services in their own MultiService and
the internet services in their own MutiService container?

Like I said, more showcasing of use cases would help suggest answers to
these kind of questions. I see a lot of possibilities, but haven't put in
the time in the trenches to try them all out ;)

Thanks,
-Dorian



> David Kao
>
>
>
> On Fri, Sep 30, 2011 at 5:28 AM, Dorian Raymer <deldotdr at gmail.com> wrote:
> > Hello,
> > I am confused about how the FingerService gets started in the
> > finger/finger.py makeService function. It is never assigned a service
> parent
> > and it can't service getUser requests if its startService method isn't
> > called; you get an AttributeError: "no attribute 'users'" the way it
> works
> > now. Is this a bug?
> > The most sensible thing, in this case, seems to be setting the
> FingerService
> > parent to be the MultiService container that all of the interent services
> > are added to in the makeService function. Is that correct?
> > I'm wondering if their are any other strategies people use for starting
> > non-internet services.
> > For instance, if instead of reading a file to get to my user db, I might
> > want to start a redis client and read from redis to service a getUser
> call.
> > But that means I'd want to make sure the startService call succeeded (the
> > redis connection was made) before one of the internet services tries to
> use
> > the FingerService (a timing problem that doesn't exist when just reading
> a
> > file)...or something. Maybe I'm really just wondering how the twisted
> > application framework handles exceptions that occur for startService
> calls.
> > Maybe I need something else to handle that kind of thing?
> > I'm also wondering if it's ever a good idea to make a extension
> > of ServerFactory (that adapts a Service) that calls Service.startService
> > when Factory.startServer is called (and Service.stopService when
> > Factory.stopFactory is called)?
> > And I'm looking for more use cases/projects (beyond the finger tutorial)
> > that really showcase the twisted application framework. Any references
> would
> > be appreciated!
> > Thanks!
> > -Dorian
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> >
> >
>
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