[Zope] Zope Persistence (was: XML-RPC within ZOPE)

[Jan-Ole Esleben]

> I don't understand the problem.  How is using XML-RPC incompatible
> with persistence?  What are you trying to exclude?

I'm sorry, but I don't understand _that_ question. What am I trying to
_exclude_?

XML-RPC and (the concept of) persistence aren't incompatible. XML-RPC
(on a ZOPE server) and ZOPE persistence are, to the extent described
by others in this thread, not by me. I just described a couple more
cases (and specifically one case) where it is bad that things are this
way (and cannot be solved by better design).

My point is: this doesn't happen _within_ single "programs", but
everywhere there is even the slightest bit of communication there is a
chance of it happening.

Ole



>
> - C
>
>
> On Dec 16, 2005, at 6:40 AM, Jan-Ole Esleben wrote:
>
> > Thanks; this is a problem we are well aware of. Our solution is to
> > increase the amount of workers, obviously.
> >
> > However, I'm increasingly getting a feeling that for a rather big
> > range of unlikely situations that are nonetheless to be expected, Zope
> > doesn't work _at all_. In a WebServices setting, situations like the
> > one I described, with one server calling back to another server within
> > a call from that latter server and both not knowing anything about the
> > implementation of the other, it would most certainly be extremely hard
> > to foresee the exact setup of such situations and impossible to
> > exclude them for persistent objects that actually _do_ change state
> > (unlike mine). The solution is to not have state in your objects, and
> > thus lose instantly most of what Zope is.
> >
> > However, as I see it, the problem is that what Zope actually _is_
> > (i.e. mostly the ZODB) is an unhealthy way of coupling data and
> > implementation (which is _exactly_ why my implementation didn't work
> > immediately). This of course comes from its origins in TTW development
> > where there wouldn't actually have been many user made products.
> >
> > Please tell me if I'm wrong with my assumption above, and why. I'm not
> > trying to be inflamatory, this just has me really worried.
> >
> > Ole
> >
> >
> > 2005/12/15, Dieter Maurer <dieter at handshake.de>:
> >> Jan-Ole Esleben wrote at 2005-12-11 19:10 +0100:
> >>> Is it at all impossible to use XML-RPC _within_ a ZOPE architecture?
> >>
> >> In principle yes.
> >>
> >> Be careful however: it is easy to introduce deadlocks!
> >>
> >>   When during request processing you call back into the same
> >>   Zope via XML-RPC, the calling out request will not complete
> >>   until the XML-RPC returns a result (this always holds for
> >>   calls, XML-RPC or other, to an external or internal server).
> >>
> >>   Zope has a limited amount of workers (the default is 4) able
> >>   to execute requests. If there are no free workers, requests
> >>   have to wait for one.
> >>
> >>   Now imagine that as many requests arrive as there are workers
> >>   and each of them wants to perform an XML-RPC against the
> >>   same Zope. Then you have a deadlock: none of the XML-RPC requests
> >>   will finish, because there are no free workers. An no worker
> >>   will ever become free again, because each of them waits for
> >>   its XML-RPC to finish.
> >>
> >> Therefore, you should directly call internal resources (rather
> >> than use XML-RPC).
> >>
> >>
> >> --
> >> Dieter
> >>
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