[Glass] Class var and class inst var caches in libraries

[monty via Glass]

> Sent: Friday, October 07, 2016 at 6:13 PM
> From: "Paul Baumann" <plbaumann at gmail.com>
> To: monty <monty2 at programmer.net>
> Cc: glass at lists.gemtalksystems.com
> Subject: Re: [Glass] Class var and class inst var caches in libraries
> With a full gs server (rather than gem-limited glass) it is uncommon that someone attempts forks within a gem rather than doing work in a separate gem. I had developed a framework for easily forking work in separate synchronous or asynchronous gems. That approach was safer and faster (due to custom gem configurations). You might consider licensing more glass gems rather than forging a less traveled path risking transaction view surprises.

The question was in the context of libraries with GS ports and how to eliminate write conflicts and achieve #fork safely with class/class inst behavior. The concern is for the users of my libraries.

> What of your code is unavoidably sensitive to a momentary size inconsistency with session temps? Perhaps more than one might start a long lazy-initialization process? That could be handled with another session temp to hold a semaphore or recording an initialization state flag, but that also needs pre-initialization.

The #size issue was the first thing that came to mind, there could be others issues. And a Dictionary confused about its size could in theory cause issues by itself, like during resizing/rehashing, but without studying the implementation this is just speculation.

> Pre-initialization of variables avoids the structure changes risks discussed by lazy-initialization. Similarly, it is a good idea to avoid lazy-initialization in persistent shared variables due to commit conflicts between competing gems. Forks within a gem are not common though, so don't be too surprised if you start encountering problems that few had seen even if using semaphores properly.
> Paul Baumann
>
> On Oct 7, 2016 3:13 PM, "monty via Glass" <glass at lists.gemtalksystems.com[mailto:glass at lists.gemtalksystems.com]> wrote:My mail client wrongly sent this response to Dale, not to the list, so I'm forwarding it
>
> > Sent: Friday, October 07, 2016 at 3:05 PM
> > From: monty <monty2 at programmer.net[mailto:monty2 at programmer.net]>
> > To: dale.henrichs at gemtalksystems.com[mailto:dale.henrichs at gemtalksystems.com]
> > Subject: Re: [Glass] Class var and class inst var caches in libraries
> >
> > Thanks, I think this will do, but I have one more question. If a class method lazily initializes a SessionTemp to the same value (using at:ifAbsentPut: for example), but that method is executed at roughly the same time in different processes (due to fork), is there a possibility that "SessionTemps current" itself could get corrupted? For example, that it could report an inaccurate #size, or that its internal state in some other way could get corrupted due to the concurrent update?
> >
> > > Sent: Tuesday, October 04, 2016 at 3:21 PM
> > > From: "Dale Henrichs via Glass" <glass at lists.gemtalksystems.com[mailto:glass at lists.gemtalksystems.com]>
> > > To: glass at lists.gemtalksystems.com[mailto:glass at lists.gemtalksystems.com]
> > > Subject: Re: [Glass] Class var and class inst var caches in libraries
> > >
> > > Yes, lazy initialization for shared variables in a multi-gem system is
> > > not a good idea.
> > >
> > > Since it sounds like you do not need the values persisted and shared
> > > between gems, you should be using the class SessionTemps. Here's an
> > > example from Collection class>>randomForPicking:
> > >
> > > randomForPicking
> > > | random |
> > > random := SessionTemps current
> > > at: #'COLLECTION_RANDOM_FOR_PICKING'
> > > otherwise: nil.
> > > random == nil
> > > ifTrue: [ random := Random new.
> > > SessionTemps current at: #'COLLECTION_RANDOM_FOR_PICKING' put:
> > > random ].
> > > ^ random
> > >
> > > SessionTemps is a subclass of SymbolDictionary and can be used for
> > > storing session-specific globals.
> > >
> > > If you have the need to sharing persistent values then an explicit
> > > initialization in the class initialization method is the right answer.
> > > The initialize method gets run during install and at least the
> > > initialization is safe from commit conflicts.
> > >
> > > Dale
> > >
> > >
> > > On 10/04/2016 11:56 AM, monty via Glass wrote:
> > > > Lots of libraries use caches stored in class vars and class isnt vars that are either explicitly or lazily initialized. But with multiple Gems accessing the same repo, this can produce write-write conflicts. In fact, simply sending #initialize to the same class from different Gems can cause them. What are the best ways to implement these caches in GS? Write-locks seem like overkill, so I'm interested in simple ways of making them non-persistent.
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