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

[Dale Henrichs via Glass]

Monty,

If there is the chance for concurrent updates by different GsProcesses 
running in the same vm, then the access should be protected by a 
Semaphore ...

Instances of Semaphore cannot be persisted, but you can persist a 
reference to a TransientSemaphore instance (created during class 
initialization). The field values in TransientSemaphore are not 
persisted while the TransientSemaphore itself _is_ persisted ...

Dale


On 10/7/16 12:12 PM, monty via Glass 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>
>> To: 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>
>>> To: 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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