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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Ada package registry?
Date: Wed, 10 Feb 2016 19:37:52 -0600
Organization: JSA Research & Innovation
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Date: 2016-02-10T19:37:52-06:00
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"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message 
news:n9c9a8$mt9$1@gioia.aioe.org...
> On 09/02/2016 00:02, Randy Brukardt wrote:
>> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message
>> news:n94cbv$19ed$1@gioia.aioe.org...
>
>>> For persistence layer you need dereference primitive operations. One for
>>> read access in order to cache data and one write access to mark the 
>>> cache
>>> dirty.
>>
>> The first is the function call (to Reference, if you're looking at the 
>> Ada
>> containers), and the second is the Finalize call that happens when the
>> object returned from Reference goes away. That was the whole point of the
>> design. (Of course, you could have written that in Ada 95, but without 
>> the
>> syntactic sugar that makes it easier to read and write.)
>
> It is not same. A reference points to a proxy object, a cached copy of the 
> external object in the persistent storage. When the proxy object is 
> updated via any reference, it must be marked dirty. So that when the 
> object at some point gets finalized or the transaction is committed it 
> would be written back to the storage.

But actually they are the same, because the reference can only exist for a 
very short time. (You can't assign a reference because the accessibility is 
too shallow.) Thus, the proxy object can be marked dirty (if you created a 
writable reference, which is only created when you actually use the 
reference in a writable context) and written back to the persistent storage. 
You do have to make sure that only one task is potentially writing a proxy 
object at a time, but that seems like a good thing. (Obviously, sharing 
proxy objects between read-only references is fine.)

                                      Randy.