Re: Distributed Systems Annex, data sharing between programs

[Maciej Sobczak <see.my.homepage@gmail.com>]

On 4 Cze, 20:49, Adam Beneschan <a...@irvine.com> wrote:

> I haven't had much need to look at the Distributed Systems Annex until recently.
[...]

These are very interesting observations.
I would like to add my two cents to them, as they are related in
particular to this:

> Also, the note in E(7) says that the resulting execution is semantically equivalent if the program is partitioned differently;

DSA is intended to provide transparent "distributability", where
transparent means that the physicality of distribution is as hidden as
possible (ideally it should be undetectable). The problem that I see
is that Ada distinguishes potentially blocking operations from those
that never block and allows to use only the latter in protected bodies
- now, if we repartition the program in such a way that a given
operation (Do_Something_For_Me) ends up in a separate partition, it
might become potentially blocking by virtue of the hidden physicality
of remote call. In short, it might become a I/O operation even though
it is not visible anywhere. If such operation is used within a
protected body, then the program can be legal or illegal depending on
the partitioning scheme, which is not expressed in code.

That's just against what I understand as Ada spirit.

Things like this lead me to conclusion that DSA is essentially broken
and is not a proper way to handle distribution in the system.

> I'm confused by examples such as http://www.adacore.com/...

A worse observation is that AdaCore might be confused, too. Which only
confirms that DSA is broken.

> If my understanding is correct, my next question is: is there any language-defined support for sharing data between programs

Do you expect it to be language-defined? Why? Ada is not the only
language (not even the only ISO-stamped one) and heterogeneity is a
natural property of systems composed of multiple programs. Defining
data sharing between programs written in the same language would be a
wasted effort (better spent elsewhere in the standard). Instead, allow
others (third-parties) to define it so that programs written in
different languages will be able to share data - this will have much
bigger benefits.

--
Maciej Sobczak * http://www.msobczak.com * http://www.inspirel.com