SDL-News: Complete systems

Subject: SDL-News: Complete systems
From: William H. Skelton (
Date: Wed Dec 17 2003 - 21:28:19 GMT

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Dear Andreas (and SDL News),

Thanks for your feedback regarding gates on the environment of a system;
comments below...

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>Date: Wed, 17 Dec 2003 16:25:29 +0100
>From: Andreas Prinz <>
>Subject: Re: [SDLTF-Members] Communication Process
>Dear William,
>>Logical conclusion: If a system is to be complete, it cannot have any
>>gates on its environment.
>In fact, this was just the question. The systems that we describe are by
>nature incomplete, in that they communicate with other systems.
>If this is the case, then it does not make sense to restrict ourselves to
>complete systems.

So do they have to communicate through an undefined mechanism outside of
the system?

This is a case of either you have an undefined mechanism outside a system
or you have an undefined mechanism somewhere in a process, for example to
read to/write from an IP socket.

For the task force definition the beauty of the complete system is it
allows us to make a difference between a system, which is complete,
self-contained and executable, and a block, which is a sub-system to be
used somewhere else, and not executable by itself.

This definition has so much beauty I think we should try to find reasons
for it, rather than against it, especially as when we look at the reasons
against, it turns out they can be handled in a clean way inside a process

>>Logical consequence: If a system needs to communicate with another system
>>it can use a block/process designed for that purpose, like an IP stack.
>Of course, you can always hide interfaces using processes with appropriate
>underlying functionality. However: why should we hide interfaces? I think
>it is most important to clearly show interfaces!

I fully agree and in fact they are not hidden at all!

We have exactly the same gate/interface specifications to a process as you
can on the environment of a system.

>Best regards,
> Andreas
>Prof. Andreas Prinz
>Agder University College
>Open Systems Development Group
>Tel: +47 3725 3220

William H. Skelton, Engineering Dept.
SOLINET GmbH Solutions for Innovative Networks
Mittlerer Pfad 26, 70499 Stuttgart, Germany
Tel +49 711 1398 1377, Fax +49 711 866 1240
Mobile +49 171 247 6688,

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