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From: darrell@sdcsvax.UUCP
Newsgroups: mod.os
Subject: Re: definitions of distributed
Message-ID: <2700@sdcsvax.UCSD.EDU>
Date: Wed, 11-Feb-87 20:32:13 EST
Article-I.D.: sdcsvax.2700
Posted: Wed Feb 11 20:32:13 1987
Date-Received: Fri, 13-Feb-87 00:39:38 EST
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Organization: NASA Ames Research Center, Mountain View, CA
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In article <2699@sdcsvax.UCSD.EDU> cogito@cs.vu.nl (Robbert van Renesse)
writes:
>When is	an application ``distributed?''
(justification omited)
>=======================================
>
>    Definition.  A distributed application is an application
>		 carried out by two or more processors.
>
>This leaves the	terms ``application'' and ``processor''	 to  be
>defined.

The problem with this definition is that it allows nearly every application
to be defined as distributed.  The pc I'm using to write this on, for example
has a keyboard processor, a graphics processor, a floating point co processor
and a cpu, so that I can argue that any application it runs is distributed.

>What is	a distributed system?
>=============================
(justification omited)
>
>    Definition.  A distributed system consists of two or more
>		 processors which have the ability to commun-
>		 icate with one another.

Since the advent of kermit, almost any two processors have the ability to
communicate with each other, provided a long enough cable. (;-)

These definitions, although acurate are overly broad.  Unfortunately, I don't
have an alternative definition which is any better ("If it walks like a
duck and it quacks like a duck, it must be a duck.")

Or maybe, the definitions are right and all applications are distributed; so
we are trying to discuss the wrong things.

An alternative way to view a distributed system is in terms of packaging;
I claim that my PC isn't a distributed system because all of the
processors were intentionally packaged into a system which won't work
without them.  Multiprocessor systems aren't distributed by this definition
because they were packaged as such; but multiple workstations running a
file server are a distributed system.

The problem with this definition comes from two grey areas.  First, if a
vendor were to offer a turnkey distributed system would it become no longer
a distributed system?  Second, some systems are designed to be packed in
many ways.  A single VAX 11/780 isn't really a distributed system, even
though it has at least two processors (the VAX CPU and the LSI-11,) and a
VAX 11/785, which is really two 11/780s and a shared memory is sold as a
multiprocessor; while a vax cluster is viewed as either a multiprocessor or
as a distributed system, depending on the way the software is configured.

Oh well, needs more thought.

Marty