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From: bsanden@gmuvax2.gmu.edu (Bo Sanden)
Newsgroups: comp.object,comp.software-eng
Subject: Re: programming design methodologies for object-oriented programming
Summary: Time-dependent entities ala JSD and time independent objects
	 are both necessary
Message-ID: <580@gmuvax2.gmu.edu>
Date: 17 Nov 89 02:14:31 GMT
Article-I.D.: gmuvax2.580
References: <910@gorath.cs.utexas.edu> <135300011@p.cs.uiuc.edu> <617@ajpo.sei.cmu.edu>
Followup-To: comp.software-eng
Organization: George Mason University, Fairfax Va.
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Paul S.R. Chisholm (psrc@pegasus.ATT.COM) started this discussion 
of Jackson System Development (JSD) and OOD. Matt Atterbury
(matt@bacchus.esa.oz) pointed out that JSD entities are time-ordered, 
while actions on objects should have no time constraints. (Bertrand 
Meyer sees the operations on a data structure as a "shopping
list of available actions without any prescribed order"). Ed
Berard (eberard@ajpo.sei.cmu.edu) elaborated on this difference
and noted that concurrency is the norm in JSD.

I have applied a JSD-like approach to real-time problems
with concurrent tasks for a number of years. I refer to it by the
more generic name "entity-life modeling". (Refs 1 and 2.) I also 
use an object-based approach.

Some people see a conflict between the time-independent
objects ala Booch and Meyer and the time-dependent JSD entities,
and feel that we must go for one or the other. I think this is 
unfortunate. Depending on the problem at hand you need either
one, or both. In an interactive application you can get away with 
mostly time indpendent objects of the shopping list variety, if 
a human operator defines the overall sequencing and timing by 
means of commands. 

In a realtime application without operator, the timing must be 
built into the software itself usually by means of tasking. 
As Berard points out, the principles of JSD lead exactly to this,
i.e., the time-dependent entities of JSD map naturally onto tasks. 

On the other hand, even in a real-time application there will be
objects without important "time dimensions", which can be
operated on in arbitrary order. Applying the JSD apparatus to 
such objects isn't productive; they don't have any meaningful
life histories. 

I find it very useful to see concurrent programs as networks of
tasks operating on objects. In the spirit of JSD, the tasks are
based on the behaviors or life histories of some entities. I don't
apply JSD to other entities in the problem environment, which do
not exhibit a strong time dependency. 


Bo Sanden, ISSE
George Mason University
Fairfax, VA 22030-4444
(703) 764-6291

Refs:

1. Bo Sanden, An entity-life modeling approach to the design of
concurrent software, CACM 32:3 (March 1989) 330-343

2. -"-  Entity-life modeling and structured analysis -
comparison of approaches to real-time software design, CACM 32:12
(Dec. 1989)