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Re: [ontolog-forum] Asynchronous processes -- was NASA-Ontolog OKDMS

To: edbark@xxxxxxxx, "[ontolog-forum]" <ontolog-forum@xxxxxxxxxxxxxxxx>
From: "John F. Sowa" <sowa@xxxxxxxxxxx>
Date: Fri, 02 Nov 2007 11:10:11 -0500
Message-id: <472B4BE3.8070901@xxxxxxxxxxx>
Ed,    (01)

I completely agree:    (02)

 > In the more general case, two or more processes are "asynchronous"
 > if they are simultaneously "enabled" (allowed to start) but their
 > actual start times may depend on other events.  (And "enabled" is
 > a state, not an action, although it may result from an action; some
 > (autonomous) processes are always enabled.)  They may run concurrently
 > and along the way they may produce "broadcast signals" or "directed
 > signals" that enable other processes in the set to determine that
 > some state of the overall world has been achieved.  Starting and
 > coming to an end are just some of those states.  And such processes
 > may then also "wait" at some point in their own activity sequence
 > for the receipt of such a signal from a sibling process.  Some
 > engineers speak of these "waits" as "synchronization points"; others
 > would argue that they are only really "synchronizations" if there
 > is an exchange of signals that confirms that all the relevant
 > processes have reached the same point before any of them continues.    (03)

Unfortunately, it is very hard to explain all those abstract points.
The reason why I like Petri nets is that you can illustrate each
of those points with very simple, clearly understandable diagrams.    (04)

Actually, I use moving diagrams.  In the days of overhead projectors,
I used plastic sheets with unmarked circles for the states.  Then I
brought in my collection of tokens (which I kept in a Petri dish)
and moved them along nodes of the of the net.  (I bought my tokens
in Germany -- for one pfennig each.  Too bad that they no longer
have pfennige.)    (05)

I still find the plastic sheets and movable tokens more effective
than any computer aids.    (06)

John    (07)

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