Subject: Re: SDL-News: transition atomicity in SDL
Date: Tue Apr 01 1997 - 12:34:30 GMT
The originator of this message is responsible for its content.
-----From Rick Reed TSE <rickreed#tseng.co.uk> to sdlnews -----
>sleue#swen.uwaterloo.ca (Stefan Leue) wrote:
>The notion of atomicity has been well-known...
> ... When using an interleaving semantics, which is
>a suitable and probably the most convenient way to represent
>concurrency in distributed systems, atomic events are those that cannot
>be interleaved by other concurrent events.
But SDL processes are not interleaved (according to Z.100), even (from the
quick check I have just done) in the formal semantic model of SDL (Z.100
Annex F.3). They are meant to be truly concurrent: each process exists
In any case, the formal semantics are only a MODEL of the SDL language - it
is not expected that SDL is necessarily implemented in this way - only that
the model should exhibit the same observable behaviour as the SDL.
I did know what "atomic" usually means, but then it is still open to
argument whether a transition can be divided or not. Once started the
transition will finish, but on the other hand the transition takes some
(probably small) time, so that there is a possibility that between two
outputs an event may occur in another process. Does this count as division
of the transition or not? Is it in fact meaningful to compare that sequence
of events in one process with a sequence in another process, unless and
until there is an event that concerns the both processes (such as the input
of a signal by one previously output by the other)?
It is still unclear to me whether SDL transitions can be classed as
"atomic" or not. It seems that any transition that is independent of an
external value is atomic, whereas transitions that include a non-atomic
procedure, Now or view are non-atomic.
Rick Reed, TSE Limited
13 Weston House, 18-22 Church Street
Lutterworth Leicestershire LE17 4AW United Kingdom
Tel +44 14 55 55 96 55; Fax +44 14 55 55 96 58
-----End text from Rick Reed TSE <rickreed#tseng.co.uk> to sdlnews -----
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