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   loosely and tightly coupled systems and type annotation

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If you don't want to read yet another rant against *imposing* type
annotation in XML please delete...

Thinking again and again on what's the difference between loosely and
tightly coupled systems and how this apply to XML...

In real life there is a clear distinction between loosely and tightly
coupled systems: when I press on a power switch to put the light on it's
a tightly coupled system but when I do a phone call to order a book it's
a loosely coupled system.

In a tightly coupled system, I have a direct control over the operation
and if I had access to the button controling the printer which will
print the book I order, this would be a tightly coupled system too.

Tightly coupled systems are usually fast and safe in that the risk of
transmission errors is very low.

Loosely coupled systems are usually more error prone (the ISBN number of
the book I want to order may be misunderstood by the clerk who will
enter it) but usually much more flexible and subject to added value: if
I don't remember the ISBN number I can still say I want the latest book
on W3C XML Schema by a guy with a Dutch name and a publisher which puts
animals on all its books and get a chance to be understood.

In computer systems, the situation is not so clear: both tightly and
loosely coupled do exchange sequences of 0 and 1s and the difference is
more subtle.

For instance, everyone agrees that XML enables loosely couple systems
and that this is a Good Thing, but that doesn't mean that using XML is
enough to build a loosely coupled system and using XML to reinvent Corba
IMO doesn't make a system loosely coupled.

The distinction is thus more subjective than this and lies in the
ability to enable and facilitate different interpretations of the
documents which are exchanged.

Any feature which decreases the number of possible interpretations (such
as type annotation) tighten the coupling between systems and if it is
legitim and needed for some applications, this shouldn't be imposed to
all the applications...

See you in San Diego.
Eric van der Vlist       http://xmlfr.org            http://dyomedea.com
(W3C) XML Schema ISBN:0-596-00252-1 http://oreilly.com/catalog/xmlschema


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