Re: [xml-dev] Backward and forward compatible schemas ... RelaxNG --> Y

[David Carver <d_a_carver@yahoo.com>]

Costello, Roger L. wrote:
> Hi Folks,
>  
> I am trying to characterize the types of changes to schemas which
> enable backward and forward compatibility.
>   
I even RelaxNG I tend to only think of these things as being in Forwards 
compatibility.  Backwards compatibility can be maintained if items are 
optional.   Meaning from my stand point, regardless of what schema 
language you use XSD, RNG, SchemaTron, or whatever, it's only backwards 
compatibile if you generate an instance that the older version of the 
schema understands.

Forwards compatibility is of a more important issue to me when evolving 
a data model of any type.  Meaning that the instances created in an 
older version should still be valid in a newer data model.    Java is a 
pretty good example in this case, APIs mostly used in JAVA written for 
1.1, 1.2, 1.3, 1.4...can still run on a JAVA 5 run time without 
change.   However, JAVA 5 that uses JAVA 5 specific constructs doesn't 
necessarily run on a prior version of the runtime.

Maintaining forward compatibility is key is an Industry Standard schema, 
as it allows trading partners to exchange information without the older 
implementation necessarily having to upgrade.  They only need to upgrade 
when newer fields need to be there.

The key here is keeping an eye on what you make required and as was 
stated before what is changed on the occurrence front.  As for the other 
comments regarding industry standard schemas, part of the problem with 
industry standard schemas is that they take too long to reach the public 
again, by that time most business requirements have changed.   The 
industry standards need to move at a faster pace and take a page from 
some of the more agile approaches to development to start releasing 
milestone and release candidates instead of waiting every 3 years to get 
a standard out.   If the industry standard organization took that 
approach, it might encourage users to contribute their changes back to 
the organizations that maintain the standards.