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Re: change the question slightly maybe...schemas, leveraging their object orientedness??

From: Dean Hiller <dhiller@avaya.com>
Date: Tue, 21 Oct 2003 12:24:25 -0600
Message-ID: <3F9579D9.20006@avaya.com>
To: noah_mendelsohn@us.ibm.com
Cc: Dare Obasanjo <dareo@microsoft.com>, xmlschema-dev@w3.org
I see your point.  I think what you state is very interesting.  thanks 
very much for the insight.  I hope we can start something out of this.

noah_mendelsohn@us.ibm.com wrote:

>Dean Hiller writes:
>>>If XSD was object-oriented, it would only care about the superclasses 
>unless Company C had companyB's xsd so they could validate the new feature 
>>>I think XSD would be so much easier to go from version to version of all 
>the different protocols out there if XSD was more object oriented. 
>I am the first to agree that versioning of vocabularies is crucial to the 
>success of XML, and I have for three years been outspoken in my 
>disappointment that the W3C community hasn't done a better job of tackling 
>that issue.  I think that (a) doing this right is a very, very hard 
>problem (b) there is quite a range of important use cases ranging from 
>handling of small bug fixes, such as allowing a new attribute or new 
>attribute value, to rather major changes to parts of a vocabulary and (c) 
>there is some reason to suspect that in an ideal world, the solutions 
>would involve not just schema, but perhaps changes to the way namespaces 
>work, and perhaps new models of processing XML.
>That said, I wouldn't leap too quickly to the assumption that "if only 
>schema had exploited the well known principles of object orientation, we'd 
>all be fine by now."  First of all, months of study were given to the use 
>of object orientation in schemas, and when you look carefully, there 
>really are a number of reasons that people use object orientation, and 
>interop across versions is just one.   For example, object orientation is 
>commonly used to allow for reuse and evolution at the source level.  I 
>believe that XML schema succeeds at this to a degree, both in the type 
>system and with substitution groups.
>The more fundamental point I wanted to make is that I think your statement 
>misses a key point about object orientation.  Indeed, it probably took a 
>year of work on XML and schema for me to learn or at least notice this: in 
>fundamental ways, XML (not just schema) and object orientation are at 
>odds.  The fundamental premise of object orientation in programming is 
>that state is hidden behind behavior.  You don't expose data, you expose 
>methods, and typically you expose those methods  by name.   In certain 
>respects, XML is a reaction to object orientation:  when people tried to 
>loosely couple the method-oriented object structure of their systems using 
>COM and Corba, the resulting contracts were too detailed for some 
>purposes.  It turns out that when I want to order a book from Amazon, I 
>don't want to know about the object structure (if any) if their 
>implementation, I just want to send them a document representing a book 
>order.  Indeed, many different operations can be performed on that 
>document including signing it, filing it, updating it with costs and 
>expected shipping dates etc, but unlike an object, a document carries no 
>Why does this relate to the versioning issue?  Well, look at how object 
>oriented systems achieve the behavior to which you refer.  They impose a 
>level of indirection, often referred to as virtual method calling, between 
>those looking for data and those providing it.  Thus, subclasses can come 
>and go, as long as the contract at the method level is preserved. If you 
>have interfaces, then sometimes only the pertinent subset of the contract 
>needs to be preserved as the system evolves (I.e. you need to support the 
>interface of interest.)
>XML is completely different.  I can write all the schemas I want to check 
>your XML document, but when it's handed to your application the data is 
>right there for you to see.  It's not just the schema language that needs 
>to adapt to versions, it's your application.  Consider a simple example 
>where someone extends a phone-number element to include a country code 
>attribute.  With some work one can write a schema for the original 
>application that will accept the new, previously unknown country code 
>element when it shows up.  The question is, what does the app do with it? 
>The schema may ignore it, but a dialing applicatoin better not.  Indeed, 
>in the US, it would need to know to prefix it with an 011 if the code is 
>other than country code 1, and dial that country code.  In an 
>object-oriented programming system, you'd have a new dial method that 
>would overload the old one, and would preserve the interface.
>Now you can see one of the reasons we didn't do multiple inheritance. With 
>single inheritance we can at least have the (somewhat messy) convention 
>that extensions are at the end of the structure.  What do you do with 
>multiple inheritance?  How do you help an application keep track of where 
>one base class is contributing content and then another?  I'm not saying 
>it's impossible, but I am saying the issues are quite different from what 
>you find in an OO system, where behavior wraps data.
>Finally, I believe this is one of the reasons that refinement and 
>extension are called out separately in schema, something which is less 
>common in OO systems.  Java doesn't particularly distinguish a subclass 
>which just provides a subset of the data of its base (e.g. restricts 
>integers to values < 65535) from subclasses that provide extended 
>behavior;  both are declared in the same way.  In a data system like 
>schema, it's much more helpful to call out "this subclass is really a 
>subset of the base", because that's the case where you know that an old 
>application can handle the new data.
>So, I agree versioning is important.  I'm less convinced that the answer 
>for XML is primarily to be found in the world of object orientation.  XML 
>is data without behavior, somewhat the opposite of OO.
>Noah Mendelsohn                              Voice: 1-617-693-4036
>IBM Corporation                                Fax: 1-617-693-8676
>One Rogers Street
>Cambridge, MA 02142
Received on Tuesday, 21 October 2003 14:24:28 UTC

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