Re: [xml-dev] Caution using XML Schema backward- or forward-compatibilit

["Fraser Goffin" <goffinf@googlemail.com>]

My feeling is that :-

you should NOT change the semantics of any information item unless :-

a. You 'own' all of the applications that make use of it and can
understand the impacts and implement any required changes in a
controlled way.

b. You are aware of all of the users of the information item and have
gained agreement and a timetable for implementing the change in a 'big
bang' fashion.

c. You make the change using a method that EXPLICITLY BREAKS exisitng
uses (possibly coupled with support for each INcompatible version for
a period of time).

Personally I would go for (c) most of the time.

Fraser.

On 27/12/2007, Costello, Roger L. <costello@mitre.org> wrote:
> Excellent discussion!
>
> Michael has brought into the discussion a very useful idea: semantic
> drift.  He asserts that it "happens naturally in the real world".
>
> I assert that it also occurs naturally and often in data versioning.
>
> Here are two examples of semantic drift in data versioning:
>
> EXAMPLE #1
>
> Consider this simple XML document:
>
>    <distance>100</distance>
>
> In the v1 XML Schema the <distance> element is declared as follows:
>
>    <element name="distance" type="nonNegativeInteger"/>
>
> The data specification document defines distance as:
>
>    "Distance represents the length measurement from center of town."
>
> In the v2 XML Schema there is no change to the declaration of the
> <distance> element:
>
>    <element name="distance" type="nonNegativeInteger"/>
>
> However, the data specification document redefines distance:
>
>    "Distance represents the length measurement from the town line."
>
> The we have an example of two versions that are "validation-compatible"
> but "semantic incompatible."
>
> The semantics of "distance" has drifted from v1 to v2.
>
> EXAMPLE #2
>
> Consider the same simple XML document:
>
>    <distance>100</distance>
>
> In the v1 XML Schema it is declared differently:
>
>    <element name="distance">
>        <complexType>
>            <simpleContent>
>                <extension base="nonNegativeInteger">
>                    <attribute name="units" fixed="miles"/>
>                </extension>
>            </simpleContent>
>        </complexType>
>    </element>
>
> The <distance> element now has a "units" attribute which is fixed at
> "miles."
>
> The data specification document defines distance as:
>
>    "Distance represents the length measurement from center of town."
>
> In the v2 XML Schema the declaration of the <distance> element is
> modified; the units attribute is fixed at "kilometers":
>
>    <element name="distance">
>        <complexType>
>            <simpleContent>
>                <extension base="nonNegativeInteger">
>                    <attribute name="units" fixed="kilometers"/>
>                </extension>
>            </simpleContent>
>        </complexType>
>    </element>
>
> The data specification document is unchanged in its definition of
> distance:
>
>    "Distance represents the length measurement from center of town."
>
> Thus, we see a second example of two versions that are
> "validation-compatible" but "semantic incompatible."
>
> The semantics of "distance" has drifted from v1 to v2.
>
> COMMENTS
>
> 1. I think that these examples illustrate two common changes in data.
> Do you agree?
>
> 2. In the examples, the XML instance document:
>
>    <distance>100</distance>
>
> validates fine against both the v1 and v2 XML Schemas.  But if the
> applications that process the XML instance aren't changed, then the
> processing results may be incorrect.
>
> CAUTION
>
> Just because an application can validate an XML instance document,
> doesn't mean it can process the XML instance document.
>
> QUESTION
>
> Can you state in one sentence the fundamental lesson to be learned in
> our discussion?
>
> /Roger
>
>
>
>
>
>
>
>
> -----Original Message-----
> From: Michael Kay [mailto:mike@saxonica.com]
> Sent: Thursday, December 27, 2007 6:13 AM
> To: 'Stephen Green'; Costello, Roger L.; xml-dev@lists.xml.org
> Subject: RE: [xml-dev] Caution using XML Schema backward- or
> forward-compatibility as a versioning strategy for data exchange
>
> > e.g. because an element wasn't made optional it
> > cannot be removed and so there is a temptation to change its
> > semantics - to reuse it for something else rather than remove
> > it.
>
> Yes, "semantic drift" is a big problem and of course it happens even in
> the
> absence of schema change.
>
> Semantic drift happens naturally in the real world, for example credit
> card
> numbers which once identified an account might start to identify a
> specific
> card with access to that account. It's not surprising that it happens,
> because if a system is capable of meeting new requirements without
> requiring
> any software changes then people will use it creatively in new ways to
> meet
> those requirements. One of the challenges in designing schemas (or
> database
> integrity constraints) is knowing whether you should try to resist
> semantic
> drift as a menace to information integrity, or whether you should allow
> your
> system to ride the waves, thus increasing its flexibility and
> longevity.
>
> System designers often underestimate the creativity of users in
> applying
> semantic overloading to data structures. I saw one system where users
> were
> marking certain records for review the following day, simply by
> entering a
> particular code that was known to be invalid and would therefore appear
> in
> tomorrow's validation report. The system designers helpfully introduced
> stronger validation at data-entry time, and chaos ensued because the
> users
> had to invent a new process.
>
> Michael Kay
> http://www.saxonica.com/
>
>
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