[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] Re: Caution using XML Schema backward- or forward-compatibilit
There are two cautions I am happy with here: 1. That forwards-compatibility places great constraints on versioning - that these constraints mean a lot of planning has to go into the earliest version to reduce such constraints, e.g. avoidance of making things mandatory because they cannot then be made optional later without breaking forwards-compatibility (in terms of instance/schema validation). The problem with this seems to be that it is in these early stages that designers are least likely to foresee such future requirements for which they must plan. 2. That mistakes made in these early stages can lead to designers wanting to change semantics to try to compensate for such mistakes, 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. This is what I believe Roger is warning is not actually any better than breaking perceived forwards-compatibility. It is just breaking compatibility but hiding the fact of doing so from the older schema. > Designing a new version of an XML Schema to be forward-compatible with > an old version necessitates that the only changes made in the new > version are "subset" changes, such as: > > - constrain an element's or attribute's datatype > - reduce the number of occurrences of an element > - eliminate an optional element or attribute > - remove an element from a choice > > This is very restrictive. And to what avail? Answer: to enable > validation of new XML instance documents against an old XML Schema. > But as described above, just because data can be validated doesn't mean > it can be processed. > -- Stephen Green Partner SystML, http://www.systml.co.uk Tel: +44 (0) 117 9541606 http://www.biblegateway.com/passage/?search=matthew+22:37 .. and voice
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