[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
> 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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