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Which is better: overload the semantics of XML axes or createauxiliary e

  • From: "Costello, Roger L." <costello@mitre.org>
  • To: "xml-dev@lists.xml.org" <xml-dev@lists.xml.org>
  • Date: Thu, 29 Jun 2017 14:18:49 +0000

Which is better: overload the semantics of XML axes or createauxiliary e

Hi Folks,

Is it better to overload the semantics of XML axes or is it better to create auxiliary elements and attach semantics to them?

Please, allow me to explain with an example.

Upon entering an airport’s terminal airspace, an aircraft flies an approach transition followed by a final approach. There is one final approach. There may be multiple ways to get to the start of the final approach; that is, there may be multiple approach transitions.

How to model multiple approach transitions funneling into one final approach?

Approach #1: Overload the semantics of the following-sibling axis

One approach is to add a semantic to the XML following-sibling axis: applications are to interpret a following sibling element to mean it occurs temporally and geospatially after its preceding sibling element. Here is an XML instance that illustrates this approach:


That has two disadvantages:

1. There are other elements in the XML document (not shown); the following siblings in those other elements are not to be interpreted with the semantic. So, the model is inconsistent: the XML following sibling axis means one thing in one part of the XML and another thing in another part of the XML.

2. Recall that “there may be multiple approach transitions.” How to model that? One might do this:


But that is nonsensical under the semantic: an approach transition does not occur temporally and geospatially after another approach transition.

Approach #2: Create auxiliary elements and attach semantics to them

In this approach the following-sibling axis has no additional meaning; a following sibling element merely means that, in the XML document, it sequentially follows another element.

Here is an instance that illustrates the second approach:


The alternativeApproachTransition element is created and assigned this semantic: The child elements are to be interpreted as alternative approach transitions.

The funnelsInto element is assigned this semantic: The child element is to be interpreted as a final approach that is to occur temporally and geospatially after the preceding stuff.

This second approach has a disadvantage: It is more verbose than the first approach.


When modeling, in XML, the concept “this occurs after that” it is better to:

  1. Overload the semantic of the following-sibling axis.
  2. Create auxiliary elements and attach semantics to them
  3. Something else (what?)
  4. It doesn’t matter—approach#1 and approach#2 are equally good/bad



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