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Re: [xml-dev] Which is better: overload the semantics of XML axesor create auxiliary elements and attach semantics to them?

I don't see any issue here. The "semantics" are given entirely by the <approachTransition> element's existence, not by its placement or order as a child element: by virtue of being an "approach transition" it is automatically earlier in time or landing sequence than a "final approach". And the processing code will have to know this no matter what schema is used.

So go for simplicity.


On 6/29/2017 10:18 AM, Costello, Roger L. wrote:
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 #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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