Identifying markup using namespaces versus using data (was: 4approaches

["Costello, Roger L." <costello@mitre.org>]

Hi Folks

One way of identifying an XML building block (data component) is by namespace. For example, this list component is identified by the namespace http://week.days.org

   <DaysOfTheWeek xmlns="http://week.days.org";>
       <Day>Sunday</Day>
       <Day>Monday</Day>
       <Day>Tuesday</Day>
       <Day>Wednesday</Day>
       <Day>Thursday</Day>
       <Day>Friday</Day>
       <Day>Saturday</Day>
   </DaysOfTheWeek>

This list is identified by the namespace http://meetings.org

   <Meetings xmlns="http://meetings.org";>
       <Meeting>Dentist</Meeting>
       <Meeting>Doctor</Meeting>
       <Meeting>Boss</Meeting>
   </Meetings>

Applications can be built that are namespace-aware.

Different data components can be mashed together into a single document and still be extracted and processed individually because each is in a namespace.


There is an alternate way of identifying an XML building block (data component): by embedding an identifier within the document, as data. The weekday list could be expressed like this:

    <List>
        <Identifier>http://week.days.org</Identifier>
        <li>Sunday</li>
        <li>Monday</li>
        <li>Tuesday</li>
        <li>Wednesday</li>
        <li>Thursday</li>
        <li>Friday</li>
        <li>Saturday</li>
    </List>

And the meetings list could be expressed like this:

    <List>
        <Identifier>http://meetings.org</Identifier>
        <li>Dentist</li>
        <li>Doctor</li>
        <li>Boss</li>
    </List>

Things to note:

1. Namespaces are not being used. (I formatted the identifier as a namespace, but I didn't have to do that)

2. The list is identified by the content of <Identifier>

3. The same XML vocabulary is used for both lists. (In fact, the same XML vocabulary is used for all lists)

The two lists can be brought together into a single document and still be processed individually. Applications partition the document based on the value in <Identifier>


xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
        ANALYSIS
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

The namespace approach has the benefit of being widely adopted. Most XML tools, parsers, and technologies are based on namespaces. For example, NVDL is entirely based on using namespaces to partition a compound document; an XSLT processor processes a document based on the XSLT namespace.

Ken nicely summarizes the advantages of the second approach:

> [because the same XML vocabulary is used for all lists it]
> makes all list-processing algorithms and code independent of 
> the content, allowing one to leverage a single investment in software 
> and access all code lists. 


Do you agree with this analysis? What else would you add?

/Roger