Thoughts about XML structure: elements vs. attributes

Processing matters

XML is meant to be both human and machine readable. Would you agree that "machine readable" matters more, because every day machines automatically process tons of XML data?

For example, the first steps of processing XHTML [1] may look like:

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|----------|   |------------|   |-----------------|
| XML data |-->| XML parser |-->| XHTML processor |--> ...
|----------|   |------------|   |-----------------|

Note, that the XML parser knows nothing about the data except that it is XML. It doesn't care whether the data is described via the elements or attributes. In fact, the XML parser is able to parse any data, whether it is an XHTML document, RSS channell, OpenDocument container and so on. The XML parser transforms the input data into objects in memory which are passed to the XHTML processor which contains the logic of dealing with these objects.

XML data

Consider an ordered list which starts with a "100" bullet:

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<ol start="100">
  <li>Coffee</li>
  <li>Milk</li>
</ol>

The XHTML processor, knows that the <ol> tag represents an ordered list, and the nested <li> tags are the list elements with the data. The processor also expects that the <ol> tag data may contain specific attribute which describe the list, e.g. "start" Note, that changing the value of the start attribute does not interfere with the data (the list items).

Now, let's create an unordered list:

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<ul>
  <li>Tee</li>
  <li>Soda</li>
</ul>

The XHTML processor will also parse this data as a list, because it is aware of the <ul> tag and its contents. The HTML standard has the third <dt> [2] tag which describes a definision list.

Let's think of an interesting design task: how would you design an XML format which stores any kind of lists unknown to the processor module?

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|----------|   |------------|   |----------------|
| XML data |-->| XML parser |-->| Our  processor |--> ...
|----------|   |------------|   |----------------|

And that is where attributes can be very useful:

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<users type="list" item="user">
  <user>
    <name>Alex</name>
    <lastname>Black</lastname>
  </user>

  <user>
    <name>John</name>
    <name>Brown</name>
  </user>
</users>

The processor is not aware of the <users> tag. But it knows that if an element contains the type="list" attribute (and value), then the item attribute would describe the list items to look for. What if we wanted to make a list with multiple selected items? The attributes can handle this situation as well:

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<users type="list" item="user">
  <user selected="true">
    <name>Alex</name>
    <lastname>Black</lastname>
  </user>

  <user>...</user>
  <user selected="true">...</user>
  <user>...</user>
</users>

Attributes as metadata

Metadata is a data which describes some other data. In all the examples above, the attributes contained the metadata information. Removing the type="list" and item="user" attributes from the <users> tag will affect the way how the data is processed, but the original data is intact.

In my opinion the attributes are perfect for metadata, and a person designing an XML document format should simply take the discussion of "Attributes vs. elements" to the "Data and metadata" level.

Eager to hear your comments!

Footnotes