Best practices: XML attribute vs XML element - When should I use elements and when should I use attributes? [duplicate]

Answer 1

There aren't usually right or wrong answers for this sort of thing. Mostly it depends on how you need to access your data.

One nice thing about the first one is that it easily, perhaps later, supports multiple pages in a section and multiple sections in a category. In the second one that information is spread across pages.

Answer 2

IBM has posted an article titled Principles of XML Design that provides some guidelines on when to use attributes vs. elements. I found this article useful, your mileage may vary.

Answer 3

A simple rule of thumb: if you can implement a data structure as an unordered map of name/value pairs, you can use an element's attributes to represent it. If you can't (if, for instance, you'll have multiple names, or a given name will have multiple associated value, or the ordering of the name/value pairs is significant), then an element with attributes is the wrong representation.

Two other things that can make this the wrong representation:

• The values contain markup. This can be represented in attribute values, but it's awkward, because all of the markup characters have to be escaped into entities. Also, the markup won't be parsed.
• You're using XML Schema validation, and there is more than one allowable set of name/value pairs. XML Schema can only define one set of allowable attributes for an element, whereas it can define multiple mutually-exclusive sets of allowable child elements.

The obvious benefit to using attributes is that they result in terser XML. They're (very) marginally faster to parse than elements, too.

Answer 4

The two examples are not equivalent, because they form different hierarchies. Is a sitemap a list of categories, like the first example? Or is it a list of pages like the second example?

The answer to that is orthogonal to the element vs attribute question.

On the Element vs Attribute question: Here is your second example transformed to an attribute approach:

<sitemap>
 <page    
  category='Animals'
  section='Dogs'
  title='Great Dane'
  url='/pics/greatdane.jpg'
  /> 
</sitemap>

The above and your second case are equivalent. One consideration for choosing one versus the other is based on whether you may modify the schema in the future. Adding an attribute to the url element as in the following example would likely be a backward compatable change. The semantically same modification would be impossible in the attribute approach, as you cannot attach an attribute to an attribute.

<sitemap>
 <page>    
  <category>Animals</category>
  <section>Dogs</section>    
  <title>Great Dane</title>    
  <url nofollow="true">/pics/greatdane.jpg</url>
 </page> 
</sitemap>