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The oldest question asked by adopters of XML is when to use elements and when to use attributes in XML design. As with most design issues, this question rarely has absolute answers, but developers have also experienced a lack of very clear guidelines to help them make this decision. In this article, Uche Ogbuji offers a set of guiding principles for what to put in elements and what to put in attributes.

Several frequently pondered questions of DTD design in SGML have followed the legacy to its offshoot, XML. Regardless of what XML schema language you use, you might find yourself asking:

• When do I use elements and when do I use attributes for presenting bits of information?
• When do I require an order for elements, and when do I just allow arbitrary order?
• When do I use wrapper elements around sequences of similar elements?

In my experience working with users of XML, the first question is by far the most common. In some cases the answer is pretty unambiguous:

• If the information in question could be itself marked up with elements, put it in an element.
• If the information is suitable for attribute form, but could end up as multiple attributes of the same name on the same element, use child elements instead.
• If the information is required to be in a standard DTD-like attribute type such as ID, IDREF, or ENTITY, use an attribute.
• If the information should not be normalized for white space, use elements. (XML processors normalize attributes in ways that can change the raw text of the attribute value.)