XML Attributes

XML elements can have attributes.

Attributes often provide information that is not a part of the data. In the example below, the file type is irrelevant to the data, but important to the software that wants to manipulate the element:

<file type="gif">computer.gif</file>

Quote Styles, "female" or 'female'?

Attribute values must always be enclosed in quotes, but either single or double quotes can be used. For a person's sex, the person tag can be written like this:

<person sex="female">

or like this:

<person sex='female'>

Use of Elements vs. Attributes

Data can be stored in child elements or in attributes.

Take a look at these examples:

<person sex="female">
  <firstname>Anna</firstname>
  <lastname>Smith</lastname>
</person>

<person>
  <sex>female</sex>
  <firstname>Anna</firstname>
  <lastname>Smith</lastname>
</person>

In the first example sex is an attribute. In the last, sex is a child element. Both examples provide the same information.

There are no rules about when to use attributes, and when to use child elements. Attributes are handy in HTML, but in XML you should try to avoid them.

Avoid using attributes?

Should you avoid using attributes?

Here are some of the problems using attributes:

attributes cannot contain multiple values (child elements can)
attributes are not easily expandable (for future changes)
attributes cannot describe structures (child elements can)
attributes are more difficult to manipulate by program code
attribute values are not easy to test against a DTD

If you use attributes as containers for data, you end up with documents that are difficult to read and maintain. Try to use elements to describe data. Use attributes only to provide information that is not relevant to the data.

Don't end up like this ( if you think this looks like XML, you have not understood the point):

<note day="12" month="11" year="2002"
to="Tove" from="Jani" heading="Reminder" 
body="Don't forget me this weekend!">
</note>

An Exception to my Attribute rule

Rules always have exceptions.

My rule about attributes has one exception:

Sometimes I assign ID references to elements. These ID references can be used to access XML elements in much the same way as the NAME or ID attributes in HTML. This example demonstrates this:

<messages>
  <note id="p501">
    <to>Tove</to>
    <from>Jani</from>
    <heading>Reminder</heading>
    <body>Don't forget me this weekend!</body>
  </note>

  <note id="p502">
    <to>Jani</to>
    <from>Tove</from>
    <heading>Re: Reminder</heading>
    <body>I will not!</body>
  </note> 
</messages>

The ID in these examples is just a counter, or a unique identifier, to identify the different notes in the XML file, and not a part of the note data.

What I am trying to say here is that metadata (data about data) should be stored as attributes, and that data itself should be stored as elements.