The GraphML default extension mechanism allows to declare so-called GraphML attributes that can be used to conveniently store additional information in a GraphML file. In its original form, this mechanism directly supports additional data of type bool, int, long, float, double, and string.

A GraphML attribute is declared using the <key> element which comprises a unique identifier as well as scope, name, and the domain of values (i.e., the actual type) for the attribute. Table 4.1, “XML attributes for GraphML <key> element” lists the set of XML attributes and describes their function within the declaration of a GraphML attribute. Note that in a GraphML file, <key> elements must be placed before any <graph> element.

Actual values for a GraphML attribute are defined using the <data> element which is nested within the GraphML elements <graph>, <node>, <edge>, or <port>. Required with each <data> element is XML attribute key which is used by a look-up mechanism to find the proper GraphML attribute declaration for the contents of a given <data> element. See also Table 4.2, “XML attribute for GraphML <data> element” for a brief explanation of XML attribute key.

This GraphML attribute look-up mechanism is responsible for matching the value given for the key attribute of a <data> element to the unique ID defined by the id attribute of a <key> element. The mapping that is established by this match is essential for delegating the parsing of a given <data> element to the proper parser code that handles the specific contents. Example 4.3, “GraphML attribute look-up mechanism” shows the correct set-up to refer to a specific GraphML attribute when defining its values using the <data> element.

<!-- Declaration of a GraphML attribute. -->
<key id="someValidIdentifier" ... />

<!-- Referring to a specific GraphML attribute by matching the (unique) -->
<!-- identifier of its declaration. -->
<data key="someValidIdentifier">
...
</data>