Example of JDO file. This file is used to define the persistent classes that are used in SigPath to represent end-users. Four persistent classes are shown: Address, Affiliation, User and UserRole. The <field> element can be used to refer to specific fields of persistent classes (such as the username field of class User on this example). The userRoles field is described to be a collection that contains elements of type UserRole. Elements called <extension> make it possible to provide vendor specific directives, such as to define indices on a persistent field.