1. A method for defining asset classes in a digital library, comprising: defining at least one asset class to include at least one attribute; defining attributes for each asset class to have an attribute object type; defining the attribute object type to indicate one of a plurality of different data structure formats that are searchable through separate application programs, wherein the attribute object types in one asset class are implemented in different data structure formats; generating an asset object instance for each asset class; and generating information in the asset object instance on a file location of attribute objects providing the attributes for the generated asset object instance.