ABSTRACT
Metadata means many things to many people. Metadata started with a simple definition, i.e., data dictionary or schema. Initially, three-schema architecture was proposed for centralized databases. Then, for distributed databases, the schema architecture was extended to five layers. Additional considerations arose for heterogeneous and federated databases such as export schemas. Then came the Internet and the Web, and metadata included information about usage patterns, policies, and procedures as well as administration information.
