A database directory is used to store the information needed for accessing each database in the multidatabase system. The database directory stores the name of each component database and a pointer to the metadata files employed by the OPM query translator for accessing the database. In addition, the database directory contains for each database the list of its classes, the address of the server, and information for accessing it remotely.
For each class appearing in a multidatabase query, the OPM multidatabase query language (OPM*QL) parser accesses the database directory to determine the database containing that class. If a class name appears in multiple databases, the class name should be qualified with a database name in the query.
The database directory also contains information about known links between classes in different databases. These links are used in forms-based query systems for constructing join expressions across databases.
Part of the database directory for GSDB-HGD is shown in Figure 2.
Figure 2: Part of the Database Directory for GSDB-GDB HGD
Information required for a inter-database link includes the names of the databases and classes involved in the link, and the information necessary to express the link. The inter-database links are still being developed and precise details of the information required has not yet been finalized.