The Global Data Dictionary
In Table 5.1 we see that the notion of a global data dictionary is common to all but the federated model of distributed database systems. The data dictionary catalogs all objects in the distributed schema, is available at every site, and is accessed identically no matter where it is viewed. It defines the distributed database and shields users, including application developers, from the details of where data resides and how it is accessed.
Obviously, there are a number of challenges in concealing the seams of the distributed database from the users while respecting the objectives of a distributed system. Issues that must be addressed include:
Placement of the global data dictionary
The local data dictionary
Management of interdatabase integrity constraints
Management of user accounts and privileges
These issues are discussed in the sections that follow.
Placement of the Global Data Dictionary
The challenge here is how to make the dictionary available and identical to all sites and respect the 12 objectives of a successful distributed database. A centralized catalog violates the “No reliance on a single site” objective. Yet storing a complete copy of the dictionary at all sites violates the local autonomy objective since local changes must be propagated to all participating sites. Another option is to make each site responsible for its portion of the catalog only; although this meets the objectives of a distributed database, it is generally not practical ...