There are two alternative approaches to the design of
data distribution, the top-down and the bottom-up approaches.
1) Top-Down Approach:
In the top-down approaches, we start by designing, the global schema, and we proceed by designing the fragmentation of the database, and then by allocating the fragments to the sites, creating the physical images. The approach is completed by performing, at each site the physical design of the data which is allocated to it.
This approach is the most attractive for system which are developed from scratch, since it allows performing the design rationally. Therefore, this approach will be followed by distributed database.
Bottom-Up Approach: This approach requires:
i) The selection of a common database model for describing the global schema of the database.
ii) The translation of each local schema into the common data model.
iii) The integration of the local schemata into a common global schema.
Thus, the bottom-up approach requires solving three problems which are not peculiar to distributed database, but are present also in centralized system.
