JNTU MCA 2021 DISTRIBUTED DATABASES Previous Question Paper (February - 2021 - R17)

The reference architecture for distributed databases is designed to manage data across multiple sites while maintaining the illusion of a single, centralized database for the user. It is structured into three main levels of abstraction to ensure data independence and transparency.

The architecture is defined by the following components:

• External Schema Level: This level consists of various user views or application-specific schemas. It provides the necessary data transparency by allowing users to interact with the database without needing to know the physical location or distribution of the data.
• Conceptual Schema Level: This is the global logical schema that describes the entire database as if it were centralized. It defines all entities, relationships, and constraints, serving as the integration point for all distributed data fragments.
• Internal Schema Level: This level deals with the physical distribution and storage of data. It maps the global conceptual schema to the local schemas at individual sites. It includes details about data fragmentation (horizontal or vertical), replication, and the specific data models used at each local site.

The mapping between these levels is managed by the Distributed Database Management System (DDBMS) through the following mechanisms:

• Fragmentation Schema: Defines how global relations are partitioned into smaller fragments.
• Allocation Schema: Specifies the physical location of each fragment across the network nodes.
• Local Mapping: Maps the fragments to the physical storage structures at each local database site.

This layered approach enables Distribution Transparency, where the system hides the complexity of data distribution, fragmentation, and replication from the end-user, ensuring that queries are executed efficiently regardless of where the data resides.