JNTU MCA 2020 DISTRIBUTED DATABASES Previous Question Paper (October/ November - 2020 - R17)

The reference architecture for distributed databases is defined by the ISO/ANSI SPARC three-level schema architecture, extended to accommodate distribution. It consists of three primary levels:

1. External Schema: Defines the user views of the data, providing a customized perspective for specific applications or users.
2. Conceptual Schema: Represents the global logical view of the entire database, independent of physical storage or distribution details.
3. Internal Schema: Describes how data is physically stored and distributed across multiple sites. This includes the Global Mapping (mapping conceptual to local schemas) and Local Schemas (defining data at individual sites).

This architecture ensures data independence by separating the global logical structure from the physical distribution and local site-specific storage mechanisms.

Distribution Transparency is the property of a distributed database system that hides the physical distribution of data from the user or application. It allows users to interact with the database as if it were a single, centralized system.

Key types of distribution transparency include:

• Fragmentation Transparency: Users are unaware that data is divided into fragments (horizontal or vertical). The system automatically reconstructs the data when queried.
• Location Transparency: Users do not need to know the physical location (site) of the data. Queries are executed without specifying node names.
• Replication Transparency: Users are unaware that multiple copies of data exist at different sites. The system handles consistency and updates across all replicas automatically.

By providing these transparencies, the system simplifies application development and maintenance, as changes to the physical distribution do not require modifications to application code.