54.1 Advanced Database Management >> Distributed Databases

1. Transparency: The distribution should be transparent to users. They shouldn’t be aware of where the data is located, how data is replicated, etc.
2. Scalability: Systems can be easily expanded by adding more machines to the network.
3. Reliability: Due to data redundancy, if one site fails in a distributed database, a user can still access the same data from another site.
4. Autonomy: Each location in the distributed system can operate independently.

1. Homogeneous Distributed Databases: Every site uses the same DBMS product, and data structures are consistent across sites.
2. Heterogeneous Distributed Databases: Different sites might use different schemas, and even different DBMS products.

1. Reliability and Availability: If one site fails, only the local data is rendered inaccessible. The remaining sites can continue to function.
2. Improved Performance: Data is located near the site of greatest demand.
3. Easier Expansion: Systems can grow by adding more machines to the network without interruptions.

1. Complexity: It’s harder to design and manage distributed systems as compared to a centralized system.
2. Security Concerns: Data distributed across various sites can have security vulnerabilities.
3. Integrity Control: Ensuring data consistency across all nodes/sites can be a challenge.
4. Cost: Initial setup and the cost of maintaining integrity can be high.

1. Data Fragmentation: The database is divided into fragments and stored across multiple sites. This can be done based on relation, row, or column.
2. Data Replication: The same data is stored at multiple sites to ensure data availability.
3. Data Allocation: Determines where each data fragment/replica should be located.

1. Distributed Query Processing: A strategy to decompose and distribute the query to get the fastest response time.
2. Distributed Transaction Management: Ensures the consistency and integrity of a transaction across distributed sites.
3. Concurrency Control: Manages concurrent access to data to ensure consistency.
4. Commit Protocols: Ensures that a transaction is reflected in multiple sites or none at all.

Distributed databases are integral in today’s world, especially with the rise of global applications and the need for high availability. However, they bring with them unique challenges that need to be addressed for efficient and reliable operations. Proper design, consistent policies, and robust technologies are essential for maintaining and operating such systems effectively.