Distributed Database - Important Considerations

Important Considerations

Care with a distributed database must be taken to ensure the following:

• The distribution is transparent — users must be able to interact with the system as if it were one logical system. This applies to the system's performance, and methods of access among other things.
• Transactions are transparent — each transaction must maintain database integrity across multiple databases. Transactions must also be divided into sub-transactions, each sub-transaction affecting one database system.

There are mainly two approaches to store a relation r in a distributed database system:

A) Replication

B) Fragmentation

A) Replication: In replication, the system maintains several identical replicas of the same relation r in different sites.

• Data is more available in this scheme.
• Parallelism is increased when read request is served.
• Increases overhead on update operations as each site containing the replica needed to be updated in order to maintain consistency.

B) Fragmentation: The relation r is fragmented into several relations r₁, r₂, r₃....r_n in such a way that the actual relation could be reconstructed from the fragments and then the fragments are scattered to different locations. There are basically two schemes of fragmentation:

• Horizontal fragmentation - splits the relation by assigning each tuple of r to one or more fragments.
• Vertical fragmentation - splits the relation by decomposing the schema R of relation r.