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....rn 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.
Read more about this topic: Distributed Database
Famous quotes containing the word important:
“A poem is like a person. Though it has a family tree, it is important not because of its ancestors but because of its individuality. The poem, like any human being, is something more than its most complete analysis. Like any human being, it gives a sense of unified individuality which no summary of its qualities can reproduce; and at the same time a sense of variety which is beyond satisfactory final analysis.”
—Donald Stauffer (b. 1930)