Replication is the process of maintaining several identical replicas (copies) of the relation; each replica is stored at a different site, resulting in data replication. Replication allows for increased local data availability. The advantages of local data availability are:
1. Access of a non-update type of information is cheaper.
2. Even if access to remote site is not possible, access to local data is available.
Advantage of Data Replication
There are a number of advantages of replication.
If one of the sites containing relation R fails, then the relation R may be found in another site. Thought the system may continue to process queries involving R despite the failure of the one site.
In the case where the majority of access to the relation R results in only the reading of the relation, the several sites can process queries involving R in parallel. The more replicas or R there are, the greater the chance that the data is found in the site where the transaction is executing. Hence replication minimizes movement of data between sites.
Disadvantage of Data Replication
Increased Overhead on Update
The system must ensure that all replicas of a relation R are consistent since otherwise erroneous computations may results. This implies that whenever R is updated, this update must be propagated to all sites containing replicas, resulting in increased overhead. For example, in a banking system, where account information is replicated in various sites, it is necessary that transaction assure that the balance in a particular account accessible in all sites.
In general, replication enhances the performance of read operations and increases the availability of data to read transaction. However, update transactions incur greater overhead. The problem of controlling concurrent updates by several transactions to replicated data is more complex than the centralized approach to concurrency control. We may simplify the management of replicas of relation R by choosing one of them as the primary copy of R. For example, in a banking system, an account may be associated with the site in which the account has been opened. Similarly, in an airline reservation system, a flight may be associated with the site at which the flight originates.