Replication Server depends on data servers to provide the transaction processing services needed to protect their stored data. To guarantee the integrity of distributed data, data servers must comply with such transaction-processing conventions as atomicity and consistency.
Data servers that store primary data provide most of the concurrency control needed for the distributed database system. If a transaction fails to update a table with primary data, Replication Server does not distribute the transaction to other sites. When a transaction does update primary data, Replication Server distributes the changes and, unless a failure occurs, the update succeeds at all sites that have subscribed to the data.
Replication Server uses optimistic concurrency control to maintain replicated data consistency. This method differs from a pessimistic distributed concurrency control method—such as the two-phase commit—because it processes failures after they occur.
Optimistic concurrency control has these advantages in a replication system:
It promotes high availability of data because it does not lock the data for the duration of the distributed transaction.
It requires fewer system resources to process a transaction.
It does not require data servers to have special distributed transaction processing features in order to participate in a distributed transaction.
