The local orders are forwarded each day to the central shipping department. While the inventory and shipping departments are centrally located, the clerks are spread across the country for the convenience of the customers. The key goals of a distributed database system are availability, accuracy, concurrency, and recoverability. When you design a distributed database system, the location of the data is the most important factor. You must ensure that local clients have quick access to the data they use most frequently, and that remote operations do not occur often.
Replication is one means of dealing with the issue of data location.
A parallel server allows multiple machines to have separate instances all accessing the same database. This configuration greatly enhances data throughput. Figure  5-6 illustrates the Parallel Server option of the Oracle Server. When you configure a system with the Parallel Server option, your key concern is data contention among the various nodes. Each node that requires updatable data must first obtain a lock on that data to ensure data consistency.