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any applications can benefit from distributed systems based on multiple computers interconnected by a communication network. Distributed systems use inexpensive high-performance computers and can be configured closely to the application. Information can be replicated on several processors to improve performance and to provide fault tolerance. However, programming distributed applications is difficult, particularly when replicated information must remain consistent as it is updated in the presence of faults. Since many messages may be required, recovery from faults may introduce delays, making real-time performance objectives difficult to achieve.