الفهرس 
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Abstract
Distributed database is preferred for many distributed applications like banking, telecommunication,and book systems than the traditional client sever model. For distributed services, replication plays an important role in distributed database. Data replication consists of maintaining multiple copies of data, called replica, on separate computers. Replicas are stored at several different sites and may be far apart geographically region, country, or even in the world. Although replication improves availability and performance, it has a potential problem in maintaining the consistency of replicas against updates.