13 Consistency

In a distributed database system with replicated data, consistency has a wider connotation than consistency in single-server systems (as in the ACID properties; see Chapter 2). For example, a user may write a new value for a data item on one replica (residing at one server); but the same user may later on read an older value from another server where the replica has not yet been updated due to delays or failures in the system. Moreover, different users might try to update data items concurrently on different replicas leading to a conflict. Different notions of consistency have been devised to specify desired properties in replicated database systems. A database system without replication (a “one-copy” database) is however the gold ...

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