The process of sharing information so as to ensure consistency between redundant resources to improve reliability, fault-tolerance, or accessibility.  Replication can be in the form of date where the same data is stored in multiple storage devices or computations where the same computing task is executed many times. Replication can be divided into active and passive.  Active replication is executed by processing the same request at every replica whereas in passive each request is processed on a single replica and then its state is transferred to other replicas.