Traditional pessimistic replication systems try to guarantee from the beginning that all of the replicas are identical to each other, as if there was only a single copy of the data all along. Optimistic replication does away with this in favor of eventual consistency, meaning that replicas are guaranteed to converge only when the system has been quiesced for a period of time. As a result, there is no longer a need to wait for all of the copies to be synchronized when updating data, which helps concurrency and parallelism. The trade-off is that different replicas may require explicit reconciliation later on, which might then prove difficult or even insoluble.
