[3] 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.
For example, personal digital assistants (PDAs) allow users to edit data either on the PDA or a computer, and then to merge these two datasets together.
Application developers must be very careful about the assumptions they make about the effect of a database update, and must be sure to simulate lag in their testing environments.