Optimistic Replication

Optimistic replication

(also known as lazy replication) is a strategy for replication in which replicas are allowed to diverge. 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.
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.
Data replication is a key technology in distributed data sharing systems, enabling higher availability and performance.
Optimistic replication algorithms that allow replica contents to diverge in the short term, in order to support concurrent work practices and to tolerate failures in low-quality communication links
The importance of such techniques is increasing as collaboration through wide-area and mobile networks becomes popular

Optimistic replication techniques are different from traditional "pessimistic" ones

Instead of synchronous replica coordination
- an optimistic algorithm propagates changes in the background
- discovers conflicts after they happen
- reaches agreement on the final contents incrementally
Needed is a solution space for optimistic replication algorithm
Key challenges facing optimistic replication systems:
- ordering operations
- detecting and resolving conflicts
- propagating changes efficiently
- bounding replica divergence
- providing a comprehensive survey of techniques developed to addressing these challenges

Presentation