Optimistic Consistency

Optimistic schemes takes the other extreme towards limiting availability for guaranteeing consistency : they don't limit availability at all. Any copy can be read or updated at anytime. This is important for several applications that wish to have almost continuous and guaranteed access to data.

Conflicts

These read-any, write-any schemes that allow updates to any available copy can introduce inconsistencies during partitions. Inconsistencies that arise due to conflicting updates are resolved when the network partitions heal. The time lag between the update and resolution of the inconsistent update depends on the length of the network partition.

Problem

While one partition updates the data, another could be reading or worse still updating the data. Thus users could read stale or conflicting data. This breaks the single copy consistency model where users always read and write the latest data and increases the complexity of the system for the user.

Solutions After the Fact

Since inconsistencies can occur during partitions, optimistic systems provide a conflict detection and resolution scheme. When updates are conflicting, the detection scheme detects the writes that were conflicting. The resolution scheme can be of two types:

Undo-redo: The updates that were conflicting are undone by running compensating actions and then a set of corrective updates are applied.
Corrective: No updates are undone. Only corrective updates are applied.