Tunable Consistency is a phenomenal characteristic that makes Cassandra a favored database choice of Developers, Analysts and Big data Architects. Consistency refers to the up-to-date and synchronized data rows on all their replicas. Cassandra’s Tunable Consistency allows users to select the consistency level best suited for their use cases. It supports two consistencies -Eventual and Consistency and Strong Consistency.
The former guarantees consistency when no new updates are made on a given data item, all accesses return the last updated value eventually. Systems with eventual consistency are known to have achieved replica convergence.
For Strong consistency, Cassandra supports the following condition:
R + W > N, where
N – Number of replicas
W – Number of nodes that need to agree for a successful write
R – Number of nodes that need to agree for a successful read
Hope this helps.