Tunable consistency is a phenomenal character 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 consistency and strong consistency.
The former guarantees consistency when no new updates are made on a given data item, i.e., 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