Eventual consistency


Eventual consistency is a consistency model used in distributed Data Store to achieve high availability that informally guarantees that, if no new updates are made to a given data item, eventually all accesses to that item will return the last updated value.

Eventual consistency is widely deployed in distributed Data Stores, often under the moniker of Optimistic replication, and has origins in early mobile computing projects.

A system that has achieved Eventual consistency is often said to have converged, or achieved replica convergence or reach a consistent state

Eventual consistency is a weak guarantee – most stronger models, like linearizability are trivially eventually consistent, but a system that is merely Eventual consistency does not usually fulfill these stronger constraints.

Eventual consistency services are often classified as providing BASE (Basically Available Soft state Eventual consistency) semantics, in contrast to traditional ACID (Atomicity, Consistency, Isolation, Durability) guarantees.

Eventual consistency is sometimes criticized as increasing the complexity of distributed software applications. This is partly because Eventual consistency is purely a liveness guarantee (reads eventually return the same value) and does not make safety guarantees: an Eventual consistency system can return any value before it has achieved replica convergence.

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