Examples of read consistency levels 

Read request examples with different consistency levels.

Rapid read protection diagram shows how the speculative retry table property affects consistency.

A single datacenter cluster with a consistency level of QUORUM 

In a single datacenter cluster with a replication factor of 3, and a read consistency level of QUORUM, 2 of the 3 replicas for the given row must respond to fulfill the read request. If the contacted replicas have different versions of the row, the replica with the most recent version will return the requested data. In the background, the third replica is checked for consistency with the first two, and if needed, a read repair is initiated for the out-of-date replicas.

A single datacenter cluster with a consistency level of ONE 

In a single datacenter cluster with a replication factor of 3, and a read consistency level of ONE, the closest replica for the given row is contacted to fulfill the read request. In the background a read repair is potentially initiated, based on the read_repair_chance setting of the table, for the other replicas.

A two datacenter cluster with a consistency level of QUORUM 

In a two datacenter cluster with a replication factor of 3, and a read consistency of QUORUM, 4 replicas for the given row must respond to fulfill the read request. The 4 replicas can be from any datacenter. In the background, the remaining replicas are checked for consistency with the first four, and if needed, a read repair is initiated for the out-of-date replicas.

A two datacenter cluster with a consistency level of LOCAL_QUORUM 

In a multiple datacenter cluster with a replication factor of 3, and a read consistency of LOCAL_QUORUM, 2 replicas in the same datacenter as the coordinator node for the given row must respond to fulfill the read request. In the background, the remaining replicas are checked for consistency with the first 2, and if needed, a read repair is initiated for the out-of-date replicas.

A two datacenter cluster with a consistency level of ONE 

In a multiple datacenter cluster with a replication factor of 3, and a read consistency of ONE, the closest replica for the given row, regardless of datacenter, is contacted to fulfill the read request. In the background a read repair is potentially initiated, based on the read_repair_chance setting of the table, for the other replicas.

A two datacenter cluster with a consistency level of LOCAL_ONE 

In a multiple datacenter cluster with a replication factor of 3, and a read consistency of LOCAL_ONE, the closest replica for the given row in the same datacenter as the coordinator node is contacted to fulfill the read request. In the background a read repair is potentially initiated, based on the read_repair_chance setting of the table, for the other replicas.