Examples of read consistency levels

Diagrams illustrating 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 ((3/2)+1 = 2) 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the third replica is checked for consistency with the first two, and if needed, a read repair is initiated for the out-of-date replicas.

Figure 1: Single datacenter cluster with 3 replica nodes and consistency set to QUORUM

Coordinator node

Chosen node

Read response

Read repair

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.

Figure 2: Single datacenter cluster with 3 replica nodes and consistency set to ONE

Coordinator node

Chosen node

Read response

Read repair

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. If needed, a read repair is initiated for the out-of-date replicas.

Figure 3: Multiple datacenter cluster with 3 replica nodes and consistency level set to QUORUM

Coordinator node

Chosen node

Read response

Read repair

A two-datacenter cluster with a consistency level of LOCAL_QUORUM

In a two-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. If needed, a read repair is initiated for the out-of-date replicas.

Figure 4: Two-datacenter cluster with 3 replica nodes and consistency set to LOCAL_QUORUM

Coordinator node

Chosen node

Read response

Read repair

A two-datacenter cluster with a consistency level of ONE

In a two-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.

Figure 5: Two-datacenter cluster with 3 replica nodes and consistency set to ONE

Coordinator node

Chosen node

Read response

Read repair

A two-datacenter cluster with a consistency level of LOCAL_ONE

In a two-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.

Figure 6: Two-datacenter cluster with 3 replica nodes and consistency set to LOCAL_ONE

Coordinator node

Chosen node

Read response

Read repair