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and if needed, a read repair is initiated for the out-of-date replicas.
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.
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initiated for the out-of-date replicas.
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for the out-of-date replicas.
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.
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.
Coordinator node
Chosen node
Read
response
Read repair