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 data center cluster with a consistency level of
QUORUM
In a single data center 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initiated for the out-of-date replicas.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 data center cluster with a consistency level of ONE
In a single data center 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 data center cluster with a consistency level of QUORUM
In a two data center 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 data center. 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initiated for the out-of-date replicas.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 data center cluster with a consistency level of
LOCAL_QUORUM
In a multiple data center cluster with a replication factor of 3, and a read consistency of
LOCAL_QUORUM, 2 replicas in the same data center 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initiated for the out-of-date replicas.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 data center cluster with a consistency level of ONE
In a multiple data center cluster with a replication factor of 3, and a read consistency of
ONE, the closest replica for the given row, regardless of data center, 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 data center cluster with a consistency level of
LOCAL_ONE
In a multiple data center cluster with a replication factor of 3, and a read consistency of
LOCAL_ONE, the closest replica for the given row in the same data center
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.
