Consistency

With data replicated across multiple nodes, the number of replicas read for a read operation or the number of replica acknowledgments awaited for a write operation affects latency, data accuracy, and availability. This replica count is configurable in Cassandra and defined as Consistency Level (CL), specified for each read and write operation. Thus, based on the requirements Cassandra allows the client to decide the consistency level, providing great flexibility. 

Read consistency

For a read operation, the consistency level specifies the minimum number of replica nodes required to respond with the data before the coordinator can send the data back to the client.

If CL cannot be reached for the read operation, the operation fails.