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The number of nodes that must acknowledge a read or write operation before it is considered successful.
For data to be successfully written or read back to the client, a sufficient number of nodes need to agree on the correct state of the data. Note that while you can set consistency levels for the cluster overall, Scylla allows each operation (each read or write) to have its own consistency level, a feature known as tunable consistency.
- CL of 1: Wait for response from one replica node
- CL of ALL: Wait for response from all replica nodes
- CL LOCAL_QUORUM: Wait for floor((#dc_replicas/2)+1), meaning that if a DC has 3 nodes replica in the cluster the application will wait for response from 2 replica nodes
- CL EACH_QUORUM: For multi DC, each DC must have a LOCAL_QUORUM. This is unsupported for reads.
- CL ALL: All replica nodes must respond. Provides the highest consistency but lowest availability.
Because data may have changed on a coordinator node, but it may not have yet been recorded and stored on all the required replicas, a Scylla cluster offers eventual consistency. With eventual consistency all of the replica nodes will eventually arrive at the same data state; operations may continue updating the database in the meantime.
More information about high availability, replication and consistency can be found in this lesson.