Information is distributed around a cluster using a series of replicas. For Couchbase buckets you can configure the number of replicas(complete copies of the data stored in the bucket) that should be kept within the Couchbase Server Cluster.
In the event of a failure in a server (either due to transient failure, or for administrative purposes), you can use a technique called failover to indicate that a node within the Couchbase Cluster is no longer available, and that the replica vBuckets for the server are enabled.
The failover process contacts each server that was acting as a replica and updates the internal table that maps client requests for documents to an available server.
Failover can be performed manually, or you can use the built-in automatic failover that reacts after a preset time when a node within the cluster becomes unavailable.
For more information, see Section 5.1, “Node Failover”.