Intra-cluster replication involves replicas that are placed on another node in the same cluster.
Replicas are copies of data that are placed on another node in a cluster. The source of the replicated vBucket data is called the active vBucket. Active vBuckets perform read and write operations on individual documents. The destination vBucket is called the replica vBucket. Replica vBuckets receive a continuous stream of mutations from the active vBucket through the Database Change Protocol (DCP). Although replica vBuckets are not accessed typically, they can respond to read requests.
Within the data service, active vBuckets are spread across the cluster for even distribution of data and best performance. Additional replicas are optional. As an administrator, you can create between one and three additional copies (replica vBuckets) of the active vBuckets. Replica vBuckets are also spread across the cluster for best availability and can use failure-domain definitions to guide distribution. For example, to protect against the node failure-domain, Couchbase Server places an active vBucket and its corresponding replica vBucket on separate nodes or separate racks.