The way data is stored within Couchbase Server is through the distribution offered by the vBucket structure. If you want to expand or shrink your Couchbase Server cluster then the information stored in the vBuckets needs to be redistributed between the available nodes, with the corresponding vBucket map updated to reflect the new structure. This process is called rebalancing.
Rebalancing is an deliberate process that you need to initiate manually when the structure of your cluster changes. The rebalance process changes the allocation of the vBuckets used to store the information and then physically moves the data between the nodes to match the new structure.
The rebalancing process can take place while the cluster is running and servicing requests. Clients using the cluster read and write to the existing structure with the data being moved in the background between nodes. Once the moving process has been completed, the vBucket map is updated and communicated to the smart clients and the proxy service (Moxi).
The result is that the distribution of data across the cluster has been rebalanced, or smoothed out, so that the data is evenly distributed across the database, taking into account the data and replicas of the data required to support the system.