The rebalance process is managed through a specific process called the orchestrator. This examines the current vBucket map and then combines that information with the node additions and removals in order to create a new vBucket map.
The orchestrator starts the process of moving the individual vBuckets from the current vBucket map to the new vBucket structure. The process is only started by the orchestrator - the nodes themselves are responsible for actually performing the movement of data between the nodes. The aim is to make the newly calculated vBucket map match the current situation.
Each vBucket is moved independently, and a number of vBuckets can be migrated simultaneously in parallel between the different nodes in the cluster. On each destination node, a process called ebucketmigrator is started, which uses the TAP system to request that all the data is transferred for a single vBucket, and that the new vBucket data will become the active vBucket once the migration has been completed.
While the vBucket migration process is taking place, clients are still sending data to the existing vBucket. This information is migrated along with the original data that existed before the migration was requested. Once the migration of the all the data has completed, the original vBucket is marked as disabled, and the new vBucket is enabled. This updates the vBucket map, which is communicated back to the connected clients which will now use the new location.