FAQ: Why do I see outgoing migrations on a new node that is being introducted to the cluster?
When introducing a new node to a cluster, I see that it not only has incoming partitions to be migrated, but also a few outgoing partitions to be migrated. As this node is empty, why does it have outgoing partitions?
This will usually occur when the prefer-uniform-balance option is enabled on a namespace (which is a configurable option from version 4.3 and is enabled by default on versions 4.7 onwards), but can also be seen with prefer-uniform-balance disabled and introducing more than one node to the cluster at the same time. This situation arises when the primary copy of the partition is being moved to the newly introduced node and a prole copy of the partition is also scheduled to be moved to another node. In this instance, the cluster will first do a fill migration of the primary partition to the new node, before then migrating this partition out to one or more prole nodes, as required.
MIGRATIONS PARTITIONS OUTGOING PREFER-UNIFORM-BALANCE