If a node is dead and cannot be brought back, tablet migrations are
stuck, until the node is explicitly marked as "permanently dead" /
"ignored node" / "excluded" (name differs in different contexts).
Currently, this is done during removenode and replace operations but
it should be possible to only mark the node as dead, for the purpose
of unblocking migrations or other topology operations, without doing
the actual removenode, because full removal might be currently
impossible, or not desirable due to lack of capacity or priorities.
This patch introduces this kind of API:
nodetool excludenode <host-id> [ ... <host-id> ]
Having this kind of API is an improvement in user experience in
several cases. For example, when we lose a rack, the only viable
option for recovery is to run removenode with an extra
--ignore-dead-nodes option. This removenode will fail in the tablet
draining phase, as there is no live node in the rack to rebuild
replicas in. This is confusing to the operator. But necessary before
ALTER KEYSPACE can proceed in order to change replication options to
drop the rack from RF.
Having this API allows operators to have more unified procedures,
where "nodetool excludenode" is always the first step of recovery,
which unblocks further topology operations, both those which restore
capacity, but also auto-scaling, tablet split/merge, load balancing,
etc.
Fixes#21281
55ecd92feb