Load balancer will recognize decommissioning nodes and will
move tablet replicas away from such nodes with highest priority.
Topology changes have now an extra step called "tablet draining" which
calls the load balancer. The step will execute tablet migration track
as long as there are nodes which require draining. It will not do regular
load balancing.
If load balancer is unable to find new tablet replicas, because RF
cannot be met or availability is at risk due to insufficient node
distribution in racks, it will throw an exception. Currently, topology
change will retry in a loop. We should make this error cause topology
change to be aborted. There is no infrastructure for
aborts yet, so this is not implemented.
Closes#15197
* github.com:scylladb/scylladb:
tablets, raft topology: Add support for decommission with tablets
tablet_allocator: Compute load sketch lazily
tablet_allocator: Set node id correctly
tablet_allocator: Make migration_plan a class
tablets: Implement cleanup step
storage_service, tablets: Prevent stale RPCs from running beyond their stage
locator: Introduce tablet_metadata_guard
locator, replica: Add a way to wait for table's effective_replication_map change
storage_service, tablets: Extract do_tablet_operation() from stream_tablet()
raft topology: Add break in the final case clause
raft topology: Fix SIGSEGV when trace-level logging is enabled
raft topology: Set node state in topology
raft topology: Always set host id in topology
a3d73bfba7