Currently, when attempting to send a hint, we might choose its
recipients in one of two ways:
- If the original destination is a natural endpoint of the hint, we only
send the hint to that node and none other,
- Otherwise, we send the hint to all current replicas of the mutation.
There is a problem when we decommission a node: while data is streamed
away from that node, it is still considered to be a natural endpoint of
the data that it used to own. Because of that, it might happen that a
hint is sent directly to it but streaming will miss it, effectively
resulting in the hint being discarded.
As sending the hint _only_ to the leaving replica is a rather bad idea,
send the hint to all replicas also in the case when the original
destiantion of the hint is leaving.
Note that this is a conservative fix written only with the decommission
+ vnode-based keyspaces combo in mind. In general, such "data loss" can
occur in other situations where the replica set is changing and we go
through a streaming phase, i.e. other topology operations in case of
vnodes and tablet load balancing. However, the consistency guarantees of
hinted handoff in the face of topology changes are not defined and it is
not clear what they should be, if there should be any at all. The
picture is further complicated by the fact that hints are used by
materialized views, and sending view updates to more replicas than
necessary can introduce inconsistencies in the form of "ghost rows".
This fix was developed in response to a failing test which checked the
hint replay + decommission scenario, and it makes it work again.
Fixesscylladb/scylla-dtest#4582
Refs scylladb/scylladb#19835
61ac0a336d