Benedict created CASSANDRA-14768:
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Summary: Transient Replication: Consistency Level Semantics
Key: CASSANDRA-14768
URL: https://issues.apache.org/jira/browse/CASSANDRA-14768
Project: Cassandra
Issue Type: Improvement
Components: Coordination
Reporter: Benedict
Fix For: 4.0
For a keyspace without transient replication, we will always attempt (and write
hints for) all logical endpoints, including those that seem to be alive but are
not responding (or perhaps dropping some messages). With transient
replication, in this scenario we only write to the transient replicas if a
certain period of time elapses _and we have not met our consistency level_.
This doesn’t lead to the same logical behaviour, although technically the
guarantees are the same. In the past, you could expect that all DCs would
reach their own local quorum promptly, if say only a single node is failing.
Now, you could reach QUORUM with only one DC + 1 remote node, and the remote DC
will stay out of whack until repair runs. This is even worse for e.g.
LOCAL_\{QUORUM,ONE\}.
While the guarantees of the system are the same, the actual behaviour is
suboptimal - while the coordinator and remote DCs are healthy, in my opinion we
should do our best to ensure each DC reaches its own quorum, just as a normal
write would.
This probably entails having our write callback handle failure to not only
write a hint for the endpoint, but also decide if a mutation should immediately
be sent to a corresponding transient replica.
At the very least, we should discuss this before 4.0, even if we opt to take no
action before 4.x.
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