Richard Low created CASSANDRA-9753:
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Summary: LOCAL_QUORUM reads can block cross-DC if there is a
digest mismatch
Key: CASSANDRA-9753
URL: https://issues.apache.org/jira/browse/CASSANDRA-9753
Project: Cassandra
Issue Type: Bug
Components: Core
Reporter: Richard Low
When there is a digest mismatch during the initial read, a data read request is
sent to all replicas involved in the initial read. This can be more than the
initial blockFor if read repair was done and if speculative retry kicked in.
E.g. for RF 3 in two DCs, the number of reads could be 4: 2 for LOCAL_QUORUM, 1
for read repair and 1 for speculative read if one replica was slow. If there is
then a digest mismatch, Cassandra will issue the data read to all 4 and set
blockFor=4. Now the read query is blocked on cross-DC latency. The digest
mismatch read blockFor should be capped at RF for the local DC when using
CL.LOCAL_*.
You can reproduce this behaviour by creating a keyspace with
NetworkTopologyStrategy, RF 3 per DC, dc_local_read_repair=1.0 and ALWAYS for
speculative read. If you force a digest mismatch (e.g. by deleting a replicas
SSTables and restarting) you can see in tracing that it is blocking for 4
responses.
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