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From "Richard Low (JIRA)" <j...@apache.org>
Subject [jira] [Created] (CASSANDRA-9753) LOCAL_QUORUM reads can block cross-DC if there is a digest mismatch
Date Tue, 07 Jul 2015 21:27:04 GMT
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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