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https://issues.apache.org/jira/browse/CASSANDRA-9753?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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C. Scott Andreas updated CASSANDRA-9753:
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Component/s: Coordination
> 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: Coordination
> Reporter: Richard Low
> Priority: Major
>
> 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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