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From "Aleksey Yeschenko (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-13880) Fix short read protection for tables with single columns PKs
Date Mon, 18 Sep 2017 16:36:00 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-13880?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16170260#comment-16170260
] 

Aleksey Yeschenko commented on CASSANDRA-13880:
-----------------------------------------------

Branches with fixes: [3.0|https://github.com/iamaleksey/cassandra/commits/13880-3.0], [3.11|https://github.com/iamaleksey/cassandra/commits/13880-3.11],
[4.0|https://github.com/iamaleksey/cassandra/commits/13880-4.0]. An isolated dtest [here|https://github.com/iamaleksey/cassandra-dtest/commits/13880].

The issue only triggers with a per partition limit set, doesn't with a regular limit. I am
investigating why, but whatever the reason is, this fix is wanted/needed anyway - it does
save us a pointless roundtrip in many scenarios.

> Fix short read protection for tables with single columns PKs
> ------------------------------------------------------------
>
>                 Key: CASSANDRA-13880
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-13880
>             Project: Cassandra
>          Issue Type: Bug
>          Components: Coordination
>            Reporter: Aleksey Yeschenko
>            Assignee: Aleksey Yeschenko
>            Priority: Critical
>             Fix For: 3.0.x, 3.11.x
>
>
> CASSANDRA-12872 fixed counting replica rows, so that we do now fetch more than one extra
row if necessary.
> Fixing the issue caused consistency_test.py:TestConsistency.test_13747 to start failing,
by exposing a bug in the way we handle empty clusterings.
> When {{moreContents()}} asks for another row and {{lastClustering}} is {{EMPTY}}, the
response again (and again) contains the row with {{EMPTY}} clustering.
> SRP assumes it’s a new row, counts it as one, gets confused and keeps asking for more,
in a loop, again and again.
> Arguably, a response to a read command with the following non-inclusive {{ClusteringIndexFilter}}:
> {code}
> command.clusteringIndexFilter(partitionKey).forPaging(metadata.comparator, Clustering.EMPTY,
false);
> {code}
> ... should return nothing at all rather than a row with an empty clustering.
> Also arguably, SRP should not even attempt to fetch more rows if {{lastClustering ==
Clustering.EMPTY}}. In a partition key only column
> we shouldn’t expect any more rows.
> This JIRA is to fix the latter issue on SRP side - to modify SRP logic to short-circuit
execution if {{lastClustering}} was an {{EMPTY}} one instead of querying pointlessly for non-existent
extra rows.



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