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From Andrés de la Peña (JIRA) <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-8272) 2ndary indexes can return stale data
Date Mon, 24 Apr 2017 16:37:05 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-8272?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15981476#comment-15981476
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Andrés de la Peña commented on CASSANDRA-8272:
----------------------------------------------

If we send a {{RangeTombstoneMarker}} each time we find a deleted index entry, the coordinator
will be able to discard the false positives returned the stale node. The problem is that read
repair will send back the tombstones to the nodes, corrupting not only the index but also
the indexed table. Possible solutions could be to disable read repair for index queries or
sending a new type of tombstone that read repair would ignore. 

As an alternative solution, the index could return also the rows pointed by the deleted index
entries, without any information about the staleness of the index entries, and use {{Index.postProcessorFor(ReadCommand)}}
to discard those rows that doesn't satisfy the index expression after reconcilliation. This
would solve the consistency problem without any changes in read repair, or in the coordinator
in general. The downside is that we should read in the base table, and possibly send, all
the rows pointed by deleted index entries satisfying the expression since last gc. 

I'm working in this last approach here:

||[3.0|https://github.com/apache/cassandra/compare/cassandra-3.0...adelapena:8272-3.0]|[utests|http://cassci.datastax.com/view/Dev/view/adelapena/job/adelapena-8272-3.0-testall/]|[dtests|http://cassci.datastax.com/view/Dev/view/adelapena/job/adelapena-8272-3.0-dtest/]|

The patch is still uncomplete, I'm posting it just to illustrate the approach. I have not
yet added dtests for the scenario described by this ticket, although I've tried it manually,
and existing dtests pass. There are many unit tests failing because {{CQLTester}} relies on
{{QueryProcessor.executeInternal}}, that doesn't use {{Index.postProcessorFor(ReadCommand)}},
and I guess it should do it.

> 2ndary indexes can return stale data
> ------------------------------------
>
>                 Key: CASSANDRA-8272
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-8272
>             Project: Cassandra
>          Issue Type: Bug
>            Reporter: Sylvain Lebresne
>            Assignee: Andrés de la Peña
>             Fix For: 2.1.x
>
>
> When replica return 2ndary index results, it's possible for a single replica to return
a stale result and that result will be sent back to the user, potentially failing the CL contract.
> For instance, consider 3 replicas A, B and C, and the following situation:
> {noformat}
> CREATE TABLE test (k int PRIMARY KEY, v text);
> CREATE INDEX ON test(v);
> INSERT INTO test(k, v) VALUES (0, 'foo');
> {noformat}
> with every replica up to date. Now, suppose that the following queries are done at {{QUORUM}}:
> {noformat}
> UPDATE test SET v = 'bar' WHERE k = 0;
> SELECT * FROM test WHERE v = 'foo';
> {noformat}
> then, if A and B acknowledge the insert but C respond to the read before having applied
the insert, then the now stale result will be returned (since C will return it and A or B
will return nothing).
> A potential solution would be that when we read a tombstone in the index (and provided
we make the index inherit the gcGrace of it's parent CF), instead of skipping that tombstone,
we'd insert in the result a corresponding range tombstone.  



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