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Date: Mon, 22 May 2017 19:32:04 +0000 (UTC)
From: "Sylvain Lebresne (JIRA)" <jira@apache.org>
To: commits@cassandra.apache.org
Message-ID: <JIRA.12753516.1415308953000.265639.1495481524294@Atlassian.JIRA>
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Subject: [jira] [Commented] (CASSANDRA-8272) 2ndary indexes can return stale
 data
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archived-at: Mon, 22 May 2017 19:32:11 -0000


    [ https://issues.apache.org/jira/browse/CASSANDRA-8272?page=3Dcom.atlas=
sian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=3D=
16020052#comment-16020052 ]=20

Sylvain Lebresne commented on CASSANDRA-8272:
---------------------------------------------

I can refer you to [my comment on 8273|https://issues.apache.org/jira/brows=
e/CASSANDRA-8273?focusedCommentId=3D16007964&page=3Dcom.atlassian.jira.plug=
in.system.issuetabpanels:comment-tabpanel#comment-16007964] on that aspect =
since it's largely the same aspect, but the short version is that you are r=
ight, but at the same time 1) anyone caring about performance should probab=
ly use token-aware clients and that make such CL.ONE optimization not reall=
y needed, 2) we actually don't ship the consistency with {{ReadCommand}} to=
 replicas, so such optimization cannot be done before 4.0 at best (it requi=
res a protocol change) and 3) the reason why this ticket is "easier" than C=
ASSANDRA-8723 is that we will likely not transfer _that_ much additional ro=
ws: we'll only sent row that use to be valid entries for the query but aren=
't anymore and this since less than gc_grace, which, except maybe for very =
specific cases, is unlikely for represent a huge overhead for any particula=
r query.

Anyway, not really opposing such optimization, but given it's not useful if=
 you follow performance best practices and given it's a tiny bit more invol=
ved than it sounds (and it adds a bit of complexity to the code after all b=
y creating a special case), I'd be happy focusing on correction here and le=
aving that to a follow-up.=20

> 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: Andr=C3=A9s de la Pe=C3=B1a
>             Fix For: 3.0.x
>
>
> When replica return 2ndary index results, it's possible for a single repl=
ica 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 ar=
e done at {{QUORUM}}:
> {noformat}
> UPDATE test SET v =3D 'bar' WHERE k =3D 0;
> SELECT * FROM test WHERE v =3D '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 (sinc=
e 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), ins=
tead of skipping that tombstone, we'd insert in the result a corresponding =
range tombstone. =20


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