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Date: Fri, 15 Jan 2016 23:22:39 +0000 (UTC)
From: "DOAN DuyHai (JIRA)" <jira@apache.org>
To: commits@cassandra.apache.org
Message-ID: <JIRA.12931603.1452900134000.127123.1452900159872@Atlassian.JIRA>
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Subject: [jira] [Created] (CASSANDRA-11023) Restrict mutation consistency
 level to QUORUM+ for LWT
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DOAN DuyHai created CASSANDRA-11023:
---------------------------------------

             Summary: Restrict mutation consistency level to QUORUM+ for LW=
T
                 Key: CASSANDRA-11023
                 URL: https://issues.apache.org/jira/browse/CASSANDRA-11023
             Project: Cassandra
          Issue Type: Bug
          Components: Coordination
            Reporter: DOAN DuyHai


5 nodes cluster, RF =3D 3

=E2=86=92 LWT consistency =3D SERIAL
=E2=86=92 Mutation consistency =3D ONE

1. Client 1 sends a INSERT INTO test (partition_key, value)  VALUES(*1*, *=
=E2=80=98ONE=E2=80=99*) IF NOT EXISTS
2. Paxos round successfully gets ballot1
3. The IF NOT EXISTS condition is validated (reading with QUORUM on all rep=
licas)
4. Paxos round validated
5. Mutation applied using consistency level ONE. Mutation pushed to replica=
s A, B and C=20
6. Replica A sends back acks. *Replicas B and C did not receive the mutatio=
n* (temporary network issue). The LWT operation is successful since we do n=
ot wait for acks from B and C
This LWT is considered successful
7. Client 2 starts a LWT : INSERT INTO test (partition_key, value)  VALUES(=
*1*, *=E2=80=98TWO=E2=80=99*) IF NOT EXISTS
8. Paxos round successfully gets ballot2 (ballot2 > ballot1)
9. The IF NOT EXISTS condition is validated (*reading with QUORUM and repli=
ca B and C reply that partition 1 does not exist*)
10. The semantics of LWT is violated because indeed the partition already e=
xists (because first LWT succeeded) with value =3D =E2=80=98ONE=E2=80=99

 I'm not saying that there is a bug in our LWT implementation, it works as =
designed. The problem here is that I can't see any legit/sensible use-case =
for LWT with mutation CL=3DONE since it opens the door for edge cases like =
the one described above and it defeats the purpose of Compare And Swap. Fur=
thermore it can confuse a lot of people not familiar of how LWT is implemen=
ted internally.

 Ideally, we should remove/ignore mutation CL for LWT and always use QUORUM=
/LOCAL_QUORUM. But for the sake of not breaking the existing API, it would =
be sufficient enough to validate and reject mutation CL < QUORUM.


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