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From "Benedict (JIRA)" <>
Subject [jira] [Created] (CASSANDRA-14592) Reconcile should not be dependent on nowInSec
Date Wed, 25 Jul 2018 16:05:00 GMT
Benedict created CASSANDRA-14592:

             Summary: Reconcile should not be dependent on nowInSec
                 Key: CASSANDRA-14592
             Project: Cassandra
          Issue Type: Bug
            Reporter: Benedict
            Assignee: Benedict
             Fix For: 4.0

To have the arrival time of a mutation on a replica determine the reconciliation priority
seems to provide for unintuitive database behaviour.  It seems we should formalise our reconciliation
logic in a manner that does not depend on this, and modify our internal APIs to prevent this
Take the following example, where both writes have the same timestamp:
Write X with a value A, TTL of 1s
Write Y with a value B, no TTL
If X and Y arrive on replicas in < 1s, X and Y are both live, so record Y wins the reconciliation. 
The value B appears in the database.
However, if X and Y arrive on replicas in > 1s, X is now (effectively) a tombstone.  This
wins the reconciliation race, and NO value is the result.
Note that the weirdness of this is more pronounced than it might first appear.  If write
X gets stuck in hints for a period on the coordinator to one replica, the value B appears
in the database until the hint is replayed.  So now we’re in a very uncertain state - will
hints get replayed or not?  If they do, the value B will disappear; if they don’t it won’t. 
This is despite a QUORUM of replicas ACKing both writes, and a QUORUM of readers being engaged
on read; the database still changes state to the user suddenly at some arbitrary future point
in time.
It seems to me that a simple solution to this, is to permit TTL’d data to always win a reconciliation
against non-TTL’d data (of same timestamp), so that we are consistent across TTLs being
transformed into tombstones.
4.0 seems like a good opportunity to fix this behaviour, and mention in CHANGES.txt.

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