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From "John Vines (JIRA)" <>
Subject [jira] [Created] (ACCUMULO-2232) Combiners can cause deleted data to come back
Date Tue, 21 Jan 2014 17:22:21 GMT
John Vines created ACCUMULO-2232:

             Summary: Combiners can cause deleted data to come back
                 Key: ACCUMULO-2232
             Project: Accumulo
          Issue Type: Bug
          Components: client, tserver
            Reporter: John Vines

The case-
3 files with-
* 1 with a key, k, with timestamp 0, value 3
* 1 with a delete of k with timestamp 1
* 1 with k with timestamp 2, value 2

The column of k has a summing combiner set on it. The issue here is that depending on how
the major compactions play out, differing values with result. If all 3 files compact, the
correct value of 2 will result. However, if 1 & 3 compact first, they will aggregate to
5. And then the delete will fall after the combined value, resulting in the result 5 to persist.

First and foremost, this should be documented. I think to remedy this, combiners should only
be used on full MajC, not not full ones. This may necessitate a special flag or a new combiner
that implemented the proper semantics.

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