[jira] [Commented] (ACCUMULO-2232) Combiners can cause deleted data to come back

["Josh Elser (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/ACCUMULO-2232?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13877638#comment-13877638 ] 

Josh Elser commented on ACCUMULO-2232:
--------------------------------------

I've seen this one happen before, and it can be rather surprising.

Definitely on the documentation. Interesting idea about usage on full MajC.

> Combiners can cause deleted data to come back
> ---------------------------------------------
>
>                 Key: ACCUMULO-2232
>                 URL: https://issues.apache.org/jira/browse/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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