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From "Keith Turner (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (ACCUMULO-803) Add Reverse Logical Time as a Time Type
Date Wed, 17 Oct 2012 15:50:02 GMT

    [ https://issues.apache.org/jira/browse/ACCUMULO-803?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13477969#comment-13477969
] 

Keith Turner commented on ACCUMULO-803:
---------------------------------------

bq.  Since Combiners work on a column family, does that imply that we might want a different
timestamp scheme on different column families?

We might also consider setting an option on mutations to reverse system set time.  This gives
users the ability to target this in very tailored ways.  Users could apply it to certain column
family prefixes, to column qualifiers, etc.  It gives a lot of flexibility w/o any server
side code.  Could add something like the following to mutation.

{code:java}
  
enum TimestampTransformations {
   NONE,
   REVERSE
}

class Mutation {
  //transformation that will be applied to system set timestamps
  public void setTimestampTransformation(TimestampTransformations tt);
}


{code}

I think the only drawback with this is that it does not support setting system time for bulk
insert.  The current patch will support that.
                
> Add Reverse Logical Time as a Time Type
> ---------------------------------------
>
>                 Key: ACCUMULO-803
>                 URL: https://issues.apache.org/jira/browse/ACCUMULO-803
>             Project: Accumulo
>          Issue Type: Improvement
>          Components: tserver
>    Affects Versions: 1.5.0
>            Reporter: Drew Farris
>            Assignee: Drew Farris
>            Priority: Minor
>         Attachments: ACCUMULO-803.patch, ACCUMULO-803.patch
>
>
> In a context where we are doing aggregation/combination of multiple values for a given
key it may be useful to iterate over the values associated with that key in the order in which
the mutations were applied (FIFO), instead of the FILO order that seems to occur when using
{{TimeType.LOGICAL}}. 
> I encountered when implemeting a checkAndPut operation that would ensure that the previous
value was expected before putting a new value. In this case, if the previous value was not
as expected, the mutation would be ignored. 
> Perhaps it is useful in a general case?

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