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From "Pritam Damania (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-2376) Add special SnapshotScanner which presents view of all data at some time in the past
Date Wed, 21 Nov 2012 05:09:58 GMT

    [ https://issues.apache.org/jira/browse/HBASE-2376?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13501704#comment-13501704
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Pritam Damania commented on HBASE-2376:
---------------------------------------

I'm not sure how you would achieve it with TTL and MIN_VERSIONS, but lets take an example,
suppose the current time in millliseconds is 60 and the table has max versions set to 3.

Suppose we want to support FlashBackQueries for upto 10ms in the past. What you want is atmost
3 versions for the time between t=0 to t=50. How does MIN_VERSIONS achieve keeping atmost
3 versions in that time range ? Does MIN_VERSIONS apply only to expired kvs ?

Also an issue with TTL and MIN_VERSIONS is that you cannot support something like if I want
a TTL of 6 days but a FlashBack upto 8 days. The FlashBack and TTL time have to be the same
which some applications might not want. Some applications might want to keep all their other
parameters the same and just specify that they want to do a read back in time for 'x' days.
Changing the TTL value for an application to provide this functionality would also change
what a scan returns since although you are pushing TTL back to retain enough data to do a
read in the past, your queries in the current time are also affected since they will surface
all kvs which are within TTL. 
                
> Add special SnapshotScanner which presents view of all data at some time in the past
> ------------------------------------------------------------------------------------
>
>                 Key: HBASE-2376
>                 URL: https://issues.apache.org/jira/browse/HBASE-2376
>             Project: HBase
>          Issue Type: New Feature
>          Components: Client, regionserver
>    Affects Versions: 0.20.3
>            Reporter: Jonathan Gray
>            Assignee: Pritam Damania
>
> In order to support a particular kind of database "snapshot" feature which doesn't require
copying data, we came up with the idea for a special SnapshotScanner that would present a
view of your data at some point in the past.  The primary use case for this would be to be
able to recover particular data/rows (but not all data, like a global rollback) should they
have somehow been messed up (application fault, application bug, user error, etc.).

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