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From "Gary Helmling (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-11292) Add an "undelete" operation
Date Thu, 28 Aug 2014 22:11:11 GMT

    [ https://issues.apache.org/jira/browse/HBASE-11292?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14114463#comment-14114463
] 

Gary Helmling commented on HBASE-11292:
---------------------------------------

bq. Yet another way to look at it is: why undelete at all? Ignoring failed transactions needs
to be implemented anyway, so instead of undo one could just continue to ignore the transaction.

Yes, for the transaction case specifically, that is a fair approach to take.  We can (and
do) ignore writes from the failed transaction, but we also need a way to clear transaction
IDs from the invalid set so that it doesn't continue to grow.  This is difficult, but it does
need to be solved, regardless of whether or not we have undeletes.  If we can do this efficiently
enough, then there's really no further need to attempt to rollback failed transactions at
all.

However, from the HBase perspective, it has been a long standing issue (or constraint at least)
that deletes cannot be undone.  Just ordering operations by seqid so that you can redo any
deleted puts is not a symmetric solution, for the reasons described above (in the case of
a single row delete marker, you have to rewrite all of the columns in the row).

> Add an "undelete" operation
> ---------------------------
>
>                 Key: HBASE-11292
>                 URL: https://issues.apache.org/jira/browse/HBASE-11292
>             Project: HBase
>          Issue Type: New Feature
>          Components: Deletes
>            Reporter: Gary Helmling
>              Labels: Phoenix
>
> While column families can be configured to keep deleted cells (allowing time range queries
to still retrieve those cells), deletes are still somewhat unique in that they are irreversible
operations.  Once a delete has been issued on a cell, the only way to "undelete" it is to
rewrite the data with a timestamp newer than the delete.
> The idea here is to add an "undelete" operation, that would make it possible to cancel
a previous delete.  An undelete operation will be similar to a delete, in that it will be
written as a marker ("tombstone" doesn't seem like the right word).  The undelete marker,
however, will sort prior to a delete marker, canceling the effect of any following delete.
> In the absence of a column family configured to KEEP_DELETED_CELLS, we can't be sure
if a prior delete marker and the effected cells have already been garbage collected.  In this
case (column family not configured with KEEP_DELETED_CELLS) it may be necessary for the server
to reject undelete operations to avoid creating the appearance of a client contact for undeletes
that can't reliably be honored.
> I think there are additional subtleties of the implementation to be worked out, but I'm
also interested in a broader discussion of interest in this capability.



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