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From "Bryan Duxbury (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HBASE-29) [hbase] HStore#get and HStore#getFull may not return expected values by timestamp when there is more than one MapFile
Date Mon, 24 Mar 2008 04:23:24 GMT

    [ https://issues.apache.org/jira/browse/HBASE-29?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12581465#action_12581465

Bryan Duxbury commented on HBASE-29:

I think we should make it a priority to get this fixed. Even if it performs worse, it's really
unacceptable to give incorrect answers.

However, I think there's a decent alternative to just getting slower wholesale. When we fixed
getClosestBefore, we decided that the assumption would always be that getClosestRowBefore
had to operate on a table where cells were always being added in ascending timestamp order,
which at least made it perform acceptably. Clearly, this issue is about situations where that
assumption isn't true. So, what I think we should do is make the default get and getRow methods
return the answer that assumes the mapfiles don't have any inherent ordering, and then make
new methods getAscending and getRowAscending (names could change?) that assume mapfiles are
sorted ascending, and are faster as a result. 

With this approach, people can make the default choice of using get and getRow, pay the performance
penalty, but get the right answer no matter what. Then, if people happen to have a use case
that matches the always-ascending constraints, then they can just switch the method call fractionally
and get the improved performance.

> [hbase] HStore#get and HStore#getFull may not return expected values by timestamp when
there is more than one MapFile
> ---------------------------------------------------------------------------------------------------------------------
>                 Key: HBASE-29
>                 URL: https://issues.apache.org/jira/browse/HBASE-29
>             Project: Hadoop HBase
>          Issue Type: Bug
>          Components: client, regionserver
>            Reporter: Bryan Duxbury
>            Assignee: Bryan Duxbury
>             Fix For: 0.2.0
>         Attachments: 29.patch
> Ok, this one is a little tricky. Let's say that you write a row with some value without
a timestamp, thus meaning right now. Then, the memcache gets flushed out to a MapFile. Then,
you write another value to the same row, this time with a timestamp that is in the past, ie,
before the "now" timestamp of the first put. 
> Some time later, but before there is a compaction, if you do a get for this row, and
only ask for a single version, you will logically be expecting the latest version of the cell,
which you would assume would be the one written at "now" time. Instead, you will get the value
written into the "past" cell, because even though it is tagged as having happened in the past,
it actually *was written* after the "now" cell, and thus when #get searches for satisfying
values, it runs into the one most recently written first. 
> The result of this problem is inconsistent data results. Note that this problem only
ever exists when there's an uncompacted HStore, because during compaction, these cells will
all get sorted into the correct order by timestamp and such. In a way, this actually makes
the problem worse, because then you could easily get inconsistent results from HBase about
the same (unchanged) row depending on whether there's been a flush/compaction.
> The only solution I can think of for this problem at the moment is to scan all the MapFiles
and Memcache for possible results, sort them, and then select the desired number of versions
off of the top. This is unfortunate because it means you never get the snazzy shortcircuit
logic except within a single mapfile or memcache. 

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