[hbase] HStore#get and HStore#getFull may not return expected values by
timestamp when there is more than one MapFile
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Key: HADOOP-2513
URL: https://issues.apache.org/jira/browse/HADOOP-2513
Project: Hadoop
Issue Type: Bug
Components: contrib/hbase
Reporter: Bryan Duxbury
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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