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https://issues.apache.org/jira/browse/HADOOP-2921?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12574246#action_12574246
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Joydeep Sen Sarma commented on HADOOP-2921:
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as i mentioned in a different jira - we don't use the key at all - the sort
field is embedded in the value itself. an interface like the partitioner
interface - that takes both the key and the value and returns an object would
do the job for us. (the reader can invoke the equals method to determine the
boundaries).
yeah - we shouldn't change the default semantics of the current reader - either
have an option that alters the semantics or a new reader.
what about text files? we don't use them much directly (always embed in
sequencefiles) - but i imagine other folks do and the same considerations can
apply ..
> align map splits on sorted files with key boundaries
> ----------------------------------------------------
>
> Key: HADOOP-2921
> URL: https://issues.apache.org/jira/browse/HADOOP-2921
> Project: Hadoop Core
> Issue Type: New Feature
> Affects Versions: 0.16.0
> Reporter: Joydeep Sen Sarma
>
> (this is something that we have implemented in the application layer - may be
> useful to have in hadoop itself).
> long term log storage systems often keep data sorted (by some sort-key).
> future computations on such files can often benefit from this sort order. if
> the job requires grouping by the sort-key - then it should be possible to do
> reduction in the map stage itself.
> this is not natively supported by hadoop (except in the degenerate case of 1
> map file per task) since splits can span the sort-key. however aligning the
> data read by the map task to sort key boundaries is straightforward - and
> this would be a useful capability to have in hadoop.
> the definition of the sort key should be left up to the application (it's not
> necessarily the key field in a Sequencefile) through a generic interface -
> but otherwise - the sequencefile and text file readers can use the extracted
> sort key to align map task data with key boundaries.
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