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From "Tim Broberg (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HADOOP-8003) Make SplitCompressionInputStream an interface instead of an abstract class
Date Fri, 17 Feb 2012 07:29:59 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-8003?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13210118#comment-13210118
] 

Tim Broberg commented on HADOOP-8003:
-------------------------------------

Actually, once I got away from the computer, I saw why seek() is the wrong tool for the job
and option #3 doesn't work. A user of the stream expects seek() to address offsets in the
*decompressed* byte stream whereas LineRecordReader wants to split the *compressed* stream
at block boundaries.

So, it's status quo or switch to return an interface in the most compatible way we can come
up with.

...or get really radical and split into fixed size decompressed chunks rather than compressed,
but that messes up locality, and we don't want that.
                
> Make SplitCompressionInputStream an interface instead of an abstract class
> --------------------------------------------------------------------------
>
>                 Key: HADOOP-8003
>                 URL: https://issues.apache.org/jira/browse/HADOOP-8003
>             Project: Hadoop Common
>          Issue Type: New Feature
>          Components: io
>    Affects Versions: 0.21.0, 0.22.0, 0.23.0, 1.0.0
>            Reporter: Tim Broberg
>
> To be splittable, a codec must extend SplittableCompressionCodec which has a function
returning a SplitCompressionInputStream.
> SplitCompressionInputStream is an abstract class which extends CompressionInputStream,
the lowest level compression stream class.
> So, no codec that wants to be splittable can reuse any code from DecompressorStream or
BlockDecompressorStream.
> You either have to duplicate that code, or not be splittable.
> SplitCompressionInputStream adds just a few very thin functions. Can we make this an
interface rather than an abstract class to allow splittable decompression streams to extend
DecompressorStream, BlockDecompressorStream, or whatever else we should scheme up in the future?
> To my knowledge, this would impact only the BZip2 codec. None of the other implement
this form of splittability yet.
> LineRecordReader looks only at whether the codec is an instance of SplittableCompressionCodec,
and then calls the appropriate version of createInputStream. This would not change, so the
application code should not have to change, just BZip and SplitCompressionInputStream.

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