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From Grzegorz Gunia <sawt...@student.agh.edu.pl>
Subject Re: CompressionCodec in MapReduce
Date Wed, 11 Apr 2012 09:23:36 GMT
I think we misunderstood here.

I'll base my question upon an example:
Lets say I want each of the files stored on my hdfs to be encrypted 
prior to being physically stored on the cluster.
For that I'll write a custom CompressionCodec, that performs the 
encryption, and use it during any edits/creations of files in the HDFS.
Then to make it more secure I'll make it so it uses different keys for 
different files, and supply the keys to the codec during its instantiation.

Now I'd like to do a MapReduce job on those files. That would require 
instantiating the codec, and supplying it with the filename, to 
determine the key used. Is it possible to do so with the current 
implementation of Hadoop?


W dniu 2012-04-11 10:44, Zizon Qiu pisze:
> If your are:
> 1. using TextInputFormat.
> 2.all input files are ends with certain suffix like ".gz"
> 3.the custom CompressionCodec already register  in configuration and 
> getDefaultExtension return the same suffix like as describe in 2.
> the nothing else you need to do.
> hadoop will deal with it automatically.
> that means the input key&value in map method are already decompress.
> But,if the origin files dose not end with certain suffix,you need 
> to write your own inputformat or subclass TextInputFormat , override 
> the createRecordReader method which return your own RecordReader.
> the InputSplit pass to the InputFormat is actually 
> FileInputSplit,which you can retrieve the input file path.
> you may also take a look at the isSplitable method declared 
> in InputFormat,if your files are not splitable.
> for more detail,refer to the TextInputFormat class implementation.
> On Wed, Apr 11, 2012 at 4:16 PM, Grzegorz Gunia 
> <sawtyss@student.agh.edu.pl <mailto:sawtyss@student.agh.edu.pl>> wrote:
>     Thanks for you reply! That clears some thing up
>     There is but one problem... My CompressionCodec has to be
>     instantiated on a per-file basis, meaning it needs to know the
>     name of the file it is to compress/decompress. I'm guessing that
>     would not be possible with the current implementation?
>     Or if so, how would I proceed with injecting it with the file name?
>     --
>     Greg
>     W dniu 2012-04-11 10:12, Zizon Qiu pisze:
>>     append your custom codec full class name in
>>     "io.compression.codecs" either in mapred-site.xml or in the
>>     configuration object pass to Job constructor.
>>     the map reduce framework will try to guess the compress algorithm
>>     using the input files suffix.
>>     if any CompressionCodec.getDefaultExtension() register in the
>>     configuration match the suffix,hadoop will try to instantiate the
>>     codec and decompress for you ,if succeed,automatically.
>>     the default value for "io.compression.codecs" is
>>     "org.apache.hadoop.io.compress.DefaultCodec,org.apache.hadoop.io.compress.GzipCodec,org.apache.hadoop.io.compress.BZip2Codec"
>>     On Wed, Apr 11, 2012 at 3:55 PM, Grzegorz Gunia
>>     <sawtyss@student.agh.edu.pl <mailto:sawtyss@student.agh.edu.pl>>
>>     wrote:
>>         Hello,
>>         I am trying to apply a custom CompressionCodec to work with
>>         MapReduce jobs, but I haven't found a way to inject it during
>>         the reading of input data, or during the write of the job
>>         results.
>>         Am I missing something, or is there no support for compressed
>>         files in the filesystem?
>>         I am well aware of how to set it up to work during the
>>         intermitent phases of the MapReduce operation, but I just
>>         can't find a way to apply it BEFORE the job takes place...
>>         Is there any other way except simply uncompressing the files
>>         I need prior to scheduling a job?
>>         Huge thanks for any help you can give me!
>>         --
>>         Greg

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