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From "He Yongqiang (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-5368) more user control on customized RecordReader
Date Tue, 14 Apr 2009 05:04:15 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-5368?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12698652#action_12698652
] 

He Yongqiang commented on HADOOP-5368:
--------------------------------------

And now we encounter a new situation that we need to get a split's start row and end. Even
though we use customized MapRunner, FileSplit, InputFormat and RecordReader, but we still
cannot find a way to get these information. 
I think a solution here maybe to give more user control on customized RecordReader. Simply
it can be done by adding a getRawRecordReader() in SkippingRecordReader and TrackedRecordReader.
Or there may be more nice and beautiful solutions.

> more user control on customized RecordReader
> --------------------------------------------
>
>                 Key: HADOOP-5368
>                 URL: https://issues.apache.org/jira/browse/HADOOP-5368
>             Project: Hadoop Core
>          Issue Type: Wish
>            Reporter: He Yongqiang
>
> Currently user can define own InputFormat and RecordReader, but the user has little control
on them. 
> An example, we input mutiple files into the mapper and want to handle them in different
ways depending on which file this mapper is working.
> This can be easily done as follows:
> {code}
>       public class BlockMapRunner implements MapRunnable {
> 	private BlockMapper mapper;
> 	@Override
> 	public void run(RecordReader input, OutputCollector output,
> 			Reporter reporter) throws IOException {
> 		if (mapper == null)
> 			return;
> 		BlockReader blkReader = (BlockReader) input;
> 		this.mapper.initialize(input);
> 		...........
> 	}
> 	@Override
> 	public void configure(JobConf job) {
> 		JobConf work = new JobConf(job);
> 		Class<? extends BlockMapper> mapCls = work.getBlockMapperClass();
> 		if (mapCls != null) {
> 			this.mapper = (BlockMapper) ReflectionUtils
> 					.newInstance(mapCls, job);
> 		}
> 	}
> }
> /*
> BlockMapper implements the Mapper and is initialized from RecordReader, from which we
get which file this mapper is working on and find the right strategy for it.
> */
> public class ExtendedMapper extends BlockMapper {
> 	private Strategy strategy;
> 	private Configuration work;
> 	@Override
> 	public void configure(Configuration job) {
> 		this.work = job;
> 	}
> 	@Override
> 	public void initialize(RecordReader reader) throws IOException {
> 		String path = ((UserDefinedRecordReader) reader).which_File_We_Are_Working_On();  
//((UserDefinedRecordReader) reader) is wrong!
> 		this.strategy = this.work.getStrategy(path);
> 	}
> 	@Override
> 	public void map(Key k, V value, OutputCollector output, Reporter reporter)
> 			throws IOException {
> 		strategy.handle(k,v);
> 	}
> }
> {code}
> {color:red}
> However, the above code does not work. The reader passed into mapper is wrapped by MapTask,
and is either SkippingRecordReader or TrackedRecordReader. We can not cast it back and we
can not pass any information through the user defined RecordReader. If the SkippingRecordReader
and TrackedRecordReader have a method for getting the raw reader, it will not have this problem.
> {color:}
> This problem could be resolved by initiating many map-reduce jobs,one job for each file.
But this apparently is what we want.
> Or there exist other solutions? 
> Appreciated for any comments.

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