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From "he yongqiang (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-5368) more user control on customized RecordReader
Date Tue, 10 Mar 2009 14:50:51 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-5368?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12680491#action_12680491

he yongqiang commented on HADOOP-5368:

Actually the FileSplit solution does not apply for everywhere, sometimes we can not determine
exactly how to process a file according its path. Currently the FileSplit solution complicates
our code a lot. In our simple situation there are many input files for maps, and these input
files are further splited into groups,  by the group we can determine how to process a file.
Sometimes we donot know in advance which group a file belongs to, so we have to find other
ways to determine its group rather than by looking at its path.

Any comments are appreciated!

> 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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