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From "He Yongqiang (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-5368) more user control on customized RecordReader
Date Sat, 16 May 2009 00:41:45 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-5368?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12710040#action_12710040

He Yongqiang commented on HADOOP-5368:

One possible solution:
1) Splitting TrackedRecordReader out of MapTask, and add in a public method getRawRecordReader()
to return the underlying recordreader.
User can cast the passed in RecordReader into TrackedRecordReader in their MapRunnable's run()
2) Since Hadoop introduced a new set of MR API, we also need to add in a public getRecordReader
method in class MapContext.

BTW, this is not a specific need, Prasad Chakka also talked a problem related to this:

I am currently using getPos() method of RecordReader to get the current
position of the row and use it in indexing. HADOOP-1230 removed this method
in the new RecordReader class in org.apache.hadoop.mapreduce package. I
didn't find any explicit reason for this in the JIRA. Are there any concerns
to adding this method back and possibly something like seek(pos)?


Besides getPos(), I think the seek function maybe also useful for user. I do not know if there
is a reason why current implementation hide RecordReader so well. 

> 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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