[
https://issues.apache.org/jira/browse/MRUNIT-165?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]
Yoni Ben-Meshulam updated MRUNIT-165:
-------------------------------------
Description:
MapReduceDriver calls the Mapper#run method for each input, causing the
Mapper#cleanup method to be called multiple times.
I believe this is a bug, since the contract in MapReduce is that, for a single
Mapper instance, the Mapper#cleanup method is only called once after all inputs
to that mapper have been processed. I might be mistaken in my assumption here.
This would not be an issue, were it not for the fact that MapReduceDriver has
only a single instance of Mapper.
One solution might be to pass the Mapper _class_ into the MapReduceDriver and
create a new instance for each input. Another solution might be to call the
MapDriver with multiple inputs (which AFAIK is not possible).
----
To reproduce, create a MapReduce job with some stateful mapper:
{code}
public class ClosedFormRegressionMapper extends Mapper<LongWritable, Text,
Text, IntWritable> {
public static final Text KEY = new Text("SomeKey");
private Int someState = 0;
/**
* Increment someState for each input.
*
* @param context the Hadoop job Map context
* @throws java.io.IOException
*/
@Override
public void map(
LongWritable key,
Text value,
Context context
) throws IOException, InterruptedException {
this.someState += 1;
}
/**
* Runs once after all maps have occurred. Dumps the accumulated state to
the output.
* @param context the Hadoop job Map context
*/
@Override
protected void cleanup(Context context) throws IOException,
InterruptedException {
context.write(this.KEY, new IntWritable(this.someState));
}
}
{code}
was:
MapReduceDriver calls the Mapper#run method for each input, causing the
Mapper#cleanup method to be called multiple times.
I believe this is a bug, since the contract in MapReduce is that, for a single
Mapper instance, the Mapper#cleanup method is only called once after all inputs
to that mapper have been processed. I might be mistaken in my assumption here.
This would not be an issue, were it not for the fact that MapReduceDriver has
only a single instance of Mapper.
One solution might be to pass the Mapper _class_ into the MapReduceDriver and
create a new instance for each input. Another solution might be to call the
MapDriver with multiple inputs (which AFAIK is not possible).
----
To reproduce, create a MapReduce job with some stateful mapper:
{code}
public class ClosedFormRegressionMapper extends Mapper<LongWritable, Text,
Text, IntWritable> {
public static final Text KEY = new Text("SomeKey");
private Int someState = 0;
/**
* Increment someState for each input.
*
* @param context the Hadoop job Map context
* @throws java.io.IOException
*/
@Override
public void map(
LongWritable key,
Text value,
Context context
) throws IOException, InterruptedException {
this.someState += 1;
}
/**
* Runs once after all maps have occurred. Dumps the accumulated state to
the output.
* @param context the Hadoop job Map context
*/
@Override
protected void cleanup(Context context) throws IOException,
InterruptedException {
context.write(this.KEY, new IntWritable(someState));
}
}
{code}
> MapReduceDriver calls Mapper#cleanup for each input instead of once
> -------------------------------------------------------------------
>
> Key: MRUNIT-165
> URL: https://issues.apache.org/jira/browse/MRUNIT-165
> Project: MRUnit
> Issue Type: Bug
> Affects Versions: 0.9.0
> Reporter: Yoni Ben-Meshulam
>
> MapReduceDriver calls the Mapper#run method for each input, causing the
> Mapper#cleanup method to be called multiple times.
> I believe this is a bug, since the contract in MapReduce is that, for a
> single Mapper instance, the Mapper#cleanup method is only called once after
> all inputs to that mapper have been processed. I might be mistaken in my
> assumption here.
> This would not be an issue, were it not for the fact that MapReduceDriver has
> only a single instance of Mapper.
> One solution might be to pass the Mapper _class_ into the MapReduceDriver and
> create a new instance for each input. Another solution might be to call the
> MapDriver with multiple inputs (which AFAIK is not possible).
> ----
> To reproduce, create a MapReduce job with some stateful mapper:
> {code}
> public class ClosedFormRegressionMapper extends Mapper<LongWritable, Text,
> Text, IntWritable> {
> public static final Text KEY = new Text("SomeKey");
> private Int someState = 0;
> /**
> * Increment someState for each input.
> *
> * @param context the Hadoop job Map context
> * @throws java.io.IOException
> */
> @Override
> public void map(
> LongWritable key,
> Text value,
> Context context
> ) throws IOException, InterruptedException {
> this.someState += 1;
> }
> /**
> * Runs once after all maps have occurred. Dumps the accumulated state to
> the output.
> * @param context the Hadoop job Map context
> */
> @Override
> protected void cleanup(Context context) throws IOException,
> InterruptedException {
> context.write(this.KEY, new IntWritable(this.someState));
> }
> }
> {code}
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