[jira] [Resolved] (MAPREDUCE-6827) Failed to traverse Iterable values the second time in reduce() method

["Daniel Templeton (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/MAPREDUCE-6827?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Daniel Templeton resolved MAPREDUCE-6827.
-----------------------------------------
    Resolution: Not A Problem

That is known, documented, and intended behavior.  The {{ValueIterator}}'s {{hasNext()}} and {{next()}} methods defer defer to the {{ReduceContextImpl}}'s {{BackupStore}} instance, so creating a new iterator won't help.  The reason we only go through the values once is to allow the data to be efficiently streamed.

> Failed to traverse Iterable values the second time in reduce() method
> ---------------------------------------------------------------------
>
>                 Key: MAPREDUCE-6827
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-6827
>             Project: Hadoop Map/Reduce
>          Issue Type: Bug
>          Components: task
>    Affects Versions: 3.0.0-alpha1
>         Environment: hadoop2.7.3
>            Reporter: javaloveme
>
> Failed to traverse Iterable values the second time in reduce() method
> The following code is a reduce() method (of WordCount):
> {code:title=WordCount.java|borderStyle=solid}
> 	public static class WcReducer extends Reducer<Text, IntWritable, Text, IntWritable> {
> 		@Override
> 		protected void reduce(Text key, Iterable<IntWritable> values, Context context)
> 				throws IOException, InterruptedException {
> 			// print some logs
> 			List<String> vals = new LinkedList<>();
> 			for(IntWritable i : values) {
> 				vals.add(i.toString());
> 			}
> 			System.out.println(String.format(">>>> reduce(%s, [%s])",
> 					key, String.join(", ", vals)));
> 			// sum of values
> 			int sum = 0;
> 			for(IntWritable i : values) {
> 				sum += i.get();
> 			}
> 			System.out.println(String.format(">>>> reduced(%s, %s)",
> 					key, sum));
> 			
> 			context.write(key, new IntWritable(sum));
> 		}			
> 	}
> {code}
> After running it, we got the result that all sums were zero!
> After debugging, it was found that the second foreach-loop was not executed, and the root cause was the returned value of Iterable.iterator(), it returned the same instance in the two calls called by foreach-loop. In general, Iterable.iterator() should return a new instance in each call, such as ArrayList.iterator().



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