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https://issues.apache.org/jira/browse/FLINK-4586?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15582283#comment-15582283
 ] 

ASF GitHub Bot commented on FLINK-4586:
---------------------------------------

Github user StephanEwen commented on a diff in the pull request:

    https://github.com/apache/flink/pull/2639#discussion_r83643344
  
    --- Diff: 
flink-core/src/main/java/org/apache/flink/api/common/accumulators/AverageAccumulator.java
 ---
    @@ -28,51 +28,52 @@
     public class AverageAccumulator implements SimpleAccumulator<Double> {
     
        private static final long serialVersionUID = 3672555084179165255L;
    -   
    -   private double localValue;
    +
        private long count;
     
    +   private double sum;
    +
        @Override
        public void add(Double value) {
                this.count++;
    -           this.localValue += value;
    +           this.sum += value;
        }
     
        public void add(double value) {
                this.count++;
    -           this.localValue += value;
    +           this.sum += value;
        }
     
        public void add(long value) {
                this.count++;
    -           this.localValue += value;
    +           this.sum += value;
        }
     
        public void add(int value) {
                this.count++;
    -           this.localValue += value;
    +           this.sum += value;
        }
     
        @Override
        public Double getLocalValue() {
                if (this.count == 0) {
                        return 0.0;
                }
    -           return this.localValue / (double)this.count;
    +           return this.sum / this.count;
        }
     
        @Override
        public void resetLocal() {
                this.count = 0;
    -           this.localValue = 0;
    +           this.sum = 0;
        }
     
        @Override
        public void merge(Accumulator<Double, Double> other) {
                if (other instanceof AverageAccumulator) {
    -                   AverageAccumulator temp = (AverageAccumulator)other;
    -                   this.count += temp.count;
    -                   this.localValue += other.getLocalValue();
    --- End diff --
    
    I guess this was the buggy line?


> NumberSequenceIterator and Accumulator threading issue
> ------------------------------------------------------
>
>                 Key: FLINK-4586
>                 URL: https://issues.apache.org/jira/browse/FLINK-4586
>             Project: Flink
>          Issue Type: Bug
>          Components: DataSet API
>    Affects Versions: 1.1.2
>            Reporter: Johannes
>            Assignee: Greg Hogan
>            Priority: Minor
>             Fix For: 1.2.0, 1.1.4
>
>         Attachments: FLINK4586Test.scala
>
>
> There is a strange problem when using the NumberSequenceIterator in 
> combination with an AverageAccumulator.
> It seems like the individual accumulators are reinitialized and overwrite 
> parts of intermediate solutions.
> The following scala snippit exemplifies the problem.
> Instead of printing the correct average, the result should be {{50.5}} but is 
> something completely different, like {{8.08}}, dependent on the number of 
> cores used.
> If the parallelism is set to {{1}} the result is correct, which indicates a 
> likely threading problem. 
> The problem occurs using the java and scala API.
> {code}
> env
>   .fromParallelCollection(new NumberSequenceIterator(1, 100))
>   .map(new RichMapFunction[Long, Long] {
>       var a : AverageAccumulator = _
>       override def map(value: Long): Long = {
>         a.add(value)
>         value
>       }
>       override def open(parameters: Configuration): Unit = {
>         a = new AverageAccumulator
>         getRuntimeContext.addAccumulator("test", a)
>       }
>   })
>   .reduce((a, b) => a + b)
>   .print()
> val lastJobExecutionResult: JobExecutionResult = env.getLastJobExecutionResult
> println(lastJobExecutionResult.getAccumulatorResult("test"))
> {code}



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