Rohini Palaniswamy created PIG-5041:

             Summary: RoundRobinPartitioner is not deterministic when order of 
input records change
                 Key: PIG-5041
             Project: Pig
          Issue Type: Bug
            Reporter: Rohini Palaniswamy
            Assignee: Rohini Palaniswamy
            Priority: Critical
             Fix For: 0.16.1

Maps can be rerun due to shuffle fetch failures. Half of the reducers can end 
up successfully pulling partitions from first run of the map while other half 
could pull from the rerun after shuffle fetch failures. If the data is not 
partitioned by the Partitioner exactly the same way every time then it could 
lead to incorrect results (loss of records and duplicated records). 

There is a good probability of order of input records changing
    - With OrderedGroupedMergedKVInput (shuffle input), they keys are sorted 
but values can be in any order as the shuffle and merge depends on the order in 
which inputs are fetched. Anything involving FLATTEN can produce different 
order of output records.
    - With UnorderedKVInput, the records could be in any order depending on 
order of shuffle fetch. 

RoundRobinPartitioner can partition records differently everytime as order of 
input records change which is very bad. We need to get rid of 
RoundRobinPartitioner. Since the key is empty whenever we use  
RoundRobinPartitioner we need to partitioning based on hashcode of values to 
produce consistent partitioning. It adds a lot of performance overhead, but 
required for correctness.

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