If one uses a combinable reduce operation which also changes the key value of the underlying data element, then the results of the reduce operation can become wrong. The reason is that after the combine phase, another reduce operator is executed which will then reduce the elements based on the new key values. This might be not so surprising if one explicitly defined ones GroupReduceOperation as combinable. However, the ReduceFunction conceals the fact that a combiner is used implicitly. Furthermore, the API does not prevent the user from changing the key fields which could solve the problem.
The following example program illustrates the problem
The expected output is
However, the actual output is
I think that the underlying problem is that associativity and commutativity is not sufficient for a combinable reduce operation. Additionally we also need to make sure that the key stays the same.