taegeonum commented on issue #142: [NEMO-257] Local combining only for 
BinaryCombineFn
URL: https://github.com/apache/incubator-nemo/pull/142#issuecomment-435013639
 
 
   `Combiner` should be commutative/associative, and thus local combining 
should be applied for all combiners. Implementing the commutative/associative 
combiners is user's responsibility.
   
   https://beam.apache.org/documentation/programming-guide/#combine
   
   > When you apply a Combine transform, you must provide the function that 
contains the logic for combining the elements or values. **The combining 
function should be commutative and associative**, as the function is not 
necessarily invoked exactly once on all values with a given key. Because the 
input data (including the value collection) may be distributed across multiple 
workers, the combining function might be called multiple times to perform 
partial combining on subsets of the value collection. The Beam SDK also 
provides some pre-built combine functions for common numeric combination 
operations such as sum, min, and max
   

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