Github user vasia commented on the pull request:

    https://github.com/apache/flink/pull/576#issuecomment-90749788
  
    Thanks for the PR @andralungu!
    
    The idea for this issue is to (a) use a combinable reduce for the simple 
(without vertex value) reduceOn* methods, because it will be more efficient and 
(b) offer groupReduceOn* versions with Collectors to allow returning more than 
one value per vertex.
    I see you have implemented (b) here, but not (a).
    
    To be more specific, the groupReduceOn* methods would be the ones with the 
Collectors. Basically, you'll just have to rename what you have already 
implemented. The reduceOn* methods will only return one value per vertex, like 
they do currently, but internally use the reduce operator (instead of 
reduceGroup).
    
    For example, if you want to compute the sum of edge values, you can use the 
`reduceOnEdges` method, same as you currently do. And if you want to compute 
the Jaccard coefficient, you need to output multiple values per vertex and for 
that you can use `groupReduceOnEdges`.
    
    Please let me know if you have any questions! Thanks again :-)


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