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ASF GitHub Bot commented on FLINK1758:

Github user vasia commented on a diff in the pull request:
https://github.com/apache/flink/pull/576#discussion_r28594064
 Diff: docs/gelly_guide.md 
@@ 269,7 +269,15 @@ Neighborhood Methods
Neighborhood methods allow vertices to perform an aggregation on their firsthop neighborhood.
`reduceOnEdges()` can be used to compute an aggregation on the neighboring edges of a
vertex, while `reduceOnNeighbors()` has access on both the neighboring edges and vertices.
The neighborhood scope is defined by the `EdgeDirection` parameter, which takes the values
`IN`, `OUT` or `ALL`. `IN` will gather all incoming edges (neighbors) of a vertex, `OUT`
will gather all outgoing edges (neighbors), while `ALL` will gather all edges (neighbors).
+`groupReduceOnEdges()` can be used to compute an aggregation on the neighboring edges
of a vertex,
+while `groupReduceOnNeighbors()` has access to both the neighboring edges and vertices.
The neighborhood scope
+is defined by the `EdgeDirection` parameter, which takes the values `IN`, `OUT` or `ALL`.
`IN` will gather all incoming edges (neighbors) of a vertex, `OUT` will gather all outgoing
edges (neighbors), while `ALL` will gather all edges (neighbors).
+
+The `groupReduceOnEdges()` and `groupReduceOnNeighbors()` methods return zero, one or
more values per vertex.
+When returning a single value per vertex, `reduceOnEdges()` or `reduceOnNeighbors()`
should be called
+as they are more efficient. Nevertheless, when the reduce on edges modifies the value
produced per vertex, for
+instance by multiplying it with a constant, `groupReduceOnEdges()` or `groupReduceOnNeighbors()`
must be used
+as illustrated in the third code snippet.
 End diff 
I would rephrase this into something like the following: "when the userdefined function
to be applied on the neighborhood is associative and commutative, it is highly advised to
use the `reduceOnEdges()` and `reduceOnNeighbors()` methods. These methods can exploit combiners
internally and significantly improve performance".
> Extend Gelly's neighborhood methods
> 
>
> Key: FLINK1758
> URL: https://issues.apache.org/jira/browse/FLINK1758
> Project: Flink
> Issue Type: Improvement
> Components: Gelly
> Affects Versions: 0.9
> Reporter: Vasia Kalavri
> Assignee: Andra Lungu
>
> Currently, the neighborhood methods only allow returning a single value per vertex. In
many cases, it is desirable to return several or no value per vertex. This is the case in
clustering coefficient computation, vertexcentric jaccard, algorithms where a vertex computes
a value per edge or when a vertex computes a value only for some of its neighbors.
> This issue proposes to
>  change the current reduceOnEdges/reduceOnNeighbors methods to use combinable reduce
operations where possible
>  provide groupReduceversions, which will use a Collector and allow returning none or
more values per vertex.

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