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From "Alessandro Presta (JIRA)" <j...@apache.org>
Subject [jira] [Created] (GIRAPH-547) Allow in-place modification of edges
Date Fri, 01 Mar 2013 19:53:12 GMT
Alessandro Presta created GIRAPH-547:
----------------------------------------

             Summary: Allow in-place modification of edges
                 Key: GIRAPH-547
                 URL: https://issues.apache.org/jira/browse/GIRAPH-547
             Project: Giraph
          Issue Type: New Feature
            Reporter: Alessandro Presta
            Assignee: Alessandro Presta


This is a somewhat long term item.

Because of some optimized edge storage implementations (byte array, primitive array), we have
a contract with the user that Edge objects returned by getEdges() are read-only.

One concrete example where this is needed: in the weighted version of PageRank, you can store
the weight sum and normalize each message sent, or you could more efficiently normalize the
out-edges once in superstep 0.

The Pregel paper describes an OutEdgeIterator that allows for in-place modification of edges.
I can see how that would be easy to implement in C++, where there is no need to reuse objects.

Giraph "unofficially" supports this if one is using generic collections to represent edges
(e.g. ArrayList or HashMap).

It may be trickier in some optimized implementations, but in principle it should be doable.

One way would be to have some special MutableEdge implementation which calls back to the edge
data structure in order to save modifications:

{code}
for (Edge<I, E> edge : getEdges()) {
  edge.setValue(newValue);
}
{code}

Another option would be to add a special set() method to our edge iterator, where one can
replace the current edge:

{code}
for (EdgeIterator<I, E> it = getEdges().iterator(); it.hasNext();) {
  Edge<I, E> edge = it.next();
  edge.setValue(newValue);
  it.set(edge);
}
{code}

We could actually implement the first version as syntactic sugar on top of the second version
(the special MutableEdge would need a reference to the iterator in order to call set(this)).

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