Alessandro Presta created GIRAPH-547:
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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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