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https://issues.apache.org/jira/browse/GIRAPH-547?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13590923#comment-13590923
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Alessandro Presta commented on GIRAPH-547:
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Exactly, the in-place change would not require seeking (i.e.
addEdge/removeEdges would remain confined to mutation requests).
Another completely different implementation would be to create a new edge data
structure: as you iterate over edges, you write them back to the new one. At
the end you delete the old one and keep the new one.
You could limit this (expensive) behavior to when it is actually needed, by
offering a special iterable: getMutableEdges().
Actually this might be a much better solution, because it can be implemented by
Giraph with no burden on the user, and works seamlessly with the current
MutableEdge type and edge-reuse policy.
> 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 in-place modification would be useful: 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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