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From "Eli Reisman (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (GIRAPH-547) Allow in-place modification of edges
Date Fri, 01 Mar 2013 20:41:14 GMT

    [ https://issues.apache.org/jira/browse/GIRAPH-547?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13590917#comment-13590917
] 

Eli Reisman commented on GIRAPH-547:
------------------------------------

I like the iterator idea, you could then choose to mutate or not mutate each edge as you iterate
along the byte buffer, without seeking or random-access to it?
                
> 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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