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From "Alessandro Presta (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (GIRAPH-547) Allow in-place modification of edges
Date Fri, 01 Mar 2013 20:47:13 GMT

    [ https://issues.apache.org/jira/browse/GIRAPH-547?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13590923#comment-13590923
] 

Alessandro Presta commented on GIRAPH-547:
------------------------------------------

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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