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https://issues.apache.org/jira/browse/JCR-2660?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12879754#action_12879754
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Jukka Zitting commented on JCR-2660:
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As discussed with Norman Maurer from the Apache James project, one good use
case for this would be to maintain per-nodetype counts of nodes in each
subtree. In James they need this information to efficiently tell the number of
message nodes within a potentially huge mailbox subtree.
> Hierarchical map-reduce
> -----------------------
>
> Key: JCR-2660
> URL: https://issues.apache.org/jira/browse/JCR-2660
> Project: Jackrabbit Content Repository
> Issue Type: New Feature
> Components: jackrabbit-core
> Reporter: Jukka Zitting
> Fix For: 2.2.0
>
>
> As discussed on a few occasions earlier, it would be great to have a feature
> that allows efficient and automatic processing of information over entire
> subtrees of content. Such a feature could be used to maintain custom reports,
> search indexes, and other summary information for selected subtrees.
> The map-reduce paradigm works great for such a purpose, as you can combine a
> per-node map operation with a reduce operation that combines the map results
> of the children of a given parent node. It's easy to generalize this model to
> cover entire subtrees. And if the reduced information is stored as a part of
> each parent node, it's even possible to incrementally update the results for
> an entire subtree when just a part of the subtree is modified.
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