[jira] [Created] (SLING-6070) Reduce temporary storage required in JcrResourceListener, call reportChanges earlier

["Stefan Egli (JIRA)" <ji...@apache.org>]

Stefan Egli created SLING-6070:
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             Summary: Reduce temporary storage required in JcrResourceListener, call reportChanges earlier
                 Key: SLING-6070
                 URL: https://issues.apache.org/jira/browse/SLING-6070
             Project: Sling
          Issue Type: Improvement
          Components: JCR
    Affects Versions: JCR Resource 2.8.0
            Reporter: Stefan Egli
             Fix For: JCR Resource 2.8.2


As noticed in OAK-4581 ([here|https://issues.apache.org/jira/browse/OAK-4581?focusedCommentId=15525601&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-15525601]) one advantage of using OakResourceListener over JcrResourceListener (hence the term 'optimize for oak' of the config parameter) is that OakResourceListener uses a NodeObserver, which calls added/removed/changed after each child traversal has finished. This will in turn be used by the OakResourceListener to call ObservationReporter.reportChanges. In the JcrResourceListener.onEvent case however this is not possible, as the events are delivered for each node/property individually, and they first have to be 'mapped' to ResourceChanges. This is currently done by keeping maps of added/removed/changed ResourceChanges and only after all events (that are passed in 1 onEvent) are processed is reportChanges invoked. This has the downside of a potentially very large temporary memory usage (these maps can get big).

A suggested improvement is to follow the same pattern as is done in the OakResourceListener/NodeObserver pair: to forward the events (by callling reportChanges) as early as possible. Since Oak uses a breadth-first tree traversal when compiling the jcr events, this fact can be used to detect the earliest possible moment to forward events, which is as soon as a child traversal has finished. That way, only parent changes need to be kept, not the entire tree of changes.



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