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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