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https://issues.apache.org/jira/browse/SLING-4751?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14567464#comment-14567464
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Stefan Egli commented on SLING-4751:
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Maybe we can group all of this into different use cases to better see what
kinds of listeners we expect and how we can support them? The 'exactly once'
for example I think should be solved via a job. The 'caching' example could be
done by listening on certain patterns (with a path prefix) instead of all child
modifications too.
The result could be that the listener would not be entirely free as to what it
wants to listen on, but would have to be more specific (internal vs external,
path vs child changes etc).
Maybe this is going too far, but whatever can be optimized to be more scalable
for the ResourceListener could in my view help.
> New Observation Support
> -----------------------
>
> Key: SLING-4751
> URL: https://issues.apache.org/jira/browse/SLING-4751
> Project: Sling
> Issue Type: Improvement
> Components: API, JCR, ResourceResolver
> Reporter: Carsten Ziegeler
> Assignee: Carsten Ziegeler
> Fix For: API 2.10.0, Resource Resolver 1.2.6
>
>
> Mail thread:
> http://mail-archives.apache.org/mod_mbox/sling-dev/201505.mbox/%3C555983F2.20402%40apache.org%3E
> Starting mail
> Right now, resources changes are propagated through event admin - which
> at the time sounded like a good idea. Over time, this has shown to be a
> bottle neck.
> Basically there are at least three problems:
> - the sender of the resource events does not know whether there is a
> receiver, therefore events for each and every change need to be sent
> - event objects are immutable and therefore all relevant data needs to
> be calculated upfront, even if it's not used. For example a resource
> event contains the resource type which needs to be fetched from the
> repository, even if no one is interested in it.
> - receivers of the events can't easily act on behalf of the user who
> initiated the change.
> I created a new listener api at [1] which defines a ResourceListener
> interface and some ways how to specify the events one is interested in.
> The user aware resource listener allows to act on behalf of the user (if
> that information is available).
> On the other side, a new service, the ObservationReporter [2] is
> defined. Resource providers report changes through this interface. The
> payload of such an event is an interface which allows for lazy retrieval
> of the information.
> We can also use this mechanism for compatibility and an implementation
> of the observation reporter might sent all events via the event admin.
> [1]
> https://svn.apache.org/repos/asf/sling/whiteboard/cziegeler/api-v3/src/main/java/org/apache/sling/api/resource/observation/
> [2]
> https://svn.apache.org/repos/asf/sling/whiteboard/cziegeler
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