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https://issues.apache.org/jira/browse/OAK-4916?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15568342#comment-15568342
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Stefan Egli commented on OAK-4916:
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One way to do this would be to make the {{BackgroundObserver}} a
{{FilteringAwareObserver}} such that it could get the 'skip' signal from the
upstream, new '{{FilteringObserver}}'. It would be another variant to what
Chetan's composition pattern, however the BackgroundObserver would still have
to maintain about the same logic in {{contentChanged}} wrt skipped changes as
now: the goal of prefiltering is to shrink the queue of the BackgroundObserver
and for that we do have to make some adaptions there. Surely there are other
variants than what I came up with, but I doubt it's possible without any change
in the BackgroundObserver.
I'll work on a next iteration based on these 2 feedbacks, Thx!
> Add support for excluding commits to BackgroundObserver
> -------------------------------------------------------
>
> Key: OAK-4916
> URL: https://issues.apache.org/jira/browse/OAK-4916
> Project: Jackrabbit Oak
> Issue Type: Technical task
> Components: core
> Affects Versions: 1.5.11
> Reporter: Stefan Egli
> Assignee: Stefan Egli
> Fix For: 1.6
>
> Attachments: OAK-4916.patch
>
>
> As part of pre-filtering commits it would be useful to have support in the
> BackgroundObserver (in general) that would allow to exclude certain commits
> from being added to the (BackgroundObserver's) queue, thus keeping the queue
> smaller. The actual filtering is up to subclasses.
> The suggested implementation is as follows:
> * a new method {{isExcluded}} is introduced which represents a subclass hook
> for filtering
> * excluded commits are not added to the queue
> * when multiple commits are excluded subsequently, this is collapsed
> * the first non-excluded commit (ContentChange) added to the queue is marked
> with the last non-excluded root state as the 'previous root'
> * downstream Observers are notified of the exclusion of a commit via a
> special CommitInfo {{NOOP_CHANGE}}: this instructs it to exclude this change
> while at the same time 'fast-forwarding' the root state to the new one.
> ** this extra token is one way of solving the problem that
> {{Observer.contentChanged}} represents a diff between two states but does not
> transport the 'from' state explicitly - that is implicitly taken from the
> previous call to {{contentChanged}}. Thus using such a gap token
> ({{NOOP_CHANGE}}) seems to be the only way to instruct Observers to skip a
> change.
> To repeat: whoever extends BackgroundObserver with filtering must be aware of
> the new {{NOOP_CHANGE}} token. Anyone not doing filtering will not get any
> {{NOOP_CHANGE}} tokens though.
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