Stefan Egli updated OAK-4916:
    Attachment: OAK-4916.v2.patch

Attaching [^OAK-4916.v2.patch] which is an alternative variant of filtering in 
the BackgroundObserver:
* as suggested by Chetan introduced a {{FilteringAwareObserver}} which is 
implemented by BackgroundObserver and thus moves the filtering _decision_ to an 
upstream Observer. Thus an element doesn't even get passed to the 
BackgroundObserver if it is filtered. However, what the BackgroundObserver must 
do is to correctly pass this exclusion to its downstream Observer - and it must 
take special care when the queue is full: as basically then exclusion doesn't 
* added a base class for doing the upstream filtering in {{FilteringObserver}}
* added several tests to the existing BackgroundObserverTest as well as added a 
new PrefilteringBackgroundObserverTest.

[~chetanm], [~mduerig], wdyt?

> 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, OAK-4916.v2.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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