[ 
https://issues.apache.org/jira/browse/OAK-4916?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15572294#comment-15572294
 ] 

Stefan Egli edited comment on OAK-4916 at 10/13/16 4:00 PM:
------------------------------------------------------------

Note that the issue with switching a filter [mentioned 
here|https://issues.apache.org/jira/browse/OAK-4796?focusedCommentId=15548765&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-15548765]
 still applies. It would require OAK-4898 to pass the FilterProvider with each 
CommitInfo into the BackgroundObserver's queue for this to work correctly.
Assuming OAK-4898 and passing the FilterProvider doesn't make it into 1.6 I'm 
wondering if this should be added as a known-issue to 
https://jackrabbit.apache.org/oak/docs/known_issues.html


was (Author: egli):
Note that the issue with switching a filter [mentioned 
here|https://issues.apache.org/jira/browse/OAK-4796?focusedCommentId=15548765&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-15548765]
 still applies. It would require OAK-4898 to pass the FilterProvider with each 
CommitInfo into the BackgroundObserver's queue for this to work correctly.

> 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.
> NOTE: See [comment further 
> below|https://issues.apache.org/jira/browse/OAK-4916?focusedCommentId=15572165&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-15572165]
>  with a new suggested approach, which doesn't use NOOP_CHANGED but introduces 
> a new FilteringAwareObserver instead.



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