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https://issues.apache.org/jira/browse/OAK-4916?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15567937#comment-15567937
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Chetan Mehrotra commented on OAK-4916:
--------------------------------------
Somehow not very comfortable with {{NOOP_CHANGE}} based protocol for
communicating filtering.
May be we introduce new interfaces
# ContentChangeFilter - This would be passed to {{BackgroundObserver}} and
would be responsible for determining if given change should be included in the
queue or not. By default {{BackgroundObserver}} would
{{ContentChangeFilter#DEFAULT}}. Observers which support prefiltering would
provide there own implementation. Kind of composition over inheritence
{code}
public interface ContentChangeFilter {
ContentChangeFilter DEFAULT = new ContentChangeFilter() {
@Override
public boolean includeChange(CommitInfo info) {
return true
}
};
boolean includeChange(CommitInfo info);
}
{code}
# FilteringAwareObserver - Any observer which allows prefiltering would also
implement this interface. And instead of {{NOOP_CHANGE}} based convention
{{BackgroundObserver}} would invoke the {{resetPreviousRoot}} to indicate that
base root state needs to be adjusted.
{code}
public interface FilteringAwareObserver extends Observer{
/**
* Invoked to enable such observers to reset there previous root
* to given NodeState
*
* @param root previous NodeState root
*/
void resetPreviousRoot(NodeState root);
}
{code}
Another approach would be to make such observer state less i.e. they do not
track the previous state and we pass in previous state and
{{BackgroundObserver}} maintains a tuple
{code}
void contentChanged(@Nullable NodeState before, @Nonnull NodeState after,
@Nullable CommitInfo info);
{code}
[~mduerig] Would have better thoughts here ;)
> 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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