[jira] [Comment Edited] (OAK-4916) Add support for excluding commits to BackgroundObserver

["Chetan Mehrotra (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/OAK-4916?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15567937#comment-15567937 ] 

Chetan Mehrotra edited comment on OAK-4916 at 10/12/16 7:39 AM:
----------------------------------------------------------------

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. This is similar to current proposed approach just using 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 ;)


was (Author: chetanm):
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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