Chetan Mehrotra commented on OAK-4796:

What a coincidence was just looking at this issue and now the patch!. Have not 
yet gone in detail into the patch but wanted to commit on approach. Instead of 
having a Validator provided by Observer determine if certain change is 
interesting for given observor we can take a more decouple approach

h3. A - Collect data
For any given commit collect set of potentially interesting things (as 
mentioned in OAK-4586)
# set of nodetype names for modified nodes
# set of property names which got modified
# Paths (upto n level) under which modification happened say upto depth 2
# set of node names which got modified

This logic can be implemented as an {{Editor}} which gets registered in 
{{IndexUpdate}} and it collects above data and stores it in {{CommitContext}} 
on _best effort basis_. The editor would be invoked for each change and there 
it can very efficiently build up this state. Complete data is collected 
irrespective if any observer is interested in that change. 

My understanding is that this state would not be very large for most of the 
commits done

By best effort means that if any data becomes too big say 1000 different 
property name which got changed then it would not empty the state and indicate 
that data is too large and observer should do the hard work. So if we are in a 
large commit then we do not collect this data (configurable limits)

h3. B - Filter out based on collected data
Each observer would then provide a Filter which would be run when any 
ContentChange gets enqueued and it only allows those changes which have changes 
which it is interested in

This approach would allow us to later serialized this collected data in 
DocumentNodeStore journal entry and gets merged when an external diff event is 
sent. Thus benefiting both local and external change processing. 

> filter events before adding to ChangeProcessor's queue
> ------------------------------------------------------
>                 Key: OAK-4796
>                 URL: https://issues.apache.org/jira/browse/OAK-4796
>             Project: Jackrabbit Oak
>          Issue Type: Improvement
>          Components: jcr
>    Affects Versions: 1.5.9
>            Reporter: Stefan Egli
>            Assignee: Stefan Egli
>              Labels: observation
>             Fix For: 1.6
>         Attachments: OAK-4796.patch
> Currently the 
> [ChangeProcessor.contentChanged|https://github.com/apache/jackrabbit-oak/blob/f4f4e01dd8f708801883260481d37fdcd5868deb/oak-jcr/src/main/java/org/apache/jackrabbit/oak/jcr/observation/ChangeProcessor.java#L335]
>  is in charge of doing the event diffing and filtering and does so in a 
> pooled Thread, ie asynchronously, at a later stage independent from the 
> commit. This has the advantage that the commit is fast, but has the following 
> potentially negative effects:
> # events (in the form of ContentChange Objects) occupy a slot of the queue 
> even if the listener is not interested in it - any commit lands on any 
> listener's queue. This reduces the capacity of the queue for 'actual' events 
> to be delivered. It therefore increases the risk that the queue fills - and 
> when full has various consequences such as loosing the CommitInfo etc.
> # each event==ContentChange later on must be evaluated, and for that a diff 
> must be calculated. Depending on runtime behavior that diff might be 
> expensive if no longer in the cache (documentMk specifically).
> As an improvement, this diffing+filtering could be done at an earlier stage 
> already, nearer to the commit, and in case the filter would ignore the event, 
> it would not have to be put into the queue at all, thus avoiding occupying a 
> slot and later potentially slower diffing.
> The suggestion is to implement this via the following algorithm:
> * During the commit, in a {{Validator}} the listener's filters are evaluated 
> - in an as-efficient-as-possible manner (Reason for doing it in a Validator 
> is that this doesn't add overhead as oak already goes through all changes for 
> other Validators). As a result a _list of potentially affected observers_ is 
> added to the {{CommitInfo}} (false positives are fine).
> ** Note that the above adds cost to the commit and must therefore be 
> carefully done and measured
> ** One potential measure could be to only do filtering when listener's queues 
> are larger than a certain threshold (eg 10)
> * The ChangeProcessor in {{contentChanged}} (in the one created in 
> [createObserver|https://github.com/apache/jackrabbit-oak/blob/f4f4e01dd8f708801883260481d37fdcd5868deb/oak-jcr/src/main/java/org/apache/jackrabbit/oak/jcr/observation/ChangeProcessor.java#L224])
>  then checks the new commitInfo's _potentially affected observers_ list and 
> if it's not in the list, adds a {{NOOP}} token at the end of the queue. If 
> there's already a NOOP there, the two are collapsed (this way when a filter 
> is not affected it would have a NOOP at the end of the queue). If later on a 
> no-NOOP item is added, the NOOP's {{root}} is used as the {{previousRoot}} 
> for the newly added {{ContentChange}} obj.
> ** To achieve that, the ContentChange obj is extended to not only have the 
> "to" {{root}} pointer, but also the "from" {{previousRoot}} pointer which 
> currently is implicitly maintained.

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