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https://issues.apache.org/jira/browse/OAK-4796?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15548765#comment-15548765
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Stefan Egli edited comment on OAK-4796 at 10/5/16 2:11 PM:
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bq. This is wrong. We need to either do prefiltering or postfiltering and can't
mix the two. Therefore for prefiltering it's essential to pass around the
applied filter in the ContentChange obj and use that later at delivery time.
Coming back to this point, there seems to be some issues with this based on the
current design: Prior to prefiltering we had only postfiltering. And changing
the FilterProvider was applied immediately - basically on all elements in the
queue. With prefiltering this is, as pointed out, not correct: those elements
in the queue already have gone through prefiltering, so postfiltering should be
done with the same FilterProvider. Which means, the ChangeProcessor - which is
in charge of postfiltering - should not use the FilterProvider set on its
instance, but use the same that was used for prefiltering. Therefore the
ChangeProcessor needs to be given the FilterProvider for each change that it
processes. The way it receives changes though is via the
Observer.contentChanged. Therefore about the only feasible place to pass the
FilterProvider from BackgroundObserver to ChangeProcessor is via the CommitInfo.
Thing now is that for external and overflow entries the CommitInfo is null. So
I'd say, as long as that's the case it's very hard to implement correctly
switching the filter.
Unless this switch is done correctly, the only thing that can be said is that:
when a filter is changed it is undefined for which changes both filters are
applied (if the queue is not empty when switching).
was (Author: egli):
bq. This is wrong. We need to either do prefiltering or postfiltering and can't
mix the two. Therefore for prefiltering it's essential to pass around the
applied filter in the ContentChange obj and use that later at delivery time.
Coming back to this point, there seems to be some issues with this based on the
current design: Prior to prefiltering we had only postfiltering. And changing
the FilterProvider was applied immediately - basically on all elements in the
queue. With prefiltering this is, as pointed out, not correct: those elements
in the queue already have gone through prefiltering, so postfiltering should be
done with the same FilterProvider. Which means, the ChangeProcessor - which is
in charge of postfiltering - should not use the FilterProvider set on its
instance, but use the same that was used for prefiltering. Therefore the
ChangeProcessor needs to be given the FilterProvider for each change that it
processes. The way it receives changes though is via the
Observer.contentChanged. Therefore about the only feasible place to pass the
FilterProvider from BackgroundObserver to ChangeProcessor is via the CommitInfo.
Thing now is that for external and overflow entries the CommitInfo is null. So
I'd say, as long as that's the case it's very hard to implement correctly
switching the filter.
Unless this switch is done correctly, the only thing that can be said is that:
when a filter is changed it is undefined if the old, the new or both filters
are applied to entries in the queue.
> 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.changeSet.patch, 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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