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From Alexander Klimetschek <aklim...@adobe.com>
Subject Observation
Date Thu, 07 Nov 2013 01:49:26 GMT
Hi everyone,

having looked at and discussed observation recently (partly on our Adobe internal list with
some of our application use cases, partly with Toby f2f), here are some things that are IMHO
worthwile to look at. I hope I can contribute a bit, but before I get pulled away again, I
wanted at least summarize and persist() my ideas:

1) listeners should not get external events by default

OAK-1121 helps, but applications must say they exclude external events, while it should be
the other way around (only local by default) - this can arguably not change because of the
jcr observation contract. The current implementation can still be optimized a bit, see my
comment in the issue.

2) filtering happens too late within listeners because jcr observation API is too simple (OAK-1133,
"observation listener plus")

The filter logic would be passed declaratively and allow more options as noted in OAK-1133:

	addEventListener(listener, Filter.Path("/libs"), Filter.PropertyValue("jcr:content/sling:resourceType",
"myValue"), ...)

(just a rough sketch; the simplest solution could simply introduce a FilterEventListener interface
that extends the existing EventListener and provides the filter in a getFilter() method; a
simple instanceof check within oak-jcr could detect that; but a completely separate API might
also make sense, see point 6 below)

This could then be evaluated when the NodeStateDiff is calculated. What is good now already
is that the base path (if specified in the jcr observation) is evaluated as soon as getting
the diff roots. But many real use cases might want to have multiple paths (/apps OR /libs)
or globbing (/content/site/*/jcr:content) - this could be easily and efficiently added here
in one place (amongst those other filters). Having one or few base paths is in the oak case
probably the best way to filter things and one should avoid listeners that register at root,
and only filter on node types for example, which requires a lot more tree walking.

In our app (CQ) we have 18 listeners that register for all events, because the JCR observation
doesn't give them the right filter options, so they have to do it themselves. I expect them
to be vastly improved with this new approach.

Also, getting events for deleted nodes could be supported here, since there is full access
to the old NodeState.

The ideal solution could also be built backwards compatible: implement the same filtering
with normal JCR API usage and automatically wrap the listener in another event listener that
filters the "manual" way. This allows application code that works with both Jackrabbit 2 or
Oak with minimal effort (would need some helper in jackrabbit-jcr-commons or so). Unless we
want to backport this to Jackrabbit 2 as well.

3) cluster events

As agreed, external events can be the real scalability issue. It was also noted, that many
cases do not need it at all (hence 1). But there are still cases that need it: in Sling for
example, code deployment (bundles, JSP scripts, etc.) is based on the JCR repository, meaning
when you put a bundle in a certain "install" folder in the JCR, all cluster instances need
to pick it up. However, such deployment events are comparably  rare (except on a developer
machine :)), so the throughput doesn't have to be high. OTOH large-scale content changes will
(and should) be handled locally, thus not requiring external propagation.

Now the question is if 2) with a filter-as-detailed-and-early-as-possible approach solves
this already (having the right external/non-external flag on all listeners, plus detailed
filters for the external listeners), or if we could make use of the fairly rare nature of
those events and try to filter on the _source_ instance already and propagate those few events
in some other, efficient way to other cluster instances. This might improve performance as
the ChangeDispatcher#externalChange() check (polling all 100ms) could then be removed completely.
But if the filtering on the NodeStateDiff is fast (because listeners are specific), then maybe
it is ok.

4) listener threads

Currently in oak each event listener gets its own thread. In our app CQ we have about 150
listeners, so you end up introducing 150 threads. I am not sure if this is a good idea.

Jackrabbit 2 had a single thread, which had the issue of one observation listener blocking
all others.

An obvious solution would be a thread pool (probably configurable number of threads). And
on top of that, if the pool is full and no thread is free anymore, one could simply kill blocked
handlers after a certain timeout (and optionally blacklist them).

5) filtering in central thread?

There was some discussion about the current filtering evaluation [0]. Each listener has its
own ChangeProcessor which in turn has its own ChangeDispatcher.Listener. This means filtering
is not in a single, central thread, but happens in each listener's thread (they each have
their own ChangeSet queue, which indeed might be different because the previous root NodeState
to compare to might be different for each).

I thought this was not optimal but it seems I was wrong - splitting that up and generating
events in a central thread and then passing them in a queue to the listeners actually turned
out to scale worse (in a quick test with many parallel listeners). See my patch at [1].

I guess it is that reading asynchronously from the immutable NodeStates is more efficient
than multiple blocking queues. Which speaks good for the underlying oak implementation :)

[0] http://markmail.org/thread/533orsfr44wllvrx
[1] https://github.com/alexkli/jackrabbit-oak/commit/aee631ded4996194a3ad0dec1fc7a9917f7123b8

6) long-running session just for observation

One problem with JCR observation is that you need one session open all the time "just for
the observation". This is done to be able to run observation under the permissions of a certain
user. But nobody in practice uses the session for anything else than reading data upon events
(which mostly is done to filter only, see 2); when you need to write things in an event, best
practice is to create a new session on demand an close it again. With Oak's refresh() policy
this is even more important.

Maybe a new API from 2) could skip the requirement of an open session. Registration would
be based upon a session to handle the credentials easily, but that session could be closed
without killing the listener. You would get back a special listener object that would need
to be kept alive and referenced to be able to clean up listeners automatically on a finalize()
if they forget to unregister.

Or you keep the session, but mark it in a special way so that Oak can internally save resources.
Or maybe the session in oak is already so light-weight that this is actually no problem at
all. I am just mentioning this because of the many warnings I get from oak in the logs about
sessions that are open very long (and don't call refresh() IIRC).

7) move code to oak-jcr (minor)

Quite a bit of the code in oak.plugins.observation is currently JCR eventing specific (EventGenerator
for example), afaics this is better suited in oak-jcr. Although with 2) it might change a
bit anyway.

Cheers,
Alex


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