Vikas Saurabh commented on OAK-4581:

I got slightly confused about what are we targeting here (sorry, haven't 
followed all the comments - just the description and the last explanation). My 
initial thought about the issue was that we're solving quicker diff elsewhere 
(segment is fast anyway. document now relies on journal, afaik can recover from 
full queue too, and can work on ranges). Given those improvements, I thought 
this issue was about persisting change pointers (rev or root-node-id as the 
need be and commit info for local changes) so that listeners work fine post 
rebounce too.

Afaics, we still want remain wary of infinite storage of pointers (whatever be 
the case - slow listeners or too many changes.. in effect rate_listener < 

Assuming (accurate??) diff processing is a different concern, I'm not sure why 
GC (assuming revGC) need to care about node states (thinking about 1A side of 
things) - I don't know about segment... but for document we can still get root 
node state. Sure if we're saying that revGC deleted node states (ie the even 
hadn't been consume in 24 hours) then is that really a case worth solving by 
retaining node states??

Anyway, assuming my original interpretation of the issue, I like 1-A-1 + 3-C... 
as you pointed out that we need to take care of OakResourceListener. I think 
we've bore a lot of pain with us improving at least diagnosing ChangeProcessor 
(jmx, configurable queue) while sling bridge kept reeling under pain with the 
default of 1k. I'd rather have improvements that work uniformly.

> Persistent local journal for more reliable event generation
> -----------------------------------------------------------
>                 Key: OAK-4581
>                 URL: https://issues.apache.org/jira/browse/OAK-4581
>             Project: Jackrabbit Oak
>          Issue Type: New Feature
>          Components: core
>            Reporter: Chetan Mehrotra
>            Assignee: Stefan Egli
>              Labels: observation
>             Fix For: 1.6
>         Attachments: OAK-4581.v0.patch
> As discussed in OAK-2683 "hitting the observation queue limit" has multiple 
> drawbacks. Quite a bit of work is done to make diff generation faster. 
> However there are still chances of event queue getting filled up. 
> This issue is meant to implement a persistent event journal. Idea here being
> # NodeStore would push the diff into a persistent store via a synchronous 
> observer
> # Observors which are meant to handle such events in async way (by virtue of 
> being wrapped in BackgroundObserver) would instead pull the events from this 
> persisted journal
> h3. A - What is persisted
> h4. 1 - Serialized Root States and CommitInfo
> In this approach we just persist the root states in serialized form. 
> * DocumentNodeStore - This means storing the root revision vector
> * SegmentNodeStore - {color:red}Q1 - What does serialized form of 
> SegmentNodeStore root state looks like{color} - Possible the RecordId of 
> "root" state
> Note that with OAK-4528 DocumentNodeStore can rely on persisted remote 
> journal to determine the affected paths. Which reduces the need for 
> persisting complete diff locally.
> Event generation logic would then "deserialize" the persisted root states and 
> then generate the diff as currently done via NodeState comparison
> h4. 2 - Serialized commit diff and CommitInfo
> In this approach we can save the diff in JSOP form. The diff only contains 
> information about affected path. Similar to what is current being stored in 
> DocumentNodeStore journal
> h4. CommitInfo
> The commit info would also need to be serialized. So it needs to be ensure 
> whatever is stored there can be serialized or re calculated
> h3. B - How it is persisted
> h4. 1 - Use a secondary segment NodeStore
> OAK-4180 makes use of SegmentNodeStore as a secondary store for caching. 
> [~mreutegg] suggested that for persisted local journal we can also utilize a 
> SegmentNodeStore instance. Care needs to be taken for compaction. Either via 
> generation approach or relying on online compaction
> h4. 2- Make use of write ahead log implementations
> [~ianeboston] suggested that we can make use of some write ahead log 
> implementation like [1], [2] or [3]
> h3. C - How changes get pulled
> Some points to consider for event generation logic
> # Would need a way to keep pointers to journal entry on per listener basis. 
> This would allow each Listener to "pull" content changes and generate diff as 
> per its speed and keeping in memory overhead low
> # The journal should survive restarts
> [1] http://www.mapdb.org/javadoc/latest/mapdb/org/mapdb/WriteAheadLog.html
> [2] 
> https://github.com/apache/activemq/tree/master/activemq-kahadb-store/src/main/java/org/apache/activemq/store/kahadb/disk/journal
> [3] 
> https://github.com/elastic/elasticsearch/tree/master/core/src/main/java/org/elasticsearch/index/translog

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