Marcel Reutegger commented on OAK-4581:

My preferred approach is I-B-1 with something similar to III-C, structured like 
a log (Kafka is probably overkill).

In other issues we so far looked at how to improve the producer side of the 
observation system, but even if the producer side is perfect, the queue could 
still fill up when the listener is too slow. And once the listener is too slow, 
it may have a ripple effect on the producer side. The node state comparisons 
may not be served from cache anymore and require I/O interfering/competing with 
other read operations in the repository. I think it's best to create the JCR 
events as soon as possible and then push them into a queue that is consumed by 
the listener. The contents of this queue will obviously have bigger space 
requirements than the current queue with just the root states, but it is 
simpler to handle. The entries are independent of the repository (unrelated to 
GC) and read and write patterns are simple linear operations.

> 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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