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From "Ian Boston (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (OAK-4581) Persistent local journal for more reliable event generation
Date Thu, 21 Jul 2016 06:50:20 GMT

    [ https://issues.apache.org/jira/browse/OAK-4581?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15387264#comment-15387264
] 

Ian Boston commented on OAK-4581:
---------------------------------

Only consider my suggestions for a WAL if the existing Journal implementation is unsuitable.
It seems the existing journal is suitable, so ignore WAL references.

---

Assuming the persisted journal capacity is only bounded by available disk space, the need
to reference a remote journal to conserve queue capacity in exchange for additional computation
or network is probably unnecessary. I assume "remote" means remote to the VM where the JVM
is running and not some other interpretation of remote.

---

Events lagging too far behind (data latency) is:
1. something that will (I assume) be caused by the commit traffic an application is subjecting
Oak to, rather than caused by Oak internal commits.
2. something an application will care about.
3. given sufficient and appropriate feedback (queue latency metric) something an application
developer will be keen to act on during dev and QE before production.



> 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
>             Fix For: 1.6
>
>
> 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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