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From Michael Dürig (JIRA) <j...@apache.org>
Subject [jira] [Commented] (OAK-4581) Persistent local journal for more reliable event generation
Date Fri, 02 Sep 2016 21:05:20 GMT

    [ https://issues.apache.org/jira/browse/OAK-4581?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15459586#comment-15459586
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Michael Dürig commented on OAK-4581:
------------------------------------

But storing the events themselves will tremendously increase the sizes of the queues. Each
commit will occupy roughly as many slots in the queue as there where changes in the commit.
Each event would need to store a path, a userId, an info map etc. Not exactly cheap. And on
top of that there is a queue per listener. In effect this would multiply the requirements
for IO bandwidth by the number of observation listeners (each commit is first stored to disk
and then distributed to N listener queues and stored again in the form of events). 

Agreed, early filtering will reduce the number of events somewhat and above picture is just
an upper bound. But storing the commit (handle) will occupy just a single slot in the queue
while storing events will occupy at least as many slots unless a commit does not generate
a single event. 

> 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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