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https://issues.apache.org/jira/browse/OAK-4581?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15522963#comment-15522963
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Vikas Saurabh edited comment on OAK-4581 at 9/26/16 12:53 PM:
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Btw, just to note (I'm unable to concretely argue if we really want to do this
or not) - no matter how we proceed, the storage would be cluster-id specific.
So, do we want to cater the case where some cluster node gets obliterated (with
pending stored events) - would/should some other node pick that up? Adding the
disclaimer again that it's a case that would hit us once we start to advertise
"stronger guarantee of delivery of observation events" - not that we must fix
it (we can as document it that the guarantee is a best-effort thing).
If we go for it, then we'd also have to handle cases such as "cluster node
crashed and burnt to never come up again", "cluster node crashed for 2 hours
but got back up again", "some other cluster node waited for lease to expire and
acquired the same cluster id as the one which got crashed recently", etc. The
more we go ahead with solving this - it seems we'd start to invade into what
sling jobs do today (have concrete notion of assignee etc with re-assignments
as required)
was (Author: catholicon):
Btw, just to note (I'm unable to concretely argue if we really want to do this
or not) - no matter how we proceed, the storage would be cluster-id specific.
So, do we want to cater the case where some cluster node gets obliterated (with
pending stored events) - would/should some other node pick that up? Adding the
disclaimer again that it's a case that would hit us once we start to advertise
"stronger guarantee delivery of observation events" - not that we must fix it
(we can as document it that the guarantee is a best-effort thing).
If we go for it, then we'd also have to handle cases such as "cluster node
crashed and burnt to never come up again", "cluster node crashed for 2 hours
but got back up again", "some other cluster node waited for least and acquired
the same cluster id as the one which got crashed", etc. The more we go ahead
with solving this - it seems we'd start to invade into what sling jobs do today
(have concrete notion of assignee etc with re-assignments as required)
> 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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