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From Michael Dürig (JIRA) <j...@apache.org>
Subject [jira] [Commented] (OAK-6276) expose way to detect "eventual consistency delay"
Date Thu, 01 Jun 2017 10:14:04 GMT

    [ https://issues.apache.org/jira/browse/OAK-6276?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16032737#comment-16032737
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Michael Dürig commented on OAK-6276:
------------------------------------

I like the term "visibility token". The contract would then be that a visibility token acquired
from one cluster node would eventually appear on all cluster nodes. Once a visibility token
appears on a cluster node each change from the cluster node the token was acquired from and
that happened before it was acquired there would be visible to the other cluster node. 
Note that visible does not mean it "is actually there". It might as well have been superseded
by merges in the meanwhile. It rather means changes from before the token was acquired are
"incorporated" in the state of the other cluster node once the token appears there. 

> expose way to detect "eventual consistency delay"
> -------------------------------------------------
>
>                 Key: OAK-6276
>                 URL: https://issues.apache.org/jira/browse/OAK-6276
>             Project: Jackrabbit Oak
>          Issue Type: New Feature
>          Components: api
>            Reporter: Stefan Egli
>
> I have a requirement to support an external messaging channel (eg Kafka) between Oak-based
instances of the same cluster. As part of handling those messages the target instance in some
cases might have to access data from the repository. 
> Now with DocumentNodeStore's eventual consistency that data might not 'travel' from the
source to the target instance as fast as is the case for the external message.
> Therefore the need arises to be able to delay such messages (on the target instance)
until the repository sees (at least) the data the source instance wrote when sending off the
message.
> This ticket is to equip Oak with any feasible way for higher level code to generally
speaking detect such an "eventual consistency delay".
> One simple idea that comes to mind is to expose the current _head revision vector_ (or
that from a particular session, but that might not be required, ie be too complicated). The
source instance could get the local head revision vector, piggyback that on the message, then
that could be compared on the target instance with its head state. If that turns out to be
older, then it could do a wait and retry. (Nicer would of course be if there would even be
a call-back - but in theory that could also be implemented ontop of an Observer).
> One means to expose the head revision vector would be via a repository descriptor (which
on access returns the current value, similar to [how discovery-lite does it|https://github.com/apache/jackrabbit-oak/blob/2634dbde9aedc2549f0512285e9abee5858b256f/oak-core/src/main/java/org/apache/jackrabbit/oak/plugins/document/DocumentDiscoveryLiteService.java#L246]).
And the format could be normalized as eg {{longs}} (eg {{\[1496071927014, 1496071926243]}}
(instead of {{\["r15c54d532ec-0-1", "r15c54d532ec-0-2"]}} to avoid leaking the revision format
explicitly).



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