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https://issues.apache.org/jira/browse/SLING-4603?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14495858#comment-14495858
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Stefan Egli commented on SLING-4603:
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Also started a discussion on oak-dev list wrt JMX vs API for this:
http://markmail.org/thread/2tdnulytfs4su6mo
> oak-documentMk based discovery.impl
> -----------------------------------
>
> Key: SLING-4603
> URL: https://issues.apache.org/jira/browse/SLING-4603
> Project: Sling
> Issue Type: New Feature
> Components: Extensions
> Reporter: Stefan Egli
> Assignee: Stefan Egli
>
> When discovery is used in a stack based on jackrabbit oak as the repository,
> the current way of discoving instances somewhat sounds like duplicating work:
> oak, or more precisely documentnodestore, itself has a low-level [lease
> mechanism|http://jackrabbit.apache.org/oak/docs/nodestore/documentmk.html]
> where it stores information about the cluster nodes including a {{leaseEnd}}
> indicating at what time others can consider a particular node as
> dead/crashed. This corresponds pretty much to the discovery.impl heartbeat
> mechanism. And in a stack which is built ontop of oak-documentMk, we could be
> making use of this fact and delegate the decision about whether a node in a
> cluster is alive or not to the oak layer. Also, with OAK-2597 the relevant
> information: {{ActiveClusterNodes}} is nicely exposed via JMX - so that can
> become the new source of truth defining the cluster view.
> When replacing discovery-owned heartbeats with oak-owned ones, there is one
> important detail to be watched out for: it can no longer easily be determined
> from another instance in the cluster, whether it has this new discovery
> bundle activated or not. Hence it is not given that when a voting happens,
> that all {{active}} nodes (as reported by oak-documentMk) are actually going
> to respond. So the 'silent instance due to deactivated discovery bundle' case
> needs special attention/handling.
> Other than that, given the normal case of all {{active}} nodes having the
> bundle activated, the voting mechanism can stay the same as in
> discovery.impl. The topology connectors can be treated the same too (by
> storing announcements to their respective
> {{/var/discovery/clusterInstances/<slingId>/announcements/<announcerSlingId>}}
> node. The properties can be handled the same too (by storing to
> {{/properties}} node. Only thing that gets replaced is the {{heartbeats}}.
> Note that in order for such an oak-based discovery.impl this oak-lease
> mechanism must be very robust (it should be so by its own interest already).
> However, there are currently a few issues that should probably first be
> resolved until discovery can be based on this: OAK-2739, OAK-2682 and
> OAK-2681 are currently known in this area.
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