[jira] [Commented] (SLING-4603) discovery.oak: oak-based discovery implementation

["Stefan Egli (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/SLING-4603?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14953201#comment-14953201 ] 

Stefan Egli commented on SLING-4603:
------------------------------------

As part of this I'm going to factor the topology-connector part of {{discovery.impl}} into its own bundle {{discovery.connectors}} - that one will then be used by both {{discovery.impl}} and {{discovery.oak}}.

> discovery.oak: oak-based discovery implementation
> -------------------------------------------------
>
>                 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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