Stefan Egli created SLING-4603:
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Summary: 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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