Stefan Egli created OAK-6276:
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Summary: 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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