Major Description
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). 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).