Michael Marth created OAK-1161:
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Summary: Simple failover for TarMK-based installations
Key: OAK-1161
URL: https://issues.apache.org/jira/browse/OAK-1161
Project: Jackrabbit Oak
Issue Type: New Feature
Reporter: Michael Marth
At the moment we have a Mongo-based MK impl that Oak users for scalable
deployments and TarMK for standalone (performant) deployments. I think it is OK
to not implement some sort of "scalability" into TarMK, even if I realize that
the hierarchical journals allow us to do that later if we want to. However, it
would even now be great to have a failover option for TarMK (MongoMK implictly
offers this through replicas). This would not be about clustering or
scalability, but only about reliability.
I think there are 2 parts to this:
# keeping a standby repository (slave) in sync and
# the actual fail over.
For the first part there could be a relatively simple way to implement this:
Let's consider that there is only one slave and that the slave does not accept
writes. Given the MVCC nature of the tar files we could simply sync the
(append-only) tar files from the master to the slave on an ongoing basis. This
could be similar to an rsync (or even use actual rsync)
The slave would keep on receiving and locally persisting these files.
Also, the slave would either need to be in a state where it is blocks writes or
even in some sort of sleep state.
I think this synchronization of files could be done a rather robust way where
shaky networks or high latency could be recovered from by choosing a proper way
of transfer.
This sync to a remote system could be implemented similarly than a tarMK-based
incremental backup (OAK-1159).
For the failover:
Ideally, we would have 2 implementations: a native failover and an external
switch (like MBean or via HTTP) that would make the slave stop accepting files
from master and start up on the last completely received revision. But simply
having the second option would be a good start.
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