Make sure the stopping of the keepalive timer and final keepalive update
are done as the last step before complete shutdown. The previous setup
could conceivably result in a node being fenced should the cleanup
operations take longer than ~45 seconds, for instance if primary node
switchover took too long or blocked, or log watchers failed to stop
quickly enough. Ensures that keepalives will continue to be run during
the shutdown process until the last possible moment.
Previously, contention could occasionally cause a flap/dual primary
contention state due to the lack of checking within this function. This
could cause a state where a node transitions to primary than is almost
immediately shifted away, which could cause undefined behaviour in the
cluster.
The solution includes several elements:
* Implement an exclusive lock operation in zkhandler
* Switch the become_primary function to use this exclusive lock
* Implement exclusive locking during the contention process
* As a failsafe, check stat versions before setting the node as the
primary node, in case another node already has
* Delay the start of takeover/relinquish operations by slightly
longer than the lock timeout
* Make the current router_state conditions more explicit (positive
conditionals rather than negative conditionals)
The new scenario ensures that during contention, only one secondary will
ever succeed at acquiring the lock. Ideally, the other would then grab
the lock and pass, but in testing this does not seem to be the case -
the lock always times out, so the failsafe check is technically not
needed but has been left as an added safety mechanism. With this setup,
the node that fails the contention will never block the switchover nor
will it try to force itself onto the cluster after another node has
successfully won contention.
Timeouts may need to be adjusted in the future, but the base timeout of
0.4 seconds (and transition delay of 0.5 seconds) seems to work reliably
during preliminary tests.
This may or may not help, but should in theory prevent the flush from
trying to run after a (locally-running) API daemon is terminated, which
could cause an API failure and a failure to flush.
This will stop systemd from killing the service in the middle of a flush
or unflush operation, which completely defeats the purpose. 30 minutes
was chosen as this is a very large but still somewhat manageable value,
which should cover even a very large very loaded cluster with room to
spare.
Most of these actions/conditionals were looking for primary state, but
were failing during node takeover. Update the conditionals to look for
both router states instead.
Also add a wait to lock flushing until a takeover is completed.
Joshua Boniface