We met a serious problem of getting STALE DATA.
This problem comes from slaveDelay and Ghost sync.
I can see this warnings, if we set the miss settings.
But we won't get this warnings, when this replica set was dull.
This problem lead to human error.
But bearable, because we can avoid it.
Simulate the network trouble.
Then 192.168.154.134 is still available !!
192.168.154.134 would change the sync target form primary(192.168.154.133) to slaveDelay secondary(192.168.154.135) and KEEP ALIVE in spite of being delayed !!
But we (mongo client) cannot realize that 192.168.154.134 is now delayed.
We think, the node should die (unreachable from client) instead of unexpected delay.
Then we (client) can read fresh data from primary.