Question

“MongoDB in Action” book says:

Imagine you issue a write to the primary node of a replica set. What happens next? First, the write is recorded and then added to the
primary’s oplog. Meanwhile, all sec- ondaries have their own oplogs
that replicate the primary’s oplog. So when a given secondary node is
ready to update itself, it does three things. First, it looks at the
time- stamp of the latest entry in its own oplog. Next, it queries the
primary’s oplog for all entries greater than that timestamp. Finally,
it adds each of those entries to its own oplog and applies the entries
to itself

So this means nodes must be time synchronized? because timestamps must be equal on all nodes.

Answer 1

In general, yes, it is a very good idea to have your hosts synchronized (NTP is the usual solution). In fact I have seen far worse issues caused than an out of sync oplog – different times on database hosts in a cluster should be considered a must.

This is actually mentioned on the Production Notes page in the docs:

http://www.mongodb.org/display/DOCS/Production+Notes#ProductionNotes-Linux

See the note about minimizing clock skew.