An obvious question that I didn't hit on in the blog is "what about host crashes"? The nice thing is, every index knows exactly which messages it covers - and we can quite quickly (within an hour or so for an entire server) scan all mailboxes and index the missing messages - it's more efficient than doing it in a real time, because you are often indexing multiple messages in the same mailbox.

Once the indexes are up to date, we can switch back to being masters again. We index on all the replicas independently so that they are always ready.