From yesterday to this morning, we had set up two MySQL servers as master-to-master configuration.

One server is located in Tokyo, the other is in Shiga.
They are replicated each other in order to get our system redundant and load-balanced. We had used
OpenVPN
for this purpose.
It is very easy to setup by using tunnelling.

Okey, we agree with that the VPN will become the neck of a bottle if our network get heavy traffic.
But fortunately or unfortunately, it is not so heavy.
So we considered that a performance issue would be still no problem, against a stability issue.