How to set up a system with openvpn to automatically switch to a backup link?

Question

linux

Hello, dear luminaries!
I have a microcontroller on linux / Debian / wheezy, which has wi-fi and a GSM modem inside. Accordingly, you can access the Internet via wi-fi (which is created by the router and the 3G modem), or you can access it via ppp on the controller's GSM modem. Inside the controller lets through openvpn (on the controller - the client, + own external openvpn server). The controller is in the country, and the Internet there often "blinks" for both providers. Accordingly, the question is:
How to properly configure this whole bunch so that the openvpn connection remains active (preferably without breaking the TCP / IP connections established through it)?
Can it be done at all?
Can this be done by just configuring the interfaces (wlan/ppp/openvpn)? How?
If you need a script for control and switching - where can I see working examples?
Thank you all in advance!

Answer 1

Well, here's an example: read .
But in general, the metric in routing can save you.