DBA shell scripting expert required; Make a mysql script replication autostart.

Hello there, We need to have mysql recuriment from our server A to server B. We have made one script to get run but in past weeks we got errors. The concept is following: Script should do mysql replication exactly every 10-15 mins from SERVER A to SERVER B. It should replace all old entries with new ones. After that to delete any temporary files. To give you more sense: we manage a custom built a payment gateway. Many sites use this to manage payments. So it is critical that this site is ALWAYS online. If it fails, it means our customer could lose hours of orders on all sites. So we have 2 servers: one server A (main server used), and a server B that is a backup server. Through cloudflare, if server A become unavallable for more than some seconds, it automatically switch on server B. This is why server B need to synch always with server A, to have a up-to date version. And if server A fails and B becomes main, I am not sure how they could make it work: ideally, we should automatically switch back to the server A when it is working again, and server A would synchronize on server B to be itself also, up-to date. So data is never loss.