We have two servers in our office. Both server are complete. Both are working HTTP server. Now server A is the master server and server B is the slave server. Server A is visible to the world. All http request will be handled by the server A. Server B will only keep syncing with server A. In case if server A go down in future we will make server B master server immediately. Then we will troubleshoot the master server and make that UP again. For this purpose you need to do two thing.
1. Just implement centos 7 rsync package to server A and B. Server B will automatically update itself from server B. This is one way not two way. Syncing will be done A > B.
2. Configure server A and B so that they can deliver outgoing emails through and external mail server.