Data Auto Sync between VMs on two separate physical machines

I have two physical machines on each of which I have Installed RHEL 7.2 on physical with 3 VMs of RHEL 7.2 (1st VM for Application, 2nd VM for Database, 3rd VM for Data). Similarly I have created it on other Physical Machines.
I want One physical machine to act as primary Server which would be accessed by the users for data entry, whereas, the other Server would act as Secondary Server and would update and auto-synchronize itself with complete data of the Primary Server. In other words, Secondary Server would be a complete replica of Primary Server and in case Primary Server fails Secondary Server would take over as Primary Server till the damaged Server is restored. Once the damaged Server is restored it should again update and synchronize the complete data with the active Server.
Just a probable step-by-step solution.
Thank You.