UNIX and Linux Applications

Hello everyone
I wanted to asked you guys for advice regarding the requirement to have Open LDAP Server in organization.
I installed Open LDAP Server, and also Client machine for testing before I put into production.
Everything works fine, along with a friendly web interface, him I add user accounts in the organization and everything's cool.
The second requirement that i that its why I turn to you here
its about the demand insert Redundancy/Backup Or whatever you want to call it
And the request is to set another server work as, say as SLAVE, and fully synchronized with the LDAP server Master.
Once each user account that I set up through the web, in the open ldap, or any change i done, also will get to the backup server.
And its main second goal its if for any reason the primary server suddenly fails and stops working, the backup server will take Control,
And we do not feel vulnerable in service, the web that manage the accounts will continue to work, and the damage will only be survival.
What is the correct way I do it?
Thanks.