Usually, you will find that when talking about PBXes, they are classified in Class 4 and Class 5. So, obviously, your next question is: What is the difference between a Class 4 and a Class 5 PBX? I will try to explain as clear as possible.
A Class 5 PBX deals with end-user. It provides final-user services such as Voice-Mails, Conference rooms, IVR and many another kind of services that a phone user may need. This kind of switches deals with the user interaction (SIP endpoints such as Soft/Hard phones). So, in general terms, you can state that principal client of a Class 5 PBX is end-user.
It is very common to find them in the retail market.
Class 4 PBX is a little different. They don't deal with end-user, but instead, they deal with other switches. If you are familiar with networking, you can see Class 4 switch as a router that is connected to other routers. Class 4 ones do not offer conference rooms, IVR or any other endpoint item; authentication is usually tied to IP rather than registration.
You can find this kind of switches on the wholesale market.
MariaDB Galera Cluster is a synchronous multi-master cluster for MariaDB. It is available on Linux only, and only supports the XtraDB/InnoDB storage engines (although there is experimental support for MyISAM - see the wsrep_replicate_myisam system variable).
Starting with MariaDB 10.1, the wsrep API for Galera Cluster is included by default. This is available as a separate download for MariaDB 10.0 and MariaDB 5.5.
MariaDB has one of the best and easiest solutions to do a Master-Master replication: Galera. Galera uses WSREP, which it is responsible for maintaining all the Database nodes in the cluster in sync. This means, after changing one record on node A, it is just a matter of seconds to see the change on node B.
I will talk in this article how I did this configuration using MariaDB 10.1.x. Remember you can install MariaDB under Centos 6 and 7 by using OKay's repository.