Comments - Is it possible to have two write-able mariadb's in a set of four?
Content reproduced on this site is the property of its respective owners,
and this content is not reviewed in advance by MariaDB. The views, information and opinions
expressed by this content do not necessarily represent those of MariaDB or any other party.
This is what would be called a multi-master setup with more than one server where writes occur. To configure it, you can use multi-source-replication.
It can work as long as you guarantee that no two writes can ever conflict between Server 1 and Server 3. This is usually quite hard to guarantee if you intend the same applications to write to both Server1 and Server 3. One option would be to use a Galera cluster but the performance might not be optimal if the nodes are located far apart.