Galera Replication is a synchronous multi-master replication plug-in for InnoDB. It is very different from the regular MySQL Replication, and addresses a number of issues including write conflicts when writing on multiple masters, replication lag and slaves being out of sync with the master. Users do not have to know which server they can write to (the master) and which servers they can read from (the slaves).
An application can write to any node in a Galera Replication cluster, and transaction commits (RBR events) are then applied on all servers, via a certification-based replication.
Certification-based replication is an alternative approach to synchronous database replication using Group Communication and transaction ordering techniques.
A minimal Galera cluster consists of 3 nodes. The reason is that, should there be a problem applying a transaction on one node (e.g., network problem or the machine becomes unresponsive), the two other nodes will have a quorum (i.e. a majority) and will be able to proceed with the transaction commit.
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