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Galera Cluster is a virtually synchronous multi-primary clustering solution for MySQL and MariaDB.
A Cluster is made up of multiple database servers(nodes), each with their own storage (shared-nothing architecture), which database changes are automatically copied to ensuring that data is always consistent and identical on each node. More information can be found on the Codership Galera Cluster project page.
Galera Cluster is powered by the MySQL-wsrep patch for MySQL and MariaDB. The patch only supports Linux/Unix operating systems and thus Galera Cluster will only work on the same.
When a node joins the cluster all database objects need to be copied to it (the joiner) from any existing synchronized system (a donor). The process of performing a full data copy to a new node is called a State Snapshot Transfer(SST). A node that has previously been a member of the cluster but is out of date on rejoining will have an Incremental State Transfer(IST) performed to bring it up to date.
SSTs methods are done using either Logical or Physical methods. A logical method interacts with database objects and contents while a physical method will interact with the underlying data files. Physical SST methods are faster when compared to Logical SSTs but require the donor node to block until the transfer is completed.
The SST methods which are built into Galera Cluster will be performed automatically if the joiner and donor are set up correctly. The supported methods as of Galera Cluster version 4 are :