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Note: If you use Microsoft SQL Server, a single database is shared by Node 1 and Node 2 (and generally installed on a third node), so database replication is unnecessary.
If you use the MySQL database, the database is replicated by the primary node sending SQL statements to the secondary node, which runs them itself on its own copy of the database. (Replication features built in to the database are not used.) During the usually short time between the original update of the primary database and the corresponding update of the secondary database, the SQL statements are stored in an encrypted file named MICSQL.blg. This buffering of SQL statements allows the replication to be done at a later time if the secondary node is down.