In Eon Mode, a primary node is a node that is a member of a primary subcluster. Primary nodes are the only nodes in the database that Vertica considers when determining whether the database is able to maintain data integrity. Vertica requires that more than half of the primary nodes in the database be up. Also, all shards must have at least one primary node as a subscriber. If either of these conditions are not met, Vertica shuts the database down to prevent potential data corruption.