mysql module provides MySQL connectivity for OpenSER. It implements the DB API defined in OpenSER, usually being referred as MySQL driver for OpenSER. It does not offer functions usable from configuration script, being used by other modules like auth_db, acc, avpops, group, usrloc, … Alternative DB drivers are dbtext, postgres, unixodbc.
However, mysql is most used DB driver in OpenSER. It takes care of reconnecting to MySQL server when the link was lost, and is able to keep the connection alive even no traffic on it.