DRIVER => [ 'DBI',
    DBH         => $authdb,
    TABLE       => 'userdb',
    CONSTRAINTS => {
      'userdb.username' => '__CREDENTIAL_1__',
      'userdb.password' => '__CREDENTIAL_2__'
      # 'MD5:userdb.password' => '__CREDENTIAL_2__'
      # 'MD5_hex:userdb.password' => '__CREDENTIAL_2__'
      # 'MD5_base64:userdb.password' => '__CREDENTIAL_2__'
    },
  ],