For a DB server:
Make a regular backup of all databases on your db server, e.g. with mysqldump plus a backup of the /etc directory and /usr/local/ispconfig. If the db server dies anytime in the future, then just install a new server incl ispconfig but dont connect it to the master server, then restore all databases from your mysqldump files e.g. by simple piping their content to the mysql command and restore the two config.inc.php files form your ispconfig backup and your db server is fully restored and connected again. No need to craete new databases or any other records in ispconfig, just restore the old server from the backup.
A alternative which micht be even simpler is to use one of the common linux backup softwatre packages, just do a regular backup and restore that wehn your server failed. there is nothing ispconfig specific here as you backup and restore the whole server.