That’s good to hear. 
I think especially @martin.schwarz’s answer to the other post explains it very good and concise.
See also my answer to this related question: Agent-initiated checks instead of server-initiated checks. The “trick” I use there avoids that half-transmitted files get cated on the server side.
The $(hostname -f) thing gets replaced with the client’s fully qualified hostname. If the client is configured with that name in checkmk then you can create the datasource rule with cat /path/to/$HOSTNAME$. checkmk will replace $HOSTNAME$ with the hostname as configured in checkmk.