If not using well-polished Node RED approach but DIY...
On a separate client device such as an or another Omega 2, or
run it directly on the MQTT server itself:
to continuously run a process you designed, which will
(a) run a copy of MQTT_client
This process will receive the topics it subscribed to,
take in the new event it received to
(b) trigger actions you want your programming logic to do.
One example of such action is to
(c) publish a new MQTT event (topic_xyz/light_1/ON) to the MQTT broker.