Using observer triggered functions with cluster

How can I make sure an observer-triggered function is only called once on an app that uses cluster to load balance multiple instances of the app?

edit: before anyone mentions making a new micro-service to handle the observer in a standalone app, it just really doesn’t fit our domain model and our codebase is getting too big to make sacrifices like that

Who first insert record to collection, that instance will have right to handle it :smiley:
Using mongo as token distributor for example.