What is the best way for a system to be notified when a change occurs in real-time (or relatively close)

I have thought of using either an observable/observe design, although I feel that this is not the best possible approach. I am curious if there is anything that actually handles this better or more elegantly. I would like to stay away from a scheduling type of check.

Scenario:

1. Waiting on a system to be in completed status. (This is the observable system)


2. Once we see that the observable system is completed, we notify a user that it is complete.

Think of this like a system checking a retail store for a certain item to drop below a desired price. If it drops below the price, send out a notification to the user.