Beacons are a very new feature in Salt, but they've already gained quite a following. In past versions of Salt, if a third-party process needed to raise an event inside Salt, it would have to explicitly make a call to Salt to do so. Beacons overcome this by allowing events to be triggered by third-party processes without having to perform any work inside that process itself.
As you can imagine, beacons were designed for monitoring and, specifically, alerting purposes. While monitoring states are fairly passive, in which they only run when called explicitly or via the scheduler, beacons are very proactive, in which they are constantly watching for changes.
Beacons are run on a regular basis on the target minion. When they pick up important changes, they will fire an event that describes these changes.
The first beacon that was ever added was for the inotify system. This is built in the Linux kernel, starting with version 2.6.13. The inotify system...