Alerting

Collecting signals is useless if no one is watching them at 3 a.m. Alerting is the active half of the watch: thresholds and rules that, when crossed, push a notification to a human instead of waiting to be found. A disk filling, a service down, a security event that shouldn’t be — each has a line, and crossing it raises a hand.

What triggers an alert, and how it reaches someone, is defined here. The collection it draws on is Signals; the outside liveness check that fires when everything else can’t is Uptime.