More than five years ago, Rob Ewaschuk created an innocuous Google doc titled “My Philosophy On Alerting”. It became kind of viral and later formed the foundation of a chapter in the famous book Site Reliability Engineering – How Google Runs Production Systems. In parallel, the metrics-based monitoring and alerting system Prometheus was developed at SoundCloud. It is the open-source tool to put Rob’s philosophy into practice. Thus, I would like to present “applied alerting philosophy” and explain how we use Prometheus at SoundCloud to create meaningful and actionable alerts. In particular, SoundCloud follows a fairly radical “you build it – you run it” approach, which requires additional care to route alerts to the right group of engineers. Prometheus’s “label everything” mantra proves to be very helpful here.