Many monitoring tools are built around the concept of the machine or instance. The increasing use of patterns like Phoenix Server and tools like Docker mean this is an increasingly unhelpful way to model infrastructure: Instances are becoming transient while services are the things that persist. Sensu allows an instance to register itself as playing a particular role, and Sensu then monitors it on that basis. Over time, different instances playing that role may come and go. Given these factors and the increasing maturity of the tool, we felt it was time to bring Sensu back on to the radar.