In general, a self-adaptive system has sensors that connect it to the world and eventually help it listen to its internal components. These sensors generate raw data that represent the physical characteristics of the world. In KnowLang, by specifying a METRIC concept we introduce a class of sensors to the KB and by specifying objects, instances of that class, we represent the real sensor. KnowLang allows the specification of four different types of metrics:

• RESOURCE - measure resources like capacity;
• QUALITY - measure qualities like performance, response time, etc.;
• ENVIRONMENT - measure environment qualities and resources;
• ENSEMBLE - measure complex qualities and resources where the metric might be a function of multiple metrics both of RESOURCE and QUALITY type.

The following example presents a specification of metrics mainly used to assist the specification of states in the specification of the SCP concept shown in the STATES Section.