Figure 2.

(a) A, representing an appliance, has a provide operation $switch()$ and a provided signal $rate$. M, representing a meter, has a provide operation $read()$, and a required signal $rate$; and (b) $A\left|\right|M$ connects the provided signal of A and the required signal of M, thus forming a component that provides operations $switch()$ and $read()$. Different ways of composing components represent different design approaches: (c) evolution of a household with a meter; and (d) evolution of an Aggregator.