CFEngine's philosophy and modus operandi is to make machines as self-reliant as possible. This is the path to scalability. Sometimes we want machines to be able to detect one another and sample each others' behavior. This can be accomplished using probes and server functions.
For example, testing whether services are up and running can be a useful probe even from a local host. CFEngine has in-built functions for generically probing the environment; these are designed to encourage decentralized monitoring.