1. A monitor device that determines a status of a service, comprising: a processor that provides a dependency tree representing (i) relationships among all the infrastructure elements in a computer system that are related to delivery of a service by a computer system, and (ii) how the infrastructure elements interact with each other in a delivery of said service, wherein a state of an infrastructure element is impacted only by states among its immediately dependent infrastructure elements of the dependency tree; and determines the state of the service by checking current states of infrastructure elements in the dependency tree that immediately depend from the service, wherein the infrastructure elements include: the service; a physical element that generates an event caused by a pre-defined physical change in the physical element; a logical element that generates an event when it has a pre-defined characteristic as measured through a synthetic transaction; a virtual element that generates an event when a predefined condition occurs; and a reference element that is a pre-defined collection of other different elements among the same dependency tree, for which a single policy is defined for handling an event that occurs within the reference element.