The monitor utilises the sensor that best matches the quality of service required, and will switch sensors if necessary to maintain that quality of service. The sensors and synthesizers simply provide a current contextual value. This is because state behaviour is artefact-dependent. For example, a person artefact needs to have their location updated a lot less frequently than a car artefact because of the slower potential rate of change. The artefact, state, sensor, synthesizer, and monitor components described thus far comprise the set of modelling components that can be used to construct a contextual model.
Clients can add new components to the catalog.
This model provides a view of the world shared by CIS clients. The four services combined form the interface through which any client program or user may construct, view, or manipulate a shared contextual model of the world. Extensibility and Reusability Issues of extensibility and reusability are paramount, as there are an infinite number of artefacts and states that a client may wish to observe. The CIS addresses these concerns by providing a catalog of components that can be augmented as required. To model a particular context the client simply selects the appropriate components from a catalog, or, alternatively, can reuse artefacts that are already in the world.