Many information-processing tasks can be part of multiple customer applications, as summarizing stock prices, integrating catolog information from several
companies in the same line of business, predicting the weather, and checking on transportation resources. We assign such sharable services to an active
middleware layer, interposed between clients and servers. We define domain-specific mediator modules to populate this layer.

Such mediating services must be of value to the customers, so that it will benefit their client applications to access mediators rather than the server sources
directly. Several categories of value can be considered: improvement in access and coverage, improvement of content, and delegation of maintenance. We
will define criteria for mediating modules: ownership by the party who assumes responsibility for the results of the services, domain-specificity to delimit the
scope of such a responsibility, and, of course, conformance with interface standards that enable interoperation.

Applications that benefit from mediation include planning and other decision-making tasks that require information from diverse resources, as databases,
reference systems, data obtained from sensors, and analyses for projecting trends into the future. The sources are often autonomous, some of them are
public, and will typically be heterogenous. The heterogeneities include representation, scope, level of abstraction, and context.

Mediated results are intented to be composable by higher level application programs, so that the applications can solve problems involving multiple subtasks
and multiple domains. Mediation requires domain expertise, and mediators are hence domain-specific. Mediators need a machine-friendly interface to
support their client applications. This interface must provide good communication, while encapsulating the mediation tasks, so that the complexity of the
composed system is not much greater than that of the individual subtasks. However, questions of effectiveness and efficiency do arise, and must be dealt
with by exploiting the processing and storage capabilities of modern hardware.

The corresponding architecture is best viewed as a generalization of a server-client model. The partitioning into layers and by domain enhances
maintainability: the applications software staff can concentrate on functional improvements, the data resource managers on operational issues, and the
technical maintenance is concentrated in the mediator layer. The concept is network-oriented, and mediating services are best provided over the network by
domain specialists. With suitable payment mechanisms, mediating services can be performed by independent entrepreneurs.