4.1 Peer-To-Peer Information Systems (P2PIS)

	Large information systems must often be developed as a network of loosely coupled autonomous components,
	services (possibly information systems) integrated using peer-to-peer principle (further P2PIS).

	The change of the architecture should be accompanied by changes in requirements specification that should
	reflect the service oriented structure of the system.

	The specification of P2PIS starts from the specification of system user interface (portal) and from the
	specifications of the services.

The specification of services starts from the definition of their interfaces.

There are two main variants of P2PIS:

		The first one is used in e-commerce where the service starting a communication must first look for
		communication partners. The partners must offer their interfaces. This schema implies the use of Internet
		and international standards like SOAP. Such systems (software) are called alliances.

		If a system S has p2p architecture, it must have structure allowing its peers/services to collaborate. The
		services must be equipped by gates connecting them to a middleware. The system S must usually be
		equipped by a user interface (portal). There can be several portals. Alliances need not have any portals as
		shown in Figure 4.1.

4.1 Peer-To-Peer Information Systems (P2PIS)

Figure 4.1: Architecture of a service oriented system (G is a gate, UC is an user interface service (portal))

4.1 Peer-To-Peer Information Systems (P2PIS)

	The properties of P2PIS depend substantially on the properties of the interfaces provided by the gates and by
	the functions of the middleware.

The most important property of the interfaces is how much they vary.

Stable interfaces increase the stability of P2PIS; reduce the development and maintenance costs.