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peer-to-peer system
subject			fact

peer-to-peer system	allows each program to function as both a client and a server
	contains software components that are distributed over several hosts and that can each be both a server and a client
	has definition A client-server system in which each program can act as both a client and a server for all the other programs
	is a subtopic of 3.4 - The Client-Server Architecture
	is a subtopic of 9.5 - Architectural Patterns
	is a kind of client-server system
	is a kind of client-server system
client-server system	generally uses multiple threads of control that can be concurrently executed to implement concurrent operations, otherwise when the client is waiting for one kind of input, it will not be able to respond to the other kind of input
	has interaction The server starts running The server listens for clients wanting to connect Clients start running and perform various operations, some of which require connecting to the server to request a service When a client attempts to connect, the server accepts the connection if it is willing The server waits for messages to arrive from connected clients When a message from a client arrives, the server takes some action in response, such as sending a message back to the client, then resumes waiting Clients and servers continue functioning in this manner until one of them decides to shut down
	has problem developers of clients are frequently forced to upgrade their clients whenever the server is changed if clients and servers are developed by different organizations
	involves at least one server and one client
	is inherently concurrent because the server runs concurrently with the clients, normally (but not necessarily) on different computers
	is prone to security violations, due to the fact that information is transmitted over a network
	is vulnerable to interruptions in communication or denial-of-service attacks
	should be forward-compatible and backward-compatible with other versions of clients and servers by designing the client-server protocols to be very general and flexible
	should incorporate encryption, firewalls and similar methods of ensuring security
distributed system	is divided up into clients and servers
system	can have specification which is then implemented by a collection of components
	exists even if its components change over the course of time, or are replaced by equivalent components
	has scope
	has part component
	has part hardware
	has part module
	has part software
	has part subsystem

Kinds of peer-to-peer system :

• Napster

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