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three-tier system
subject			fact

three-tier system	contains a server which communicates with both a client (usually through the Internet) and with a database (usually within an intranet, for security reasons) and which acts as a client when accessing the database server
	is a subtopic of 9.5 - Architectural Patterns
	is a kind of client-server system
	uses three-tier model of client-server architecture
client-server system	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

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