outline13.txt

unix/linux 编程实践一书的所有源代码

			   Lecture UDP			  Outline
_________________________________________________________________

	  Topics:  Multiple clients, servers with sensitive data

	  Approach:  Investigate license management in detail

	  Today's New Ideas:  datagrams, the ticket model, transactions

	  Outline

	       The Ideal World - Where systems never crash

		    1.	What is a product license
			 A means for controlling where/who/how many users

		    2.	What is a license server
			 A program that enforces product licensing in
			 a multi-user setting.

		    3.	The ticket model
			 How the license server limits usage of an
			 application

		    4.	The Communication Model
			 Simple transactions - a request and a response
			 Datagrams - postcards from one socket to another

		    5.	Flow of Control
			 In the client
			 In the Server

		    6.	The code itself.

	       The Real World - where bad things can happen to good programs

	       Possible problems:
		    1.	Clients can crash - natural disasters
		    2.	Servers can crash - unnatural disasters.

	       Solutions:
		    1.	Clients who crash leave tickets stuck - server
			 can "expire" these tickets.
			- The ball_move() model for ticket expiration.

		    2.	When the server crashes, the database is lost.
		       - The clients can "check in" to be sure their tickets
			 are still good.
		       - Ticket validation as a means of recovery.