英語翻譯
Consider the case of a mail-order firm shown in Figure 5.16.On the left side,customer Sanchez sent a payment on his account.At the same time the clerk begins to process the payment,Sanchez calls a second clerk and places a new order.The figure shows what happens if both transacions continue and interfere with each other.What should the final balance be?Does the computer have the correct number?To solve this problem,the application program must know that several people might try to access the same piece of data at the same time.The software locks out all users except one.When the first process is finished,the other users can try to gain access again.To keep data accurate,applications used by many people at the same time must be written to handle these concurrency problems.Early personal computers were designed for only one user,so much of the software did not prevent concurrency problems.Software designed for computer networks generally handles this issue.When you use this software,you will occasionally receive a message that says a piece of data you desire is currently being used by another person.If you get this message,simply wait for a few minutes and try again.When the first person is finished,you should be able to proceed.