tailieunhanh - DISTRIBUTED SYSTEMS principles and paradigms Second Edition phần 3

Nghiên cứu hiện tại của ông tập trung chủ yếu vào bảo mật máy tính, đặc biệt là trong hệ điều hành, mạng lưới, và diện rộng hệ thống phân phối lớn. Cùng nhau, tất cả các dự án nghiên cứu đã dẫn đến hơn 125 giấy tờ refereed trên các tạp chí và các biên bản hội nghị và năm cuốn sách, đã được dịch ra 21 ngôn ngữ. Giáo sư Tanenbaum cũng | 124 COMMUNICATION CHAP. 4 as a middleware service without being modified. This approach is somewhat analogous to offering UDP at the transport level. Likewise middleware communication services may include message-passing services comparable to those offered by the transport layer. In the remainder of this chapter we concentrate on four high-level middleware communication services remote procedure calls message queuing services support for communication of continuous media through streams and multicasting. Before doing so there are other general criteria for distinguishing middleware communication which we discuss next Types of Communication To understand the various alternatives in communication that middleware can offer to applications we view the middleware as an additional service in clientserver computing as shown in Fig. 4-4. Consider for example an electronic mail system. In principle the core of the mail delivery system can be seen as a middleware communication service. Each host runs a user agent allowing users to compose send and receive e-mail. A sending user agent passes such mail to the mail delivery system expecting it in tum to eventually deliver the mail to the intended recipient. Likewise the user agent at the receiver s side connects to the mail delivery system to see whether any mail has come in. If so the messages are transferred to the user agent so that they can be displayed and read by the user. Synchronize at Synchronize at Synchronize after request submission request delivery processing by server Server Time Figure 4-4. Viewing middleware as an intermediate distributed service in application-level communication. An electronic mail system is a typical example in which communication is persistent. With persistent communication a message that has been submitted for transmission is stored by the communication middleware as long as it takes to deliver it to the receiver. In this case the middleware will store the message at one or several .