TAILIEUCHUNG - Building Secure and Reliable Network Applications phần 8

và những tin tức tốt là cho hầu hết các thiết lập thực tế câu trả lời là các giao thức như vậy làm cho tiến độ với xác suất áp đảo cao nếu xác suất thất bại và mất tin nhắn thống nhất và độc lập trên các quy trình và tin nhắn được gửi trong hệ thống. | Chapter 18 Reliable Distributed Computing Systems 359 Using Horus it was straightforward to extend CMT with fault-tolerance and multicast capabilities. Five Horus stacks were required. One of these is hidden from the application and implements a clock synchronization protocol Cri89 . It uses a Horus layer called MERGE to ensure that the different machines will find each other automatically even after network partitions and employs the virtual synchrony property to rank the processes assigning the lowest ranked machine to maintain a master clock on behalf of the others. The second stack synchronizes the speeds and offsets with respect to real-time of the logical timestamp objects. To keep these values consistent it is necessary that they be updated in the same order. Therefore this stack is similar to the previous one but includes a Horus protocol block that places a total order on multicast messages delivered within the group. 18 The third tracks the list of servers and clients. Using a deterministic rule based on the process ranking maintained by the virtual synchrony layer one server decides to multicast the video and one server usually the same decides to multicast the audio. This set-up is shown in Figure 18-5b. To disseminate the multi-media data we used two identical stacks one for audio and one for video. The key component in these is a protocol block that implements a multi-media generalization of the Cyclic UDP protocol. The algorithm is similar to FRAG but will reassemble messages that arrive out of order and drop messages with missing. One might expect that a huge amount of recoding would have been required to accomplish these changes. However all of the necessary work was completed using 42 lines of Tcl code. An additional 160 lines of C code supports the CMT frame buffers in Horus. Two new Horus layers were needed but were developed by adapting existing layers they consist of 1800 lines of C code and 300 lines respectively ignoring the comments and .

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