tailieunhanh - DISTRIBUTED SYSTEMS principles and paradigms Second Edition phần 2

ông đã được Hiệu trưởng trường cao cấp cho máy tính và hình ảnh, một trường Interuniversity tốt nghiệp làm nghiên cứu song song tiên tiến, phân phối, và các hệ thống hình ảnh. Trong quá khứ. ông đã thực hiện nghiên cứu trên các trình biên dịch, hệ thống điều hành, mạng, và hệ thống phân phối khu vực địa phương. | SEC. SYSTEM ARCHITECTURES 53 Collaborative Distributed Systems Hybrid structures are notably deployed in collaborative distributed systems. The main issue in many of these systems to first get started for which often a traditional client-server scheme is deployed. Once anode has joined the system it can use a fully decentralized scheme for collaboration. To make matters concrete let us first consider the BitTorrent file-sharing system Cohen 2003 . BitTorrent is a peer-to-peer file downloading system. Its principal working is shown in Fig. 2-14 The basic idea is that when an end user is looking for a file he downloads chunks of the file from other users until the downloaded chunks can be assembled together yielding the complete file. An important design goal was to ensure collaboration. In most file-sharing systems a significant fraction of participants merely download files but otherwise contribute close to nothing Adar and Huberman 2000 Saroiu et al. 2003 and Yang et al. 2005 . To this end a file can be downloaded only when the downloading client is providing content to someone else. We will return to this tit-for-tat behavior shortly. Figure 2-14. The principal working of BitTorrent adapted with permission from Pouwelse et al. 2004 . To download a me a user needs to access a global directory which is just one of a few well-known Web sites. Such a directory contains references to what are called .torrent files. A .torrent file contains the information that is needed to download a specific file. In particular it refers to what is known as a tracker which is a server that is keeping an accurate account of active nodes that have chunks of the requested file. An active node is one that is currently downloading another file. Obviously there will be many different trackers although there will generally be only a single tracker per file or collection of files . Once the nodes have been identified from where chunks can be downloaded the downloading node effectively .