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1、计算机网络与分布式系统,北京大学计算机科学与技术系王建勇Email:URL:HTTP:/,Why Do We Study Distributed Systems?,规模可伸缩性(scalability)通过图灵测试(Turing test)语音到文本的转换(speech to text)文本到语音的转换(text to speech)机器视觉(machine vision)个人的“梅米克斯”(personal MEMEX),A dozen remaining IT problems proposed by James Gray:,世界的“梅米克斯”(world MEMEX)虚拟现实(TeleP
2、resence,i.e.VR)无故障系统(trouble-free systems)安全系统(secure systems)高可用系统(AlwaysUp)自动程序设计(automatic programming),教材：G.Coulouris,J.Dollimore,J.Kindberg.Distributed Systems:Concepts and Design.Addison-Wesley,1994,参考书:Larry L.Peterson and Bruce S.Davie.Computer Networks:A System Approach,Morgan Kaufmann,1996
3、.Andrew S.Tanenbaum.Distributed Operating Systems.Prentice Hall International,Inc.,1996.Andrew S.Tanenbaum.Computer Networks.Prentice Hall International,Inc.,1996.Xueliang Yang.Distributed Computer Systems.Graduate school of USTC,the Chinese Academy of Sciences,P.R.China.,成绩考核:1次编程（11月12日前完成提交），1次文献
4、阅读（论文提交的截止日期：2000/12/26），一次期末考试（占总成绩的60%）.,网络与分布式系统文献阅读URL:http:/,安全与身份验证1.Jennifer G.Steiner,Clifford Neuman,and Jerrfey I.Schiller.Kerberos:An Authentication Service for Open Network Systems.Proceedings of the 1988 USENIX Winter Conference,February 1988,Dallas,Texas,Pages 191-202.(MIT)2.Butler Lam
5、pson,Martin Abadi,Michael Burrows,and Edward Wobber.Authentication in Distributed Systems:Theory and Practice.Proceeding of the 13th Symposium on Operating Systems Principles,October 1991,Pacific Grove,CA,Pages 165-182.(DEC)3.Victor L.Voydock and Stephen T.Kent.Security Mechanisms in High-Level Netw
6、ork Protocols.ACM Computing Surveys,15(2),June 1983,Pages 135-171.,Tentative course outline,Introduction,Basic networking,ISO model,networks DFS case studies:NFS,AFS,Coda,COSMOS(or S2FS).,Tentative course outline(continue),Distributed shared memory:IVY&Munin Coordination in Distributed System:potent
7、ial causality,clock synchronization,logical time Replication:Gossip&Isis Transaction:Acid,Locks,deadlocks,nested transaction,optimistic concurrency control,timestamp ordering,distributed transaction Recovery&Fault tolerance Security in DS:DES,RSA,digital signature,Needham-Schroeder model,kerberos,Co
8、mponents of distributed system,Chapter 1 Introduction to Distributed Systems,Review of computing historyWhy should we develop distributed system Key characteristics of distributed system,1.1 Review of computing history,Physically distributed hardware Logically centralized software,1.1.1 The trend of
9、 hardware,1.1.2 The need for logically centralized software,1960s&1970s:timesharing system,1980s:personal computer&personal workstation,1990s:distributed computer systems,Users requirement:-a system built out of large numbers of powerful PCs or workstations-but which act together in a coherent way t
10、hat is as easy to use&understand as an old fashioned timesharing system.,Role of a new generation operating system(DOS):,2000s:mass distributed systems,-e.g.,Web OS,Cluster OS,Review of computing history,Figure 1.2 Milestones in Distributed Systems,1.2 Why should we develop distributed system,1.2.1
11、Most important reason is that application is a starting point and end result of development of distributed systems.,Many computer applications occur in a distributed or decentralized environment.Sharing expensive resources Exchange data between systems,1.2.4 The interface between users and Computers
12、 is more friendly,1.2.5 LAN&Internet applications stimulate DOSs development,Its the software,not the hardware that determines whether a system is distributed or not,1.2.6 Examples of distributed systems and applications,1、Distributed UNIX:Berkeley BSD UNIX+NFS+NIS Amoeba,Mach,Chorus,1.2.3 Prolifera
13、tion of low cost and high performance PCs or Workstations,2、Commercial applications airline seat reservation and ticketing automatic teller machine,Reliability,security,3、Wide area network applications Internet,ARPAnet 100=1 million Internet information service,such as Email,Web,www search engine,BB
14、S,E-commerce、digital library,4、Cluster system IBM SP2 Berkeleys NOW NCICs Dawning superserver,5、Meta Computing idle computers are ubiquitous,6、Multimedia information access and conferencing application continuous media service,such as VOD servers,video phone and video conference,their main requireme
15、nt is quality of service ATM,real time OS,continuous media servers,1.3 Key characteristics of distributed system,Whats the Distributed System?,Definition 1:A distributed system is one in which there exists a multip-licity of interconnected processing resources able to cooperate under system-wide con
16、trol on a single problem with minimal reliance on centralized procedures,data or hardware.Formulated by the organizing committee for the 1st conf.on DCS,Definition 2:A Distributed system consists of a collection of autonomous computers linked by a computer network and equipped with distributedsystem
17、 softwareFrom our textbook,1.3.1 Resource Decentralization and sharing,-Some or all of the computing resources should be decentralized in function as well as distance-and this is a prerequisite for making the distinction from other types of systems,such as time-sharing system.,1.3.2 Cooperative Auto
18、nomy,1.3.3 Concurrency(i.e.work parallelism),1.3.4 System transparency,-Some resources are very expensive,and data sharing is an essential requirement in many computer applications,-Cooperative autonomy,especially control autonomy increases the overall reliability and availability of the system,-Con
19、current vs Parallel、MIMD(Multiple Instruction Many server processes run concurrently,each responding to different request from client processes.,-it looks to its users like a centralized single computer system-but runs on multiple independent machines,i.e.Single System Image.,1.3.7 Openness,1.3.5 Fa
20、ult tolerance,1.3.6 Scalability,ISO definition:Access transparency,Location transparency,Concurrency transparency,Replication transparency,Failure transparency,Migration transparency,Performance transparency,Scaling transparency,-Two approaches to the the design of fault-tolerant computer systems:ha
21、rdware redundancy:the use of redundant components;software recovery:the design of programs to recover from faults.,-Availability is a measure of the proportion of time that it is avail-able for use.,-Scalable techniques:Re-configurable,removing performance bottleneckserverless,replicated data and se
22、rvices,caching e.g.,NFS is short of scalability.,-the characteristic that determines whether the system can be extended in various ways.-e.g.,UNIX,Open systems are characterized by the fact that their key interfaces are published;,Open distributed systems are based on the provision of a uniform inte
23、r-process communication mechanism and published interfaces for access to shared resources;,Open distributed systems can be constructed from heterogeneous hardware and software,possibly from different venders.,-To summarize:,Chapter 2 Design Goals&Issues,IntroductionBasic technical issues Users requi
24、rements Summary,Key characteristics of distributed system,Resource sharing,Openness,Concurrency,Fault tolerance,Transparency,Scalability,Key design goals,2.1 Basic design issues,Naming:-global meaning&scalabilityCommunication:-how to optimize the implementation of communication in distributed system
25、-while retaining a high-level programming model for its useSoftware structure:-how to structure a system so that new services can be introduced that will interwork fully with existing services without duplicating existing service elementsWorkload allocation:-how to deploy the processing and communic
26、ation resources in a network to optimum effect in the processing of a changing workloadConsistency maintenance:-maintenance of consistency at reasonable cost,2.1.1 Naming,name vs identifierresolved name is an identifier together with other attributes-internet communication:IP+PORT number-UNIX file s
27、ystem:index node number-Mach communication system:Port numbernaming design considerations-choose an appropriate name space-use name service to resolve names to communication identifiers-scalability considerationsname contexts are represented by tables or databases-file system:/etc/a.out vs/usr/a.out
28、names maybe structured or flat,readable or unreadable,location-independent or containing location cluesnaming schemes can incorporate security mechanism-file systems directory,2.1.2 Communication,Communication between a pair of processes involves:-transfer of data-transmission of a reply to the clie
29、nt.it can be implemented in terms of message-passing operations(send&receive)-but commonly presented at the language level as RPC,Dynamic binding in client-server model-example:DNS name server Function shipping in client-server model-example:Postscript with laser printers,2.1.2.2 Group multicast-sen
30、ding a message to the members of a specified group of processes is known as multicasting,Motivation of group multicasting-Locating an object-Fault-tolerance-Multiple update e.g.,maintaining cache coherence under write-update mechanism e.g.,Time synchronization,RAID,2.1.3 Software Structure,component
31、s of DOS-operating system kernel services extending conventional Unix kernel,like BSD Unix microkernels,like Mach,Amoeba and Chorus-open services DFS DSM other services,like electronic mail delivery service-Support for distributed programming RPC MPI or PVM,2.1.4 Workload allocation,two main workloa
32、d allocation model-processor pool model,-the use of idle workstations,2.1.4.1 The processor pool model,Figure 2.5 the processor pool model,examples:Amoeba,Plan 9,Cambridge Distributed Computing System,Dawning 2000 super server,2.1.4.2 Use of idle workstation,use of idle or under-utilized workstation
33、s as a fluctuating pool of extra computers example:Sprite,LSF,2.1.4.3 Shared-memory multiprocessors-also called Symmetric shared-memory Multi-Processor(or SMP),2.1.5 Consistency maintenance,Update consistency,-there are likely to be many users accessing shared data;-the operation of the system itsel
34、f depends on the consistency of certain databases,Replication consistency,Cache coherency-hypothesis of locality,Failure consistency,Clock consistency,User interface consistency,2.2 User requirements,Functionality-what the system should do for usersReconfigurability-the need for a system to accommod
35、ate changes without causing disruption to existing service provisionQuality of service-embracing issues of performance,reliability and security,2.2.1 Functionality,Key benefits of a distributed computer system:-economy-potential improvement in performance&reliability from distributed resource.,Enhan
36、cements to the services provided by centralized computers:-sharing across a network can bring access to a richer variety of resources than could be provided by any single computer;-utilization of the advantages of distribution enables explicit sharing,fault-tolerant or parallel applications can be p
37、rogrammed.,Three options when considering a migration from centralized computing to distributed computing:,-adapt existing operating systems for networking example:BSD Unix+NFS-move to an entirely new operating system designed specifically for distributed systems-emulation:move to a new DOS,but can
38、emulate one or more existing OS.examples:Mach&Chorus,2.2.2 Reconfigurability,Requirements of a reconfigurable distributed system:-the changes due to the scalability of a distributed system design and its ability to accommodate heterogeneity-a failed process,computer or network component is replaced
39、by another working counterpart;-computational load is shifted from over-loaded to less-loaded machines,so as to increase the total throughput of the distributed system;,2.2.3 Quality of service,Performance:in terms of the response times experienced by its users-Optimizing the performance of all of t
40、he software components that involved OSs communication services distributed programming support(e.g.,RPC)and the software that implements the service.,Security comes from two main threats,-against the privacy and integrity of users data as it travels over the network-their openness to interference w
41、ith system software:not all machines on a network can in general be made physically secure,-a fault-tolerant system is one which can detect a fault either fail gracefully(that is,predictably)or mask the fault so that no failure is perceived by users of the system.,Reliability and availability:,Chapter 3 Networking&Internetworking,Network technologiesProtocolsTechnology case studies:Ethernet,Token Ring and ATMProtocol case studies:Internet protocols and FLIP,In next class well discuss:,Thanks for your attention!,
