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1、word1. 分布式系统定义A distributed system is a collection of autonomousputing elements that appears to its users as a singlecoherent system.独立的计算机的集合，对这个系统的用户来说，系统就像一台计算机一样。2. 分布式系统的目标Making resource availableDistribution transparency(分布的透明性)Openness(开放性)Scalability(可扩展性)包含三个方面：Size: Number of users and/or
2、 processes(solution:强大的服务器)Geographical: Maximum distance between nodesAdministrative: Number of administrative domains3. 为什么需要分布式Economics经济性微处理器能提供比大型机更好的性价比Speed速度分布式系统能提供比大型机更强的计算能力Inherent distribution固有的分布性有一些应用包含空间上别离的机器Reliability可靠性当某台机器崩溃时，整个系统仍能正常工作Incremental growth可扩展性计算能力逐步增加4. 分布式系统透明
3、性含义分布式系统的透明性Transparency in a Distributed System定义：对用户和应用程序员屏蔽分布式系统组件的分散性，系统被认为是一个整体，而不是独立的组件集合。透明性对用户和应用程序员隐藏了与手头任务无直接关系的资源，并匿名使用，使得分布的某些特性对应用程序员具有不可见性，这样应用程序员只要关心特定应用的设计问题。(1)网络透明性：用户发觉不出是以网络的方式工作，所有的工作对象是在单台机器上完成。(2)操作系统透明性：使用不同操作系统的机器和用户可以自由的协同工作，不同操作系统带来的不同被掩盖。(3)语言透明性：使用不同语言编写的程序或者模块能够自由交互工作，相
4、互调用。(4)数据复制与分片透明性：分片的数据像未分片的数据一样工作，修改数据时复制在多处的数据自动更新。实现方式Access(访问)Hide differences in data representation and how a resource is accessedLocation(位置)Hide where a resource is locatedMigration(迁移)Hide that a resource may move to anotherlocationRelocation(重定位)Hide that a resource may be moved to anothe
5、r location while in useReplication(复制)Hide that a resource may be shared by several petitive usersConcurrency(并发)Hide that a resource may be shared by several petitive usersFailure(故障)Hide the failure and recovery of a resource透明度(Degree of Transparency)：Aiming at full distribution transparency may
6、be too muchUsers may be located in different continentspletely hiding failures of networks and nodes is (theoretically and practically) impossibleYou cannot distinguish a slow puter from a failing oneYou can never be sure that a server actually performed an operation before a crashFull transparency
7、will cost performance, exposing distribution of the systemKeeping Web caches exactly up-to-date with the masterImmediately flushing write operations to disk for fault tolerance5. 分布式系统开放性含义Be able to interact with services from other open systems, irrespective of the underlying environment:Systems s
8、hould conform to well-defined interfacesSystems should support portability of applicationsSystems should easily interoperateAt least make the distributed system independent from heterogeneity of the underlying environment6. 分布式操作系统、网络操作系统和基于中间件的系统定义分布式操作系统：配置在分布式系统上的操作系统，能够直接对分布式系统中的各种资源进展动态分配，并能有效地
9、控制和协调分布式系统中各任务的并行执行，同时还向用户提供了一个方便的、透明的使用整个分布式系统的界面。网络操作系统：是在网络环境下实现对网络资源的管理和控制的操作系统，是用户与网络资源之间的接口。网络操作系统是建立在独立的操作系统之上，为网络用户提供使用网络系统资源的桥梁。在多个用户争用系统资源时，网络操作系统进展资源调剂管理，它依靠各个独立的计算机操作系统对所属资源进展管理，协调和管理网络用户进程或程序与联机操作系统进展交互。基于中间件的系统：？系统名称系统描述主要目标DOS紧耦合，管理多处理器系统和同构式多计算机系统隐藏和管理硬件资源NOS松耦合，管理异构式多计算机系统向远程客户端提供本地
10、服务Middleware位于NOS通用服务实现层之上的附加层，屏蔽了底层的异构性和具体的通信协议细节，为应用程序员提供方便的编程模型。提供分布透明性7. 分布式系统类型(1)Distributed puting systemsCluster putingEssentially a group of high-end systems connected through a LAN:Homogeneous: same OS, near-identical hardwareSingle managing nodeGrid putingThe next step: lots of nodes from
11、 everywhere:HeterogeneousDispersed across several organizationsCan easily span a wide-area network(2)Distributed information systemsThe vast amount of distributed systems in use today are forms of traditional information systems, that now integrate legacy systems.A transaction is a collection of ope
12、rations on the state of an object that satisfies the following properties(ACID)Atomicity(原子性)Consistency(一致性)Isolation (隔离性)Durability(持久性)Transaction Processing MonitorIn many cases, the data involved in a transaction is distributed across several servers. A TP Monitor is responsible for coordinati
13、ng the execution of a transaction.(3)Distributed pervasive systemsEmerging next-generation of distributed systems in which nodes are small, mobile, and often embedded in a larger system, characterized by the fact that the system naturally blends into the users environment.Ubiquitous puting systemsMo
14、bile puting systemsSensor (and actuator) networks8. 分布式系统中的机制与策略Requires support for different policies:What level of consistency do we require for client-cached(客户端缓存) data?Which operations do we allow downloaded code to perform?Which QoS requirements do we adjust in the face of varying bandwidth(带
15、宽)?What level of secrecy(某某) do we require for munication?Ideally, a distributed system provides only mechanisms:Allow (dynamic) setting of caching policiesSupport different levels of trust for mobile codeProvide adjustable QoS parameters per data streamOffer different encryption(加密) algorithms9. 分布式系统架构风格(1)Organize into logically different ponents, and distribute those ponents over the various machines(2)Decoupling processes in space (“anonymous) and also time (“asynchronous) has led to alternative styles10. 分布式系统组织形式(1) CentralizedBasic Clie
