Chapter 16 Distributed System Structures Operating System Concepts

Chapter 16: Distributed System Structures n Motivation n Types of Network-Based Operating Systems n Network Structure n Network Topology n Communication Structure n Communication Protocols Operating System Concepts with Java – 8 th Edition 16. 2 Silberschatz, Galvin and Gagne © 2009

Chapter Objectives n To provide a high-level overview of distributed systems and the networks that interconnect them n To discuss the general structure of distributed operating systems v Distributed system : a collection of processes that do not share memory but it has its own local memory and communicate with one another through communication lines like LAN. v Distributed file system : file service system whose servers, users, storage devices are dispersed among the sites of a distributed systems Operating System Concepts with Java – 8 th Edition 16. 3 Silberschatz, Galvin and Gagne © 2009

Motivation n Distributed system is collection of loosely coupled processors interconnected by a communications network n The processor in a distributed system may very in size and function. They may include small microprocessors, workstations, minicomputers, and large-purpose computer systems. n Processors variously called nodes, computers, machines, hosts l Site is location of the processor or machine n Reasons for distributed systems l Resource sharing 4 sharing and printing files at remote sites 4 processing 4 using information in a distributed database remote specialized hardware devices l Computation speedup – load sharing l Reliability – detect and recover from site failure, function transfer, reintegrate failed site(if one fails does not affect others l Communication – message passing/ exchange information Operating System Concepts with Java – 8 th Edition 16. 4 Silberschatz, Galvin and Gagne © 2009

A Distributed System Operating System Concepts with Java – 8 th Edition 16. 5

Types of Distributed Operating Systems n Network Operating Systems n Distributed Operating Systems Need mechanisms for o Process synchronization o Communication o Dealing with deadlock o Handling failers Operating System Concepts with Java – 8 th Edition 16. 6 Silberschatz, Galvin and Gagne © 2009

Network-Operating Systems n Network Operating Systems: provides an environment in which users, who are aware of the multiplicity of machines, can access resources by either logging in to the appropriate remote machine or transferring data from the remote machine to their own machines. n Access to resources of various machines is done explicitly by: 4 Remote Login: An important function of a network operating system is to allow users to logging into the appropriate remote machine remotely. The internet provides the telnet facility for this purpose. 4 Remote file Transfer: Another major function of a network operating system is to provide a mechanism for remote file transfer from one machine to another. In such an environment, each computer maintains its own file system. The internet provides a mechanism foe such a transfer with the file transfer protocol (FTP) program 4 There is also remote desktop like windows Operating System Concepts with Java – 8 th Edition 16. 7 Silberschatz, Galvin and Gagne © 2009

Distributed-Operating Systems n Users not aware of multiplicity of machines l n n Data Migration – for example: suppose a user on site A wants to access data that reside at site B. The system can transfer the data by one of two basic methods: 1. transfer data by transferring entire file. 2. or transferring only those portions of the file necessary for the immediate task. Computation Migration – transfer the computation, rather than the data, across the system. l n Access to remote resources similar to access to local resources For example, consider a job that needs to access various large files that reside at different site, to obtain a summary of those files. It would be more efficient to access the files at the sites where they reside and return the desired results to the site that initiated the computation. Generally, if the time to transfer the data is linger than the time to execute the remote command, the remote command should be used. Operating System Concepts with Java – 8 th Edition 16. 8 Silberschatz, Galvin and Gagne © 2009

Distributed-Operating Systems (Cont. ) n Process Migration – execute an entire process, or parts of it, at different sites(not executed at initiate site). This scheme may be used for several reasons: l Load balancing – distribute processes across network to even the workload l Computation speedup – If a single process can be divided into a number of subprocesses that can run concurrently on different sites, then the total process turnaround time can be reduces. l Hardware preference – The process may have characteristics that make it more suitable for execution on some specialized processor. l Software preference – The process may required software that is available at only a particular site. l Data access – run process remotely, rather than transfer all data locally Operating System Concepts with Java – 8 th Edition 16. 9 Silberschatz, Galvin and Gagne © 2009

16. 3 Network Structure There are basically two types of network: 1. Local-Area Network (LAN) – designed to cover small geographical area. l The most common configuration are multiacess bus, ring, or star network l The most common links are twisted-pair and fiber optical cabling. l Communication Speed 10 – 100 megabits/second l Broadcast is fast and cheap l Nodes: 4 usually 4 a workstations and/or personal computers few (usually one or two) mainframes Operating System Concepts with Java – 8 th Edition 16. 10 Silberschatz, Galvin and Gagne © 2009

Depiction of typical LAN Operating System Concepts with Java – 8 th Edition 16.

Network structure (Cont. ) 2. Wide-Area Network (WAN) – 1960 - are physically distributed over a large geographical separated sites to communicate with the other. l Point-to-point connections over long-haul lines (often leased from a phone company) l The communication links are by default are slow and unreliable l Connection between networks use a telephone-system service called T 1, , satellite channels, … l Communication Speed 1. 544 – 45 megbits/second l For sites requiring faster internet access, T 1 are collected into multiple-T 1 units that work in parallel to provide more throughput l Broadcast usually requires multiple messages l Nodes: 4 usually a high percentage of mainframes. l Modems devices : converts digital data to analog data l Routers Operating System Concepts with Java – 8 th Edition 16. 12 Silberschatz, Galvin and Gagne © 2009

Communication Processors in a Wide-Area Network Operating System Concepts with Java – 8 th

Network Topology n Sites in the system can be physically connected in a variety of ways; they are compared with respect to the following criteria: l Installation cost - How expensive is it to link the various sites in the system? l Communication cost - How long does it take to send a message from site A to site B? l Reliability - If a link or a site in the system fails, can the remaining sites still communicate with each other? n The various topologies are depicted as graphs whose nodes correspond to sites l An edge from node A to node B corresponds to a direct connection between the two sites n The following six items depict various network topologies Operating System Concepts with Java – 8 th Edition 16. 14 Silberschatz, Galvin and Gagne © 2009

Network Topology Operating System Concepts with Java – 8 th Edition 16. 15 Silberschatz,

Communication Structure • We talked about physical aspects of networking, now internal working. • The design of a communication network must address four basic issues: n Naming and name resolution - How do two processes locate each other to communicate? n Routing strategies - How are messages sent through the network? (path) n Packet strategies – Are packets sent individually or as a sequence? (frame, datagram) n Connection strategies - How do two processes send a sequence of messages? (communication sessions to exchange information) n Contention - The network is a shared resource, so how do we resolve conflicting demands for its use? (depending on network topology) Operating System Concepts with Java – 8 th Edition 16. 16 Silberschatz, Galvin and Gagne © 2009