Chapter 16: Distributed System Structures

Chapter 16: Distributed System Structures Motivation Types of Network-Based Operating Systems Network Structure Network Topology Communication Structure Communication Protocols

Chapter Objectives To provide a high-level overview of distributed systems and the networks that interconnect them To discuss the general structure of distributed operating systems 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. Distributed file system : file service system whose servers, users, storage devices are dispersed among the sites of a distributed systems

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

A Distributed System

Types of Distributed Operating Systems Network Operating Systems Distributed Operating Systems Need mechanisms for Process synchronization Communication Dealing with deadlock Handling failers

Network-Operating Systems 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. Access to resources of various machines is done explicitly by: 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. 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 There is also remote desktop like windows

Distributed-Operating Systems Users not aware of multiplicity of machines Access to remote resources similar to access to local resources 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: transfer data by transferring entire file. 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. 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.

Distributed-Operating Systems (Cont.) 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: Load balancing – distribute processes across network to even the workload 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. Hardware preference – The process may have characteristics that make it more suitable for execution on some specialized processor. Software preference – The process may required software that is available at only a particular site. Data access – run process remotely, rather than transfer all data locally

16.3 Network Structure There are basically two types of network:- 1. Local-Area Network (LAN) – designed to cover small geographical area. The most common configuration are multiacess bus, ring, or star network The most common links are twisted-pair and fiber optical cabling. Communication Speed  10 – 100 megabits/second Broadcast is fast and cheap Nodes: usually workstations and/or personal computers a few (usually one or two) mainframes

Depiction of typical LAN

Network structure (Cont.) 2. Wide-Area Network (WAN) – 1960- are physically distributed over a large geographical separated sites to communicate with the other. Point-to-point connections over long-haul lines (often leased from a phone company) The communication links are by default are slow and unreliable Connection between networks use a telephone-system service called T1, , satellite channels,… Communication Speed  1.544 – 45 megbits/second For sites requiring faster internet access, T1 are collected into multiple-T1 units that work in parallel to provide more throughput Broadcast usually requires multiple messages Nodes: usually a high percentage of mainframes. Modems devices : converts digital data to analog data Routers

Communication Processors in a Wide-Area Network

Network Topology Sites in the system can be physically connected in a variety of ways; they are compared with respect to the following criteria: Installation cost - How expensive is it to link the various sites in the system? Communication cost - How long does it take to send a message from site A to site B? Reliability - If a link or a site in the system fails, can the remaining sites still communicate with each other? The various topologies are depicted as graphs whose nodes correspond to sites An edge from node A to node B corresponds to a direct connection between the two sites The following six items depict various network topologies

Network Topology

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

End of Chapter 16