Distributed OS

Introduction These systems are similar to multicomputers in that each node has its own private memory, with no shared physical memory in the system. Distributed systems are even more loosely coupled than multicomputers

Comparison

Middleware One way a distributed system can achieve some measure of uniformity in the face of different underlying hardware and operating systems is to have a layer of software on top of the operating system. The layer, called middleware This layer provides certain data structures and operations that allow processes and users on different machines to interoperate in a consistent way.

Middleware in Distributed System

Network Hardware Distributed systems are built on top of computer networks LAN (Ethernet) and WAN (Internet)

Network Hardware Ethernet Classic Ethernet, which is described in IEEE Standard 802.3, consists of a coaxial cable to which a number of computers are attached. The cable is called the Ethernet

Network Hardware The Internet The Internet evolved from the ARPANET

Network Services and Protocols Connection-oriented service connectionless service

Network Services and Protocols Network Protocols IP TCPs

Ways to implement Middleware World Wide Web File System-Based Middleware Shared Object-Based Middleware Coordination-Based Middleware

Ways to implement Middleware World Wide Web every computer can hold one or more documents, called Web pages When a user requests a Web page using a program called a Web browser, the page is displayed on the screen Each Web page has a unique address, called a URL (Uniform Resource Locator), of the form protocol://DNS-name/file-name. The protocol is most commonly http (HyperText Transfer Protocol), but ftp and others also exist The Web is fundamentally a client-server system, with the user being the client and the Web site being the server

Ways to implement Middleware File System-Based Middleware The basic idea behind the Web is to make a distributed system look like a giant collection of hyperlinked documents. A second approach is to make a distributed system look like a great big file system Transfer Model The Directory Hierarchy Naming Transparency

Ways to implement Middleware Transfer Model

Ways to implement Middleware Naming Transparency Naming transparency through location transparency, means that the path name gives no hint as to where the file is located. A path like /server1/dir1/dir2/x tells everyone that x is located on server 1, but it does not tell where that server is located. The server is free to move anywhere it wants to in the network without the path name having to be changed. Thus this system has location transparency