1. Distributed OS

2. 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

3. Comparison

4. 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.

5. Middleware in Distributed System

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

7. 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

8. Network Hardware
The Internet
The Internet evolved from the ARPANET

9. Network Services and Protocols
Connection-oriented service
connectionless service

10. Network Services and Protocols
Network Protocols IP
TCPs

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

12. 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

13. 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

14. Ways to implement Middleware
Transfer Model

15. 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