1. Fall 2007cs4251 Distributed Computing Umar Kalim Dept. of Communication Systems Engineering umar.kalim@niit.edu.pk http://www.niit.edu.pk/~umarkalim 31/10/2007

2. Fall 2007cs4252 Agenda Client Server Models

3. Fall 2007cs4253 Clients and Servers Processes are divided into –Servers: implementing a specific service –Clients: requesting a service from a server by sending it a request and subsequent waiting for the server’s reply Distributed across different machines Follow a request-reply

4. Fall 2007cs4254 What is a Server Definition: “A process that implements a specific service on behalf of a collection of clients”. Typically, servers are organized to do one of two things: –1. Wait –2. Service … wait … service … wait … service … wait …

5. Fall 2007cs4255 Servers Iterative –Server handles request, then returns results to the client; any new client requests must wait for previous request to complete (also useful to think of this type of server as sequential). Concurrent –Server does not handle the request itself; a separate thread or sub-process handles the request and returns any results to the client; the server is then free to immediately service the next client (i.e., there’s no waiting, as service requests are processed in parallel).

6. Fall 2007cs4256 Server States Stateless servers – no information is maintained on the current “connections” to the server. The web is the classic example of a stateless service. As can be imagined, this type of server is easy to implement. Stateful servers – information is maintained on the current “connections” to the server. Advanced file servers, where copies of a file can be updated “locally” then applied to the main server (as the server knows the state of things). These are more difficult to implement. By the way, what are cookies?

7. Fall 2007cs4257 Application Layering Traditional three-layered view User-interface layer: programs that allow end users to interact with the application; differ in their sophistication Processing layer: contains the functions of an application Data layer: contains the data that a client wants to manipulate through the application components (persistency, consistency, data independence)

8. Fall 2007cs4258 Processing Level The general organization of an Internet search engine into three different layers

9. Fall 2007cs4259 Multi-tiered Architecture

10. Fall 2007cs42510 Modern Architectures An example of horizontal distribution of a Web service. –Horizontal Splitting –Vertical Splitting

11. Fall 2007cs42511 Alternative Architectures Cooperating servers –Traditional Multi-tier systems –Replicated servers Cooperating clients –Teleconferencing Web applets Peer-to-peer networks

12. Fall 2007cs42512 Cooperating Servers Multi-tiered Systems

13. Fall 2007cs42513 Proxy Server

14. Fall 2007cs42514 Web Applets Client requests results in the downloading of applet code Client interacts with the applet

15. Fall 2007cs42515 Peer to Peer Networks

16. Fall 2007cs42516 Questions? That’s all for today!