- How to draw a clear distinction between a client and a server(there is often no clear distinction) - A server may continuously act as a client - Distinction between three levels : 1- The user-interface level 2- The processing level 3- The data level

User-interface level : Contains all that is necessary to directly interface with the user Clients typically implement this level Consists of all programs that allow end user to interact with applications

The simplest user interface program is nothing more than a character-based screen - such an interface has been used in mainframe environments - the mainframe controls all interaction, including the keyboard and monitor - in many cases,the user’s terminal does some local processing

Nowadays,even in mainframe environments, there is more advanced user interfaces - the client machine offers at least a graphical display - much of the screen controls are handled through a mouse instead of keyboard - such as : X-windows interfaces, MS-DOS PCs Modern user interfaces offer considerably more functionality by allowing applications to share a single graphical window, and to use that window to exchange data through user actions

Processing Level : Client-server applications can be constructed from three pieces: 1- a part to handle interaction with a user. 2- a part to operate on a database or file system. 3- a middle part that generally contains the core functionality of an application. This middle part placed at the processing level.

Examples : 1- Internet search engine

The user interface of the search engine is very simple: a user types in a string of keywords and is subsequently presented with the list of titles of web pages. The core of the search engine is a program that transforms the string of keywords of the user into one or more database queries. It subsequently ranks the results into a list, and transforms that list into a series of HTML pages. This retrieval part is typically placed at the processing level

2- Decision support system for a stock brokerage This system can be divided into : 1- User interface. 2- Back end for accessing a database with the financial data. 3- The analysis programs between these two. The core of a financial decision support system may even need to be executed on high- performance computers in order to achieve the throughput and responsiveness in expected from its users.

3- Desktop package consider a typical desktop package consisting of a word processor, spreadsheet application, communication facilities, and so on.. Such “office” suites are generally integrated through a common user interface that supports compound documents, and operates on files from the home directory of the user. In this case the processing level consists of relatively large collection of programs, each having rather simple processing capabilities.

Data Level Contains the programs that maintain the actual data on which the applications operate. An important property of this level is that data are persistent and it means that even if no application is running,data will be stored some where for next use. The data level is also responsible for keeping data constraints across different applications.

When database are being used, maintaining consistency means that metadata such as table descriptions, entry constraints and application- specific metadata are also stored in this level.

In traditional business-oriented environment, the data level is organized as a relational database,the data are organized independent of the applications in such a way that changes in the organization do not affect applications. Using relational database in the client-server model helps us separate the processing level from the data level, as processing and data are independent.