3. Introduction  Client/Server technology is seen by many as the solution to the difficulty of linking together the various departments of corporation.  It is the outgrowth of the traditional network that hooked a number of terminals onto the mainframe processor.

4.  Many companies have switched to the client/server approach which allows the company to connect many sites across the globe.  But Client/Server technology is immature; user report that it is expensive to maintain and manage and that there is along learning curve.  There are simply many potential points of failure.

5. Client/Server Architecture : Client/Server Architecture is made up of a number of clients ( generally desktop devices ) who request services from a number of servers ( specialized larger computers ).

6. The job of the server is to reply to all the requests made of :  requests for data  for communication messages  for updating master files.

7.  In short all the work a user needs to accomplish must be managed in terms of signals for either transaction processing or for data to support a decision.  Transaction processing ( TP ) is far more complex to carry out than is decision support, however TP is a major reason that client/server applications are needed by companies.

8.  The request from the client travels along the network, for example a local area network ( LAN ) to the server. The servers job is to determine how best to fill the request made by client.  Clients are generally terminals on the desktop operated by end users. They maybe other servers.

9. Two-Tier Model :  For some application a two-tier environment will suffice. Server Clients Client / server basic model

10. Three-Tier model :  A Three-Tier environment ( database server, application server and desktop ) improves overall response time. This approach separates functionality by placing the database on a yet third level.  If more users need to be connected to the system, more servers and terminals will be added.

11. Desktop Application Server( s ) Database Server Three - Tier Client / Server Structure

12. Middleware  Middleware is considers to be the collection of software needed to provide the connections and process the interactions between the layers.  Middleware is installed on each of the two or three tiers of the client/server structure.

13. It include such functions as :  The network operating system ( NOS )  Routers  Bridges  Gateways Server Middleware structure NOS Routers Bridges Gateways

14.  The elements of the three tiers ( database server, application server and desktop ) maybe linked by various communication protocols such as TCP/IP.  Populating the protocol are task-oriented agents whose role is that of managing communications from one server to another- from the applications server to the database server for example see the figure below.

15. Communications agents Update agents Batch agents Spool agents Dispatche r

16.  The agents are specialized and report to a dispatcher ( who sends jobs to the agents).  The dispatcher identifies the type of task ( update, batch, spool, etc.) and sends the jobs to a waiting agent, one who specializes in that type of command.  Most ERP are designed for a client/server environment. Most ERP packages are implemented in a three-tier environment, while smaller applications can be implemented in a two- tier environment.

17. The three-tier structure has the following advantages :  It is easier to increase throughput by adding servers  Input/Output intensive functions can be place on larger computer.  Workstations provide desktop computing that doesn't place a load on the system as a whole.  And an integrated system can be made available across multiple locations.

18. Of course, the advantages bring with them the difficulty of managing and maintaining a complex web of systems and protocols.