Client Server Architecture

A client is a computer system that sends request to the server connected to the network, and a server is a computer system that receives the request, processes it, and returns the requested information back to the client. Client and server are usually present at different sites. The end users (remote database users) work on client computer system and database system runs on the server. Servers can be of several types, for example, file servers, printer servers, web servers, database servers, etc. The client machines have user interfaces that help users to utilize the servers. It also provides users the local processing power to run local applications on the client side.

 

There are two approaches to implement client/server architecture. In the first approach, the user interface and application programs are placed on the client side and the database system on the server side. This architecture is called two-tier architecture. The application programs that reside at the client side invoke the DBMS at the server side.

Fig.1.9. Two-tier architecture

The second approach, that is, three-tier architecture is primarily used for web-based applications. It adds intermediate layer known asapplication server (or web server) between the client and the database server. The client communicates with the application server, which in turn communicates with the database server. The application server stores the business rules (procedures and constraints) used for accessing data from database server. It checks the client’s credentials before forwarding a request to database server. Hence, it improves database security.

When a client requests for information, the application server accepts the request, processes it, and sends corresponding database commands to database server. The database server sends the result back to application server which is converted into GUI format and presented to the client. Three-tier architecture is given bellow.