<P> Clients and servers exchange messages in a request--response messaging pattern . The client sends a request, and the server returns a response . This exchange of messages is an example of inter-process communication . To communicate, the computers must have a common language, and they must follow rules so that both the client and the server know what to expect . The language and rules of communication are defined in a communications protocol . All client - server protocols operate in the application layer . The application layer protocol defines the basic patterns of the dialogue . To formalize the data exchange even further, the server may implement an application programming interface (API). The API is an abstraction layer for accessing a service . By restricting communication to a specific content format, it facilitates parsing . By abstracting access, it facilitates cross-platform data exchange . </P> <P> A server may receive requests from many distinct clients in a short period of time . A computer can only perform a limited number of tasks at any moment, and relies on a scheduling system to prioritize incoming requests from clients to accommodate them . To prevent abuse and maximize availability, server software may limit the availability to clients . Denial of service attacks are designed to exploit a server's obligation to process requests by overloading it with excessive request rates . </P> <P> When a bank customer accesses online banking services with a web browser (the client), the client initiates a request to the bank's web server . The customer's login credentials may be stored in a database, and the web server accesses the database server as a client . An application server interprets the returned data by applying the bank's business logic, and provides the output to the web server . Finally, the web server returns the result to the client web browser for display . </P> <P> In each step of this sequence of client--server message exchanges, a computer processes a request and returns data . This is the request - response messaging pattern . When all the requests are met, the sequence is complete and the web browser presents the data to the customer . </P>

Examples of the use of the client-server model on the internet