<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> <P> This example illustrates a design pattern applicable to the client--server model: separation of concerns . </P> <P> An early form of client--server architecture is remote job entry, dating at least to OS / 360 (announced 1964), where the request was to run a job, and the response was the output . </P> <P> While formulating the client--server model in the 1960s and 1970s, computer scientists building ARPANET (at the Stanford Research Institute) used the terms server - host (or serving host) and user - host (or using - host), and these appear in the early documents RFC 5 and RFC 4 . This usage was continued at Xerox PARC in the mid-1970s . </P>

Briefly explain the role of a server when a file is requested by a client terminal