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Information - Are You Being Served?

We are familiar with ways to get information in the non-network world. We can go to a library, or buy a book in a bookshop. We can telephone companies or individuals by looking up their names in a phonebook. We can sit around and watch TV or listen to the radio.

In the networked world, there are a number of ways of carrying out the same kinds of activities, using programs that run on a PC or workstation to access information servers in the network that hopefully hold the knowledge we seek.

Information on the net used to be about as hard to retrieve as computers were to use. Nowadays it is usually pretty easy if you can find the information. First you have to know what kind of server holds it (its a bit like knowing whether a written item is in a reference book, a novel, a magazine, newspaper or shopping list). We illustrate the array of available information servers as seen from a very high level view of layered services in figure 2.1.

Different kinds of information services have different models of use and different ways they hold information. Almost all fit into the ``Client/Server'' model that has become widespread in distributed computing.

Client/Server communication is quite easy to understand in terms of roles, and is very closely analogous to what happens in a shopping situation. An assistant in a shop awaits a customer. The assistant doesn't know in advance which customer might arrive (or even how many - the store manager is supposed to make sure that enough assistants are employed to just about cope with the maximum number of shoppers arriving at any one time). A server on the network is typically a dedicated computer that runs a program called the server. This awaits requests from the network, according to some specified protocol, and servers them, one or more at a time, without regard for who they come from.

There are a variety of refinements of this model, such as requiring authentication or registration with the server before other kinds of transactions can be undertaken, but almost all the basic systems on the Internet work like this for now.

We can categorise information servers along a number of different axes:

Figure 2.1:  The Whole Shebang

next up previous contents
Next: Transport Protocols Up: The World Wide Web Previous: Policies

Jon Crowcroft
Wed May 10 11:46:29 BST 1995