Client-server computing has traditionally been done using sockets. A socket is a point of connection within a program to the Internet, and a program can start to communicate when it arranges for the socket to bind to a port of the computer that is running the program.

The Java classes java.net.DatagramSocket, java.net.ServerSocket and java.net.Socket provide easy-to-use ways of writing programs that use sockets. However, in Java, client-server computing can be performed without having to work at the level of sockets. This is because Java has several application programming interfaces (APIs) that offer higher levels of abstraction for client-server computing.

In this document, we look at three APIs: Remote Method Invocation, Common Object Request Broker Architecture and Java DataBase Connectivity. The latter is a way in which a Java program can use SQL to access database servers, whereas the other two APIs allow us to rise up above the socket-level details of client-server computing when we want to access objects distributed across different computers.

Although Java applications can use these APIs to attempt to communicate with any computer on the Internet, Java applets are normally restricted to talking to the site from which the applet was downloaded. This hinders the use of applets in client-server computing. This document explains how signed applets can be used to overcome these security restrictions.

In the past, client-server computing has usually involved just two computers talking to one another. Often a lot of the processing is going on in the client, which often means that the client is a big program. More recently, systems have been produced that use an intermediate layer: the client just provides a user interface whereas the intermediate layer contains all the complicated application logic. The final topic of this document introduces the idea of Three Tier Systems.