In the past, many distributed systems have been built that are two tier. This is a system that has a client program communicating with a server program. The examples of RMI, JavaIDL and JDBC given earlier are simple two-tier systems. Typically, the user interface is in the client and the server is there to process data, e.g., to manipulate a database. The complicated application logic (or business logic) is often put in the client (or in the server). More recently, three tier systems have come into favour. This is where an additional tier is interposed: it is there to handle the application logic leaving the client just to handle the user interface.

There are a number of advantages to this approach. For example:

• The code running on the client is small.
• When different client programs are needed to handle the differences between different client platforms, the amount of code that has to be provided is small because it only has to handle the user interface.
• It is possible to write the code running on the middle tier so that it handles requests from more than one client. In this way, requests can be more efficiently handled.