Two‐Tier (Client‐ Server) Systems

Older types of network application were always made up of a server (which did the hard work of
running the application) and multiple clients (which connected to that server). The clients provided
instructions to the server and accessed the results it produced. The clients in this situation are referred
to as thin clients, because they do not have to do much work most of the functionality is hosted on the
server.

As time went by and computing power got cheaper, the temptation was for the clients to perform more of the processing work, easing the strain on big expensive servers. When more of this processing work is passed to a client, the typical client application needs to contain all the code needed to process the required tasks. If such a client were accessing a database, for example, the client application would need to be capable of accessing the database and manipulating that data while the data alone still resides on the server. This is known as a fat client because some of the processing has been passed onto it.

While this relieved the load on the main server, any changes either to the client or to the database then have rippling effects across all client installations. Any changes that need to be made are not just made to the central server; each individual client has to be updated for each change, and potentially several different applications have to be recompiled and updated if data manipulation and access code is shared between them. As computers spread into more areas of the workplace, the demands on computer‐based applications rose rapidly. These applications are often referred to as enterprise applications, and incorporate many servers and data stores. They tend to be organic in that they are always growing, and tend to be difficult to design and to maintain.

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