Single-Tier Systems

Simple software applications are written to run on a single computer, as illustrated in Figure 1-1. All of the services provided by the application—the user interface, the persistent data access, and the logic that processes the data input by the user and reads from storage—all exist on the same physical machine and are often lumped together into the application. That monolithic architecture is called single tier, because all of the logical application services—the presentation, the business rules, and the data access layers—exist in a single computing layer.



Single-tier systems are relatively easy to manage, and data consistency is simple because data is stored in only one single location. However, they also have some disadvantages. Singletier systems do not scale to handle multiple users, and they do not provide an easy means of sharing data across an enterprise. Think of the word processor on your personal computer: It does an excellent job of helping you to create documents, but the application can be used by only a single person. Also, while you can share documents with other people, only one person can work on the document at a time.