InfoQ:

In his new article, Martin Fowler is using the 3-level model of restful maturity that was developed by Leonard Richardson to explain web-style systems. Throughout his explanation Fowler is using an example of a service for booking a doctors appointment.

According to Fowler, the starting point for the maturity model is to use HTTP purely as a transport system for remote interactions. In this case there is a single service – appointment service, which is using a single method call (POST in his example) and an XML input/output to communicate specific requests and replies.