On the IBM developerWorks website today there's a new tutorial showing how to create a REST web service with the help of the Agavi framework.

These days, every Web application worth its salt has a REST API. Flickr has one; so do Google, Bit.ly, and NetFlix, as well as many other popular applications. As an architectural pattern, REST is popular. [...] This article will walk you through the process of building a simple REST API with Agavi. If you already have an Agavi-based application, this article will explain how to leverage existing framework conventions and expose your application innards to third-party developers. If you're creating a new REST-based application, this article will demonstrate how using Agavi can make the process simpler and more efficient.

He starts with an introduction to the REST protocol for making web service requests, covering the basics of verbs and how you can map URLs. From there he steps you through the creation of a sample application with customized routing and modules to handle GET and POST requests for the example book information his service is based on. He also adds in a method that will let you swap out the XML output for JSON.