In the previous section we have introduced REST and showed how it can be used as a service for web applications. In this section we will introduce the Atom Publishing Protocol (APP), which at the time of writing this book was on its way to becoming a new IETF standard. This means that some aspects of this section might no longer be up to date by the time you read them.

APP has arisen from the Atom community as an application-level protocol on top of HTTP to allow the publishing and editing of web resources. The unit of messages between an APP server and a client is based on the Atom XML-document format defined in RFC 4287.

Although APP is not specified as being an implementation of the REST principles, the protocol does follow the same ideas, which give it a RESTful aspect. Therefore, many of the principles of the previous section will apply here; but first let's overview the Atom XML-document format.