feeds. All the same characteristics of content and (Web host)
feeds. All the same characteristics of content and metadata that are handled by the rest of a syndication system are produced by the Client Producer. But in current implementations at least, the Client Producer is the odd one out of these four categories. It may not involve any of the syndication format standards. The Client Producer part of syndication systems may in effect be integrated with the backend, server-side of the system, with content being passed from an HTML form into the database using proprietary techniques. If the Client Producer is separate from the rest of the system, chances are it will use the Blogger API (unless it s a very recent piece of software). ABlogger API is a specification that uses the XML-RPC (Remote Procedure Call) protocol, introduced earlier in the chapter. More recent software might support the Atom API, which is based on a fundamentally different approach to Web communications. There is a range of techniques that can be used when it comes to a piece of software either advertising what protocol(s) it supports or trying to discover what is supported by a service. In particular, the term introspection has been used for the advertising within Atom, and autodiscovery for the trick of pointing to service endpoint addresses. Although syndication is arguably the most widely deployed Web service, aside from autodiscovery of feeds from HTML pages there is no standard cross-system approach for discovering what is available. The following sections should give you an overview of the protocol side of Client Producer subsystems, which are usually authoring applications. In Chapter 26, you will see some code that uses these protocols to post to weblogs. XML-RPC This protocol follows one traditional approach to client-server programming, in which program procedure calls are executed on a remote system rather than locally. XML is used to wrap the calls in a form that can be delivered over HTTP. The receiver of an XML-RPC message will unwrap the XML envelope to discover what parameters should be delivered to what programming procedures. XML-RPC is a predecessor of the SOAP protocol, which now plays a key role in Web services. Its adoption in the world of syndication began when Pyra Labs, the company that used to be behind the Blogger hosting service, released the Blogger API (Application Programming Interface) in 2001 as a specification for communicating between blogging clients (that is, Client Producers) and their server-side offerings. Although it was described as experimental and alpha, developers quickly seized it to fill a gap in requirements. Unfortunately it didn t fill the gap too well (for example, entry titles were missing), and the Blogger spec was followed a few months later by the MetaWeblog API from UserLand, which added more structure to the messages echoing the components of an RSS item in procedure calls. Implementations using the Blogger and MetaWeblog APIs generally use ready-made library code to enable the construction of the message at the producer and the interpretation of the message at the consumer. This is good news for developers, because the actual data sent in this fashion can be incredibly complex due to the way the data is built. For example, to specify the title of a new post, this has to be described in terms of a procedure call, giving its data type and value. These pieces of data will be wrapped in a struct structure corresponding to a parameter in the method call. Acomplete MetaWeblog API message is too long to include here, but the following listing contains the structural elements relating to the title:
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