There is a developing trend in application protocol design toward using XML within MIME to structure requests and payloads. BEEP peers use this format for channel negotiations. Three major protocols are going the XML route throughout: XML-RPC and SOAP (Simple Object Access Protocol) for remote procedure calls, and Jabber for instant messaging and presence. All three are XML document types.

XML-RPC is very much in the Unix spirit (its author observes that he learned how to program in the 1970s by reading the original source code for Unix). It's deliberately minimalist but nevertheless quite powerful, offering a way for the vast majority of RPC applications that can get by on passing around scalar boolean/integer/float/string datatypes to do their thing in a way that is lightweight and easy to understand and monitor. XML-RPC's type ontology is richer than that of a text stream, but still simple and portable enough to act as a valuable check on interface complexity. Open-source implementations are available. An excellent XML-RPC home page points to specifications and multiple open-source implementations.

SOAP is a more heavyweight RPC protocol with a richer type ontology that includes arrays and C-like structs. It was inspired by XML-RPC, but has been plausibly accused of being an overdesigned victim of the second-system effect. As of mid-2003 the SOAP standard is still a work in progress, but a trial implementation in Apache is tracking the drafts. Open-source client modules in Perl, Python, Tcl, and Java are readily discoverable by a Web search. The W3C draft specification is available on the Web.

XML-RPC and SOAP, considered as remote procedure call methods, have some associated risks that we discuss at the end of Chapter7.

Jabber is a peer-to-peer protocol designed to support instant messaging and presence. What makes it interesting as an application protocol is that it supports passing around XML forms and live documents. Specifications, documentation, and open-source implementations are available at the Jabber Software Foundation site.