• Simple Object Access Protocol
• XML-RPC was produced by UserLand (1998)
• Data type definition mechanism was a prolonged ordeal
• Microsoft, UserLand and DevelopMentor eventually got it together with XML Schema (1999)

When XML was first discussed as a data exchange format, among the goals were structured message exchanges between separate systems. One of the first wide-spread uses were deployments of Rich Site Summaries or RSS files on web sites to provide third parties with a structure message about what was current on a web site. While there's nothing wrong with XML-RPC per se, it didn't seem to go far enough as far providing data types to make the payloads self-describing.