One of the first standards that emerged was called XML-RPC. The idea behind the project was to provide an RPC technology that would compete with the then prevalent Common Object Model (COM), and CORBA. The aim was to use HTTP as a transport protocol and make the format human-readable and human-writable as well as parsable to machines. To achieve that, XML was chosen as the data encoding format.
When using XML-RPC, the client that wants to perform a remote procedure call sends an HTTP request to the server. The request may have multiple parameters. The server answers with a single response. The XML-RPC protocol defines several data types for parameters and results.
Although SOAP features similar data types, it uses XML schema definitions, which make the messages much less readable than the ones in XML-RPC.