XML-RPC, SOAP, and data persistence

[Mark Carter]

I had a play with XML-RPC the other day, and it seemed really simple
to use; which really made an impression on me. But you can't make data
persist on the server (not without rolling your own mechanism, at any
rate). I'm thinking here about thin clients, where the main process
runs on a server.

Is there any free solution to this problem? Is SOAP the answer?