Created by: mlpvolt, Last modification: 05 Apr 2006 (19:46 UTC)


XML-RPC is a quick-and-easy way to make procedure calls over the Internet. It converts the procedure call into XML document, sends it to a remote server using HTTP, and gets back the response as XML.

This feature is currently undocumented for bitweaver. The Information below is general information about XMLRPC.

What is XML-RPC?

XML-RPC is a standard network protocol that computers can use to talk to each other in a remote procedure call fashion. Remote procedure call essentially means that a program on one computer runs a program on another computer. But a simpler way of looking at this kind of network protocol is just that you have client and servers. A client makes individual isolated requests of A server. A server sits around waiting for a request to arrive from some client, does what the request asks, and sends a response. It then goes back to waiting for the next request.

Here are some examples of remote procedure call (RPC) style communications:

  • There is a server that can measure atmospheric temperature. A client anywhere in the world can ask the server at any time what the temperature is. The "what temperature is it?" request and the "the temperature is..." response constitute an RPC transaction.
  • There is a server that can turn a light on or off. A client can tell the server to turn the light on. A request to turn the light on and the acknowledgement that the light has been turned on constitute an RPC transaction.
  • There is a server that knows the phone numbers of a million people. A client can supply a name and get back the phone number of the named person.
  • A network of 1000 computers is used to search millions of web pages. A dispatcher computer is a client and the rest are servers. The dispatcher sends each of the servers one URL at a time and some search terms. The server responds with yes or no as to whether that URL contains the search terms. The dispatcher keeps a list of the URLs that match. Each of the requests to search a particular URL is an RPC transaction.

Here are some kinds of communication that are not RPC:

  • A long-lived connection such as an SSH login session.
  • A high volume transfer such as an FTP download.
  • A one-way transmission such as a UDP packet.
  • A dialogue such as an SMTP (mail) transaction.

The original RPC protocol is the ONC RPC protocol — the one that NFS (the network fileystem protocol) uses. It's often called "Sun RPC" because Sun invented and promulgated it. The ONC RPC protocol is layered over UDP or sometimes TCP and uses a machine-friendly bits and bytes format, just like the TCP/IP layers under it.

XML-RPC differs from ONC RPC in that it encodes the information in the requests and responses in XML. That means they are human friendly — XML is human-readable text, so a human can readily see what's going on from a network trace and quickly write code to create and decipher XML-RPC streams. Of course, the tradeoff is that XML-RPC uses way more network and computation resources than ONC RPC. Because XML-RPC is meant to be used for relatively small and infrequent transactions, this is thought not to matter.

In XML-RPC, the aforementioned XML is transported via HTTP (the protocol whose principal purpose is to implement web serving — web browsing is a form of RPC, after all). HTTP is normally carried over TCP, which is carried over IP.

There are lots of servers in the world that use the XML-RPC protocol and lots of programs and programming libraries from which people can build XML-RPC-based servers and clients.

There are also other HTTP-based RPC protocols. SOAP and CORBA are the most famous.

For more information on XML-RPC, see The XML-RPC web site and The XML-RPC Howto. The latter includes information (with examples) on implementing XML-RPC clients and servers in languages other than C and C++, which is all that is covered in this document.