XML-RPC is a remote procedure call (RPC) protocol which uses XML to encode its calls and HTTP as a transport mechanism. "XML-RPC" also refers generically to the use of XML for remote procedure call, independently of the specific protocol. This article is about the protocol named "XML-RPC".
The generic use of XML for remote procedure call (RPC) was patented by Phillip Merrick, Stewart Allen, and Joseph Lapp in April 2006, claiming benefit to a provisional application filed in March 1998. The patent is assigned to webMethods, located in Fairfax, VA.
XML-RPC works by sending a HTTP request to a server implementing the protocol. The client in that case is typically software wanting to call a single method of a remote system. Multiple input parameters can be passed to the remote method, one return value is returned. The parameter types allow nesting of parameters into maps and lists, thus larger structures can be transported. Therefore XML-RPC can be used to transport objects or structures both as input and as output parameters.
Identification of clients for authorization purposes can be achieved using popular HTTP security methods. Basic access authentication is used for identification, HTTPS is used when identification (via certificates) and encrypted messages are needed. Both methods can be combined.
In comparison to REST, where resource representations (documents) are transferred, XML-RPC is designed to call methods.
XML-RPC is simpler to use and understand than SOAP because it
- allows only one method of method serialization, whereas SOAP defines multiple different encodings
- has a simpler security model
- does not require (nor support) the creation of WSDL service descriptions, although XRDL provides a simple subset of the functionality provided by WSDL
JSON-RPC is similar to XML-RPC.
Data types 
Common datatypes are converted into their XML equivalents with example values shown below:
<array> <data> <value><i4>1404</i4></value> <value><string>Something here</string></value> <value><i4>1</i4></value> </data> </array>
|Array of values, storing no keys|
|Base64-encoded binary data|
|Boolean logical value (0 or 1)|
|Date and time in ISO 8601 format|
|Double precision floating point number|
|Whole number, integer|
|String of characters. Must follow XML encoding.|
<struct> <member> <name>foo</name> <value><i4>1</i4></value> </member> <member> <name>bar</name> <value><i4>2</i4></value> </member> </struct>
|Discriminated null value; an XML-RPC extension|
An example of a typical XML-RPC request would be:
<?xml version="1.0"?> <methodCall> <methodName>examples.getStateName</methodName> <params> <param> <value><i4>40</i4></value> </param> </params> </methodCall>
An example of a typical XML-RPC response would be:
<?xml version="1.0"?> <methodResponse> <params> <param> <value><string>South Dakota</string></value> </param> </params> </methodResponse>
A typical XML-RPC fault would be:
<?xml version="1.0"?> <methodResponse> <fault> <value> <struct> <member> <name>faultCode</name> <value><int>4</int></value> </member> <member> <name>faultString</name> <value><string>Too many parameters.</string></value> </member> </struct> </value> </fault> </methodResponse>
Critics of XML-RPC argue that RPC calls can be made with plain XML, and that XML-RPC does not add any value over XML. Both XML-RPC and XML require an application-level data model, such as which field names are defined in the XML schema or the parameter names in XML-RPC. Furthermore, XML-RPC uses about 4 times the number of bytes compared to plain XML to encode the same objects, which is itself bloated compared to JSON.
- Ultra lightweight XML-RPC library for C++
- XML-RPC for C and C++
- XmlRpc C++ client for Windows
- gSOAP toolkit for C and C++ supporting XML-RPC and more
- libmaia: XML-RPC for Qt/C++
- XML-RPC Client S60: XML-RPC for Symbian/C++
- Cxxtools XML-RPC C++ Client and Server for POSIX. It's part of Tntnet project.
Objective-C / GNUstep / Cocoa 
- XML-RPC for Erlang: This is an HTTP 1.1 compliant XML-RPC library for Erlang. It is designed to make it easy to write XML-RPC Erlang clients and/or servers. The library is compliant with the XML-RPC specification published by http://www.xmlrpc.org/
- Apache XML-RPC: Open source library for Java
- XML-RPC Delight: Convenient serialisation/deserialisation for Apache XML-RPC using Java Annotations and Beans
- : Secure Apache XML-RPC
- Redstone XML-RPC Library: Redstone's Open Source Library – XML-RPC implementation in Java
- XML-RPC Library for Java ME: Open source client-side library for Java ME
- aXMLRPC: Open source XML-RPC client library for Java and Android
- pyJabberXMLRPC: Python classes for XMPP
- Jabber-RPC: Over the Extensible Messaging and Presence Protocol protocol
- RPC::XML: A set of Perl classes for core data, message and XML handling
- Frontier::RPC: Another set of Perl modules for XML-RPC client/server implementation
- XML::Compile::RPC: RPC extension to XML::Compile
- JSON/XML-RPC Client and Server: Abstract away the differences between JSON-RPC and XML-RPC
- RemObjects SDK Delphi and .NET package for XML-RPC, in addition to SOAP and others
- RealThinClient SDK: For Delphi/C++
- XML-RPC for ActionScript: For Flash ActionScript 2.0
- as3-rpclib: For Flex/Actionscript 3
- XML-RPC.NET: Open source library for .NET clients and servers
- XmlRpc-Light: Client and server library for OCaml
- S-XML-RPC: Client and server library for Common Lisp
- clj-xmlrpc: XML-RPC client for Clojure
- necessary-evil: XML-RPC Client and Ring-based Server for Clojure
- HaXR: Client and server library for Haskell
- Ruby XML-RPC library: For Ruby
- XML-RPC interface to Lua: For Lua
- android-xmlrpc: A light XML-RPC client for Google Android
- XML-RPC for Tcl: A Tcl implementation of XML-RPC providing client and server support
- : RebXR, a full client/server XML-RPC implementation for REBOL.
See also 
- Simon St. Laurent, Joe Johnston, Edd Dumbill. (June 2001) Programming Web Services with XML-RPC. O'Reilly. First Edition.
- Box, Don (1 April 2001). "A Brief History of SOAP". O'Reilly. Retrieved 27 October 2010.
- Merrick et al. (11 April 2006). "US Patent 7,028,312". Retrieved 18 September 2008.
- "What is the benefit of XML-RPC over plain XML?". Stack Overflow. 9 September 2009. Retrieved 7 April 2011.
- "An open poll on the merits of XmlRpc versus alternatives". intertwingly.net. 22 November 2006. Retrieved 7 April 2011.
- Jon Canady (14 January 2010). "If you have REST, why XML-RPC?". joncanady.com. Retrieved 7 April 2011.