What is XML-RPC?

By
admin
 – March 29, 2010Posted in: Web Services

XML-RPC is a remote procedure call protocol which uses XML to encode its calls and HTTP as a transport mechanism.[1]

Contents

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[edit] History

XML-RPC was created in 1998 by Dave Winer of UserLand Software and Microsoft[2]. As new functionality was introduced, the standard evolved into what is now SOAP.

XML-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.[3]

[edit] Usage

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 resources are transported, 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. Also the security model is simpler, and XML-RPC does not support (nor require) the creation of WSDL service descriptions. JSON-RPC is similar to XML-RPC.

[edit] Data types

Common datatypes are converted into their XML equivalents with example values shown below:

Name Tag Example Description
array 
<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 
<base64>eW91IGNhbid0IHJlYWQgdGhpcyE=</base64>
Base64-encoded binary data
boolean 
<boolean>1</boolean>
Boolean logical value (0 or 1)
date/time 
<dateTime.iso8601>19980717T14:08:55</dateTime.iso8601>
Date and time in ISO 8601 format
double 
<double>-12.53</double>
Double precision floating point number
integer 
<i4>42</i4>

or

<int>42</int>
Whole number, integer
string 
<string>Hello world!</string>
String of characters. Must follow XML encoding.
struct 
<struct>
  <member>
    <name>foo</name>
    <value><i4>1</i4></value>

  </member>
  <member>
    <name>bar</name>
    <value><i4>2</i4></value>

  </member>
</struct>
Associative array
nil 
<nil/>
Discriminated null value; an XML-RPC extension

[edit] Examples

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>

[edit] Criticism

Critics of XML-RPC argue that RPC calls can be made with plain XML, and that XML-RPC doesn’t 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 [4]. This criticism is only referenced here in one single StackOverflow post; the advantages of the XML-RPC standard seem to outweigh this criticism.

[edit] Implementations

[edit] C++

[edit] Objective-C / GNUstep / Cocoa

[edit] Erlang

  • 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/

[edit] Java

[edit] XMPP

[edit] Other

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