View source for XML-RPC

{{Short description|Data serialization format}}
{{Distinguish|text = the use of XML for RPC, independently of the specific protocol. This article is about the protocol named "XML-RPC"}}
{{More citations needed|date=October 2016}}
{{Use dmy dates|date=December 2021}}
'''XML-RPC''' is a [[remote procedure call]] (RPC) [[Communications protocol|protocol]] which uses [[XML]] to encode its calls and [[HTTP]] as a transport mechanism.<ref name="book1">Simon St. Laurent, Joe Johnston, Edd Dumbill. (June 2001) ''Programming Web Services with XML-RPC.'' O'Reilly. First Edition.</ref>

==History==
The XML-RPC protocol was created in 1998 by [[Dave Winer]] of [[UserLand Software]] and [[Microsoft]],<ref>{{cite web|last=Box|first=Don|title=A Brief History of SOAP
|publisher=[[O'Reilly Media|O'Reilly]]|date=1 April 2001|url=http://www.xml.com/pub/a/ws/2001/04/04/soap.html
|access-date=27 October 2010}}</ref> with Microsoft seeing the protocol as an essential part of scaling up its efforts in business-to-business e-commerce.<ref>{{Cite web|last=Rupley |first=Sebastian |title=XML's Next Step |work=PC Magazine |access-date=2015-11-17 |date=1999-06-30 |url=http://www.zdnet.com/pcmag/stories/trends/0,7607,2286488,00.html |url-status=dead |archive-url=https://web.archive.org/web/20000304215507/http://www.zdnet.com/pcmag/stories/trends/0,7607,2286488,00.html |archive-date=4 March 2000 }}</ref> As new functionality was introduced, the standard evolved into what is now [[SOAP (protocol)|SOAP]].<ref name=spearheads>{{Cite news|last=Walsh |first=Jeff |title=Microsoft spearheads protocol push |work=Infoworld |access-date=2015-11-17 |date=1999-07-10 |url=http://www.infoworld.com/cgi-bin/displayStory.pl?980710.whsoap.htm |url-status=dead |archive-url=https://web.archive.org/web/19990914001234/http://www.infoworld.com/cgi-bin/displayStory.pl?980710.whsoap.htm |archive-date=14 September 1999 }}</ref>

UserLand supported XML-RPC from version 5.1 of its Frontier web content management system,<ref name=spearheads /> released in June 1998.<ref>{{cite web| last = Walsh| first = Jeff| title = UserLand releases Frontier 5.1, drops freeware model | work = InfoWorld| access-date =17 November 2015| date = 29 June 1998| url = http://www.infoworld.com/cgi-bin/displayStory.pl?980629.wifrontier.htm |archive-url = https://web.archive.org/web/19990915175718/http://www.infoworld.com/cgi-bin/displayStory.pl?980629.wifrontier.htm |archive-date=15 September 1999 }}</ref>

XML-RPC's idea of a human-readable-and-writable, script-parsable standard for HTTP-based requests and responses has also been implemented in competing specifications such as Allaire's [[WDDX|Web Distributed Data Exchange]] (WDDX) and webMethod's [[WIDL (Internet Standard)|Web Interface Definition Language]] (WIDL).<ref>{{Cite news|last=Udell |first=Jon |title=Exploring XML-RPC: DCOM? CORBA? RMI? Why Not Just XML-RPC? |work=Byte |access-date=2015-11-17 |date=1999-06-07 |url=http://www.byte.com/features/1999/06/0607XML_RPC5.html |url-status=dead |archive-url=https://web.archive.org/web/20000304171225/http://www.byte.com/features/1999/06/0607XML_RPC5.html |archive-date=4 March 2000 }}</ref> Prior art wrapping [[Component Object Model|COM]], [[Common Object Request Broker Architecture|CORBA]], and [[Java remote method invocation|Java RMI]] objects in XML syntax and transporting them via HTTP also existed in DataChannel's WebBroker technology.<ref>{{Cite news|volume=20 |issue=21 |last=Walsh |first=Jeff |title=W3C gives a nod to DataChannel's WebBroker |work=Infoworld |access-date=2015-11-17 |date=1998-05-25 |url=http://www.infoworld.com/cgi-bin/displayArchive.pl?/98/21/i06-21.80.htm |url-status=dead |archive-url=https://web.archive.org/web/19990910213004/http://www.infoworld.com/cgi-bin/displayArchive.pl?/98/21/i06-21.80.htm |archive-date=10 September 1999 }}</ref><ref>{{Cite web|last1=Vizard |first1=Michael |last2=Walsh |first2=Jeff |title=DataChannel's Dave Pool talks about shaping the role of XML to suit different needs |work=Infoworld |access-date=2015-12-08 |date=1998-06-29 |url=http://www.infoworld.com/cgi-bin/displayStory.pl?/interviews/980629pool.htm |url-status=dead |archive-url=https://web.archive.org/web/19990916093829/http://www.infoworld.com/cgi-bin/displayStory.pl?/interviews/980629pool.htm |archive-date=16 September 1999 }}</ref>

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 was assigned to [[webMethods]], located in [[Fairfax, Virginia]]. The patent expired on March 23, 2019.<ref name= "merrick01">{{cite web |url=http://www.google.com/patents?id=WFV4AAAAEBAJ |archive-url=https://web.archive.org/web/20111203095836/http://www.google.com/patents?id=WFV4AAAAEBAJ |url-status=dead |archive-date=3 December 2011 |title=US Patent 7,028,312 |access-date=18 September 2008 |author=Merrick|date=11 April 2006|display-authors=etal}}</ref>

==Usage==
In XML-RPC, a client performs an RPC by sending an HTTP request to a server that implements XML-RPC and receives the HTTP response.  A call can have multiple parameters and one result.  The protocol defines a few data types for the parameters and result.  Some of these data types are complex, i.e. nested.  For example, you can have a parameter that is an array of five integers.

The parameters/result structure and the set of data types are meant to mirror those used in common programming languages.

''Identification'' of clients for authorization purposes can be achieved using popular HTTP security methods. [[Basic access authentication]] can be used for identification and authentication.

In comparison to RESTful protocols, where ''resource representations'' (documents) are transferred, XML-RPC is designed to ''call methods''.  The practical difference is just that XML-RPC is much more structured, which means common library code can be used to implement clients and servers and there is less design and documentation work for a specific application protocol. {{Citation needed|reason=This is a vague opinion that doesn't reflect commonly accepted views, XML-RPC adoption having been declining for its lack of reuse and common structure by major actors since the last 20 years|date=August 2024}} One salient technical difference between typical RESTful protocols and XML-RPC is that many RESTful protocols use the HTTP URI for parameter information, whereas with XML-RPC, the URI just identifies the server.

[[JSON-RPC]] is similar to XML-RPC.

==Data types==
Common [[datatype]]s are converted into their XML equivalents with example values shown below:
{| class="wikitable"
!Name
!Tag Example
!Description
|-
|array
|
<syntaxhighlight lang="xml">
<array>
  <data>
    <value><i4>1404</i4></value>
    <value><string>Something here</string></value>
    <value><i4>1</i4></value>
  </data>
</array>
</syntaxhighlight>
|[[Array data structure|Array]] of values, storing no keys
|-
|base64
|
<syntaxhighlight lang="xml">
<base64>eW91IGNhbid0IHJlYWQgdGhpcyE=</base64>
</syntaxhighlight>
|[[Base64]]-encoded binary data
|-
|boolean
|
<syntaxhighlight lang="xml">
<boolean>1</boolean>
</syntaxhighlight>
|[[Boolean data type|Boolean]] logical value (0 or 1)
|-
|date/time
|
<syntaxhighlight lang="xml">
<dateTime.iso8601>19980717T14:08:55Z</dateTime.iso8601>
</syntaxhighlight>
|Date and time in [[ISO 8601]] format
|-
|double
|
<syntaxhighlight lang="xml">
<double>-12.53</double>
</syntaxhighlight>
|[[Double precision]] floating point number
|-
|integer
|
<syntaxhighlight lang="xml">
<int>42</int>
</syntaxhighlight>
or
<syntaxhighlight lang="xml">
<i4>42</i4>
</syntaxhighlight>
|Signed [[integer]] coded on 4 bytes
|-
|string
|
<syntaxhighlight lang="xml">
<string>Hello world!</string>
</syntaxhighlight>
or
<syntaxhighlight lang="xml">
Hello world!
</syntaxhighlight>
|String of characters. Must follow [[XML#Characters and escaping|XML encoding]].
|-
|struct
|
<syntaxhighlight lang="xml">
<struct>
  <member>
    <name>foo</name>
    <value><i4>1</i4></value>
  </member>
  <member>
    <name>bar</name>
    <value><i4>2</i4></value>
  </member>
</struct>
</syntaxhighlight>
|[[Associative array]]
|-
|nil
|
<syntaxhighlight lang="xml">
<nil/>
</syntaxhighlight>
|[[nullable type|Discriminated null value]]; an XML-RPC [https://web.archive.org/web/20050911054235/http://ontosys.com/xml-rpc/extensions.php extension]
|
|-
|long
|
<syntaxhighlight lang="xml">
<i8>1312</i8>
</syntaxhighlight>
|Signed integer coded on 8 bytes. This is not part of the specification, but it is supported by several XML-RPC implementations<ref>{{cite web  |title=RPC::XML - A set of classes for core data, message and XML handling - metacpan.org |work=MetaCPAN |url=https://metacpan.org/pod/RPC::XML |access-date=13 April 2025}}</ref>{{,}}<ref>{{cite web |title=User manual for XML-RPC For C/C++ |url=https://xmlrpc-c.sourceforge.io/doc/libgeneral.html |access-date=13 April 2025}}</ref>
|}

==Examples==
An example of a typical XML-RPC request would be:
<syntaxhighlight lang="xml">
<?xml version="1.0"?>
<methodCall>
  <methodName>examples.getStateName</methodName>
  <params>
    <param>
        <value><i4>40</i4></value>
    </param>
  </params>
</methodCall>
</syntaxhighlight>

An example of a typical XML-RPC response would be:
<syntaxhighlight lang="xml">
<?xml version="1.0"?>
<methodResponse>
  <params>
    <param>
        <value><string>South Dakota</string></value>
    </param>
  </params>
</methodResponse>
</syntaxhighlight>

A typical XML-RPC fault would be:
<syntaxhighlight lang="xml">
<?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>
</syntaxhighlight>

== Criticism ==
Recent critics (from 2010 and onwards) 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 verbose compared to [[JSON]].<ref>
{{cite web
| url         = https://stackoverflow.com/questions/1376688
| title       = What is the benefit of XML-RPC over plain XML?
| date        = 9 September 2009
| publisher   = [[Stack Overflow]]
| access-date  = 7 April 2011
}}
</ref><ref>
{{cite web
| url         = http://www.intertwingly.net/wiki/pie/XmlRpcDiscussion?action=show&redirect=DontUseXmlRpc
| title       = An open poll on the merits of XmlRpc versus alternatives
| date        = 22 November 2006
| publisher   = intertwingly.net
| access-date  = 7 April 2011
}}
</ref><ref>
{{cite web
 |url         = http://joncanady.com/blog/2010/01/14/if-you-have-rest-why-xml-rpc/ 
 |title       = If you have REST, why XML-RPC? 
 |author      = Jon Canady 
 |date        = 14 January 2010 
 |publisher   = joncanady.com
 |access-date  = 7 April 2011 
 |url-status     = dead 
 |archive-url  = https://web.archive.org/web/20130511053512/http://joncanady.com/blog/2010/01/14/if-you-have-rest-why-xml-rpc/ 
 |archive-date = 11 May 2013 
}}
</ref>

==See also==

* [[Weblogs.com]]
* [[Pingback]]
*[[Ajax (programming)]]
*[[Software componentry#Technologies for Software Components|Component technologies]]
*[[Comparison of data serialization formats]]
*[[OPML]]
*[[JSON-RPC]]
*[[Web service]]
*[[gRPC]]

==References==
{{Reflist}}

==External links==
* {{web archive|url=https://web.archive.org/web/20130113041824/http://xmlrpc.scripting.com/default.html|title=Official website}}

{{DEFAULTSORT:Xml-Rpc}}
[[Category:XML-based standards]]
[[Category:Web services]]
[[Category:Internet protocols]]
[[Category:Remote procedure call]]