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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 XML-RPC protocol was created in 1998 by Dave Winer of UserLand Software and Microsoft, with Microsoft seeing the protocol as an essential part of scaling up its efforts in business-to-business e-commerce. As new functionality was introduced, the standard evolved into what is now SOAP.
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 Web Distributed Data Exchange (WDDX) and webMethod's Web Interface Definition Language (WIDL). Prior art wrapping COM, CORBA, and Java RMI objects in XML syntax and transporting them via HTTP also existed in DataChannel's WebBroker technology.
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 an 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.
JSON-RPC is similar to XML-RPC.
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>
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.
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