XML.com: XML From the Inside Out
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Top Ten FAQs for Web Services
by Ethan Cerami | Pages: 1, 2, 3

5. What is XML-RPC?

XML-RPC is a protocol that uses XML messages to perform Remote Procedure Calls. Requests are encoded in XML and sent via HTTP POST; XML responses are embedded in the body of the HTTP response.

More succinctly, XML-RPC = HTTP + XML + Remote Procedure Calls.

Because XML-RPC is platform independent, diverse applications can communicate with one another. For example, a Java client can speak XML-RPC to a Perl server.

To get a quick sense of XML-RPC, here is a sample XML-RPC request to a weather service (with the HTTP Headers omitted):


<?xml version="1.0" encoding="ISO-8859-1"?>
<methodCall>
   <methodName>weather.getWeather</methodName>
   <params>
      <param><value>10016</value></param>
   </params>
</methodCall>

The request consists of a simple <methodCall> element, which specifies the method name (getWeather) and any method parameters (zip code).

Here is a sample XML-RPC response from the weather service:


<?xml version="1.0" encoding="ISO-8859-1"?>
<methodResponse>
   <params>
      <param>
         <value><int>65</int></value>
      </param>
   </params>
</methodResponse>

The response consists of a single <methodReponse> element, which specifies the return value (the current temperature). In this case, the return value is specified as an integer.

In many ways, XML-RPC is much simpler than SOAP, and therefore represents the easiest way to get started with Web services.

The official XML-RPC specification is available at XML-RPC.com. Dozens of XML-RPC implementations are available in Perl, Python, Java, and Ruby. See the XML-RPC home page for a complete list of implementations.

6. What is SOAP?

SOAP is an XML-based protocol for exchanging information between computers. Although SOAP can be used in a variety of messaging systems and can be delivered via a variety of transport protocols, the main focus of SOAP is Remote Procedure Calls (RPC) transported via HTTP. Like XML-RPC, SOAP is platform independent, and therefore enables diverse applications to communicate with one another.

To get a quick sense of SOAP, here is a sample SOAP request to a weather service (with the HTTP Headers omitted):


<?xml version='1.0' encoding='UTF-8'?>
<SOAP-ENV:Envelope
   xmlns:SOAP-ENV="http://www.w3.org/2001/09/soap-envelope"
   xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
   xmlns:xsd="http://www.w3.org/2001/XMLSchema">
   <SOAP-ENV:Body>
      <ns1:getWeather 
         xmlns:ns1="urn:examples:weatherservice"
         SOAP-ENV:encodingStyle=" http://www.w3.org/2001/09/soap-encoding
         <zipcode xsi:type="xsd:string">10016</zipcode>
      </ns1:getWeather>
   </SOAP-ENV:Body>
</SOAP-ENV:Envelope>

As you can see, the request is slightly more complicated than XML-RPC and makes use of both XML namespaces and XML Schemas. Much like XML-RPC, however, the body of the request specifies both a method name (getWeather), and a list of parameters (zipcode).

Here is a sample SOAP response from the weather service:


<?xml version='1.0' encoding='UTF-8'?>
<SOAP-ENV:Envelope
   xmlns:SOAP-ENV="http://www.w3.org/2001/09/soap-envelope"
   xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
   xmlns:xsd="http://www.w3.org/2001/XMLSchema">
   <SOAP-ENV:Body>
      <ns1:getWeatherResponse
         xmlns:ns1="urn:examples:weatherservice"
         SOAP-ENV:encodingStyle="http://www.w3.org/2001/09/soap-encoding">
         <return xsi:type="xsd:int">65</return>
      </ns1:getWeatherResponse>
   </SOAP-ENV:Body>
</SOAP-ENV:Envelope>

The response indicates a single integer return value (the current temperature).

The World Wide Web Consortium (W3C) is in the process of creating a SOAP standard. The latest working draft is designated as SOAP 1.2, and the specification is now broken into two parts. Part 1 describes the SOAP messaging framework and envelope specification. Part 2 describes the SOAP encoding rules, the SOAP-RPC convention, and HTTP binding details.

7. What is WSDL?

The Web Services Description Language (WSDL) currently represents the service description layer within the Web service protocol stack.

In a nutshell, WSDL is an XML grammar for specifying a public interface for a Web service. This public interface can include the following:

  • Information on all publicly available functions.
  • Data type information for all XML messages.
  • Binding information about the specific transport protocol to be used.
  • Address information for locating the specified service.

WSDL is not necessarily tied to a specific XML messaging system, but it does include built-in extensions for describing SOAP services.

Below is a sample WSDL file. This file describes the public interface for the weather service used in the SOAP example above. Obviously, there are many details to understanding the example. For now, just consider two points.

    First, the <message> elements specify the individual XML messages that are transferred between computers. In this case, we have a getWeatherRequest and a getWeatherResponse. Second, the <service> element specifies that the service is available via SOAP and is available at a specific URL.


<?xml version="1.0" encoding="UTF-8"?>
<definitions name="WeatherService"
   targetNamespace="http://www.ecerami.com/wsdl/WeatherService.wsdl"
   xmlns="http://schemas.xmlsoap.org/wsdl/"
   xmlns:soap="http://schemas.xmlsoap.org/wsdl/soap/"
   xmlns:tns="http://www.ecerami.com/wsdl/WeatherService.wsdl"
   xmlns:xsd="http://www.w3.org/2001/XMLSchema">

   <message name="getWeatherRequest">
      <part name="zipcode" type="xsd:string"/>
   </message>
   <message name="getWeatherResponse">
      <part name="temperature" type="xsd:int"/>
   </message>

   <portType name="Weather_PortType">
      <operation name="getWeather">
         <input message="tns:getWeatherRequest"/>
         <output message="tns:getWeatherResponse"/>
      </operation>
   </portType>
   
   <binding name="Weather_Binding" type="tns:Weather_PortType">
      <soap:binding style="rpc" 
         transport="http://schemas.xmlsoap.org/soap/http"/>
      <operation name="getWeather">
         <soap:operation soapAction=""/>
         <input>
            <soap:body
               encodingStyle="http://schemas.xmlsoap.org/soap/encoding/"
               namespace="urn:examples:weatherservice"
               use="encoded"/>
         </input>
         <output>
            <soap:body
               encodingStyle="http://schemas.xmlsoap.org/soap/encoding/"
               namespace="urn:examples:weatherservice"
               use="encoded"/>
         </output>
      </operation>
   </binding>

   <service name="Weather_Service">
      <documentation>WSDL File for Weather Service</documentation>
      <port binding="tns:Weather_Binding" name="Weather_Port">
         <soap:address 
            location="http://localhost:8080/soap/servlet/rpcrouter"/>
      </port>
   </service>
</definitions>

Using WSDL, a client can locate a Web service, and invoke any of the publicly available functions. With WSDL-aware tools, this process can be entirely automated, enabling applications to easily integrate new services with little or no manual code. For example, check out the GLUE platform from the Mind Electric.

WSDL has been submitted to the W3C, but it currently has no official status within the W3C. See this W3C page for the latest draft.

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