What is the Web service protocol stack?

The Web service protocol stack is an evolving set of protocols used to define, discover, and implement Web services. The core protocol stack consists of four layers:
Service Transport: This layer is responsible for transporting messages between applications. Currently, this includes HTTP, SMTP, FTP, and newer protocols, such as Blocks Extensible Exchange Protocol (BEEP).
XML Messaging: This layer is responsible for encoding messages in a common XML format so that messages can be understood at either end. Currently, this includes XML-RPC and SOAP.
Service Description: This layer is responsible for describing the public interface to a specific Web service. Currently, service description is handled via the WSDL.
Service Discovery: This layer is responsible for centralizing services into a common registry, and providing easy publish/find functionality. Currently, service discovery is handled via the UDDI.
Beyond the essentials of XML-RPC, SOAP, WSDL, and UDDI, the Web service protocol stack includes a whole zoo of newer, evolving protocols. These include WSFL (Web Services Flow Language), SOAP-DSIG (SOAP Security Extensions: Digital Signature), and USML (UDDI Search Markup Language). For an overview of these protocols, check out Pavel Kulchenko's article, Web Services Acronyms, Demystified, on XML.com.
Fortunately, you do not need to understand the full protocol stack to get started with Web services. Assuming you already know the basics of HTTP, it is best to start at the XML Messaging layer and work your way up.

 

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 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 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.