XML-RPC

What is XML RPC?

XML-RPC is a remote procedure call (RPC) protocol which encodes its calls as XML. It's a protocol for executing procedures on remote servers, making it a precursor to SOAP and REST. XML-RPC allows you to send a request to a remote server, execute a given procedure, and return the results, all encapsulated in XML format.

Principles of XML RPC

• Transport Agnostic: Similar to JSON-RPC, XML-RPC doesn't mandate a specific transport mechanism. It can be utilized over HTTP, SMTP, TCP, or any other reliable transport layer, making it versatile for different networking scenarios.

  Example: You can send an XML-RPC message via HTTP like so:

POST /RPC2 HTTP/1.0
User-Agent: myClient
Host: www.example.com
Content-Type: text/xml
Content-Length: length

<?xml version="1.0"?>
<methodCall>
  <methodName>examples.getStateName</methodName>
  <params>
    <param>
      <value><i4>41</i4></value>
    </param>
  </params>
</methodCall>

In this example, a POST request is made to the /RPC2 endpoint on www.example.com with a XML-RPC message requesting the state name for the number 41.

• Simple Protocol: XML-RPC is designed to be as simple as possible, while allowing for complex structures to be transmitted, processed, and returned.

• Structured Data Communication: XML-RPC utilizes XML encoding to enable structured data communication, ensuring a standardized data exchange format.

Benefits of XML-RPC

XML-RPC stands out due to its simple and structured communication model. It's particularly useful in environments where different platforms need to communicate with each other.

• Simplicity: XML-RPC's protocol design is straightforward, making it easy to implement and use. It's well-suited for cross-platform communication as XML is a widely accepted standard for data encoding.

  Example: An XML-RPC request to get the state name for a given state number looks like:

<?xml version="1.0"?>
<methodCall>
  <methodName>getStateName</methodName>
  <params>
    <param>
      <value><int>1</int></value>
    </param>
  </params>
</methodCall>

The methodName field specifies the operation, and the params field specifies the input values.

• Cross-platform Communication: The use of XML as a data encoding format ensures that XML-RPC can be used for communication between different platforms and programming environments.

• Transport Flexibility: Being transport agnostic, XML-RPC can be utilized over different transport mechanisms, providing flexibility in network configurations.

How does XML-RPC Work?

XML-RPC follows a simple request-response model:

1. Client Sends a Request

   Clients send a request to the server encoded in XML with a specified method and parameters.

<?xml version="1.0"?>
<methodCall>
  <methodName>examples.getStateName</methodName>
  <params>
    <param>
      <value><i4>41</i4></value>
    </param>
  </params>
</methodCall>

In this request, the methodName field indicates the operation to perform, getStateName in this case, and the params field provides the arguments for the method, here 41.

2. Server Processes the Request

The server processes the request, performs the specified method with the provided parameters, and prepares a response. It decodes the XML message, extracts the method name and parameters, performs the operation, and then prepares an XML-encoded response.

3. Server Sends a Response to the Client

The server sends a response back to the client with the result of the request.

<?xml version="1.0"?>
<methodResponse>
  <params>
    <param>
      <value><string>South Dakota</string></value>
    </param>
  </params>
</methodResponse>

In this response, the methodResponse element contains a params element with the result of the getStateName operation.

Components of an XML-RPC Request

Each XML-RPC request consists of several key components:

• methodCall: This element encloses the entire method call.

<methodCall>
  ...
</methodCall>

• methodName: The methodName element specifies the name of the method to be invoked on the server.

<methodName>examples.getStateName</methodName>

• params: The params element encloses the method parameters.

<params>
  <param>
    <value><i4>41</i4></value>
  </param>
</params>

XML RPC Error Handling

XML-RPC has a structured error handling mechanism through the fault element in the response:

Error Object:

<?xml version="1.0"?>
<methodResponse>
  <fault>
    <value>
      <struct>
        <member>
          <name>faultCode</name>
          <value><int>-32602</int></value>
        </member>
        <member>
          <name>faultString</name>
          <value><string>Invalid params</string></value>
        </member>
      </struct>
    </value>
  </fault>
</methodResponse>

Components of an XML-RPC Response

When a server sends back a response, it provides the result of the method invocation or an error object if something went wrong:

• methodResponse: This element encloses the entire method response.

  <methodResponse>
    ...
  </methodResponse>

• params or fault: The params element contains the result of the method invocation, or the fault element contains an error object if something went wrong.

<params>
  <param>
    <value><string>South Dakota</string></value>
  </param>
</params>

or

<fault>
  ...
</fault>

Real-world example of XML-RPC

To further understand XML-RPC, let's consider a real-world scenario of a "To-Do List" application similar to the previous modules. In this scenario, XML-RPC is used to manage tasks between the client and server.

API Endpoints in Action

For our hypothetical To-Do List application, the XML-RPC defines several methods, each corresponding to a specific action:

getTasks: Viewing All Tasks

This method provides a comprehensive list of all tasks stored in the application's database.

Request:

<?xml version="1.0"?>
<methodCall>
  <methodName>getTasks</methodName>
  <params>
  </params>
</methodCall>

Response:

<?xml version="1.0"?>
<methodResponse>
  <params>
    <param>
      <value>
        <array>
          <data>
            <value><struct>
              <member>
                <name>title</name>
                <value><string>Buy groceries</string></value>
              </member>
              <member>
                <name>id</name>
                <value><i4>1</i4></value>
              </member>
            </struct></value>
            <value><struct>
              <member>
                <name>title</name>
                <value><string>Walk the dog</string></value>
              </member>
              <member>
                <name>id</name>
                <value><i4>2</i4></value>
              </member>
            </struct></value>
          </data>
        </array>
      </value>
    </param>
  </params>
</methodResponse>

Conclusion

XML-RPC serves as a simple, yet powerful protocol for executing remote procedure calls, especially in a cross-platform environment. Its usage of XML for encoding requests and responses ensures structured communication, making it a reliable choice for developing web services that require remote procedure calls. Through the understanding of XML-RPC, developers can leverage its simplicity and structured communication model to build robust and interoperable web services.