XML-RPC — Simple Remote Procedure Calls Using XML Over HTTP

In this tutorial, you will learn about XML. We cover key concepts, practical examples, and best practices to help you master this topic.

XML-RPC is a remote procedure call protocol that uses XML to encode requests and responses over HTTP, providing a simpler alternative to SOAP for RPC-style Web Services.

What You'll Learn

• XML-RPC message structure and data types
• How XML-RPC compares to SOAP and JSON-RPC
• Building XML-RPC clients and servers

Why It Matters

XML-RPC was influential in the evolution of web services and is still used in some legacy systems and content management platforms.

Code Examples

<!-- XML-RPC Request -->
<?xml version="1.0"?>
<methodCall>
  <methodName>examples.getStateName</methodName>
  <params>
    <param>
      <value><int>41</int></value>
    </param>
  </params>
</methodCall>

<!-- XML-RPC Response -->
<?xml version="1.0"?>
<methodResponse>
  <params>
    <param>
      <value><string>Montana</string></value>
    </param>
  </params>
</methodResponse>

<!-- XML-RPC Fault -->
<?xml version="1.0"?>
<methodResponse>
  <fault>
    <value>
      <struct>
        <member>
          <name>faultCode</name>
          <value><int>4</int></value>
        </member>
        <member>
          <name>faultString</name>
          <value><string>Method not found</string></value>
        </member>
      </struct>
    </value>
  </fault>
</methodResponse>

# XML-RPC client in Python
import xmlrpc.client

proxy = xmlrpc.client.ServerProxy("https://api.example.com/xmlrpc")
result = proxy.examples.getStateName(41)
print(f"State: {result}")  # Montana

# Calling a method with complex types
data = proxy.data.process([
    {"name": "Alice", "age": 30},
    {"name": "Bob", "age": 25}
])

Common Mistakes

1. Using XML-RPC for Large Payloads

XML-RPC is verbose. Use JSON-RPC or REST for large data transfers.

2. Forgetting the Data Type Wrapper

XML-RPC requires explicit type wrappers like <int>, <string>, <boolean>.

3. Not Handling Fault Responses

Always check for fault responses, which have a different structure than success responses.

4. Mixing XML-RPC with SOAP Libraries

XML-RPC and SOAP use different XML structures. Use the correct library for each.

5. Assuming XML-RPC Supports All Data Types

XML-RPC has limited types (int, string, boolean, double, dateTime, base64, array, struct).

Practice Questions

1. What is the root element of an XML-RPC request?
2. How does XML-RPC encode data types differently from SOAP?
3. What is the structure of an XML-RPC fault response?
4. Which Python module supports XML-RPC client/server?
5. What are the limits of XML-RPC's type system?

Answers:

1. <methodCall> containing <methodName> and <params>.
2. XML-RPC uses value wrappers with type names; SOAP uses XML Schema types.
3. <methodResponse> contains a single <fault> element with faultCode and faultString.
4. xmlrpc.client for client and xmlrpc.server for server.
5. No support for null, date/time limited, no nested structures beyond array and struct.

Challenge: Build an XML-RPC server with three methods: add (two ints), getUser (returns struct), and listFiles (returns array). Test each method from a client.

FAQ

What's Next

Learn about JSON-RPC as a lighter-weight alternative, then build the Web Services Project.