XML-RPC — Opsalis Learn

What Is XML-RPC?

XML-RPC is a remote procedure call protocol created in 1998 by Dave Winer and Microsoft. It was the first widely-adopted protocol to use XML over HTTP for machine-to-machine communication, and it directly inspired SOAP (which evolved from XML-RPC by adding namespaces, headers, and the WS-* extensions). While SOAP grew complex, XML-RPC stayed simple — and that simplicity is why it endures today.

An XML-RPC request is an HTTP POST containing a <methodCall> XML document. Inside that document are two things: a <methodName> (the procedure to invoke, like getOfficialByName) and an optional <params> block containing the arguments. Each parameter is wrapped in a <value> tag with a type indicator: <string>, <int>, <double>, <boolean>, <dateTime.iso8601>, <base64>, <array>, or <struct>.

The server responds with a <methodResponse> document containing either a <params> block (on success) or a <fault> block (on error). A fault contains a faultCode (integer) and a faultString (human-readable message). There is no concept of WSDL or schema files — the method list and parameter types are typically documented informally or discovered through a system.listMethods introspection call.

XML-RPC is still actively used in many systems: WordPress exposes its entire remote management API via XML-RPC (/xmlrpc.php), many legacy enterprise systems and embedded devices speak XML-RPC, and some content management systems and publishing tools rely on it. If you have an XML-RPC API that works and you do not want to rewrite it, Opsalis can monetize it exactly as it is.

How XML-RPC Works with Opsalis

XML-RPC is method-based, like JSON-RPC, but uses XML for encoding. The Opsalis handler bridges the XML serialization gap so consumers work with clean JSON.

Owner provides a method list. Upload a JSON file listing each XML-RPC method name, its parameter types, and response structure. The handler uses this to generate OpenAPI endpoints and validate calls.
Handler constructs methodCall XML. When a consumer request arrives as JSON, the handler converts each parameter to the appropriate XML-RPC type (<string>, <int>, <struct>, etc.) and wraps everything in a <methodCall> document.
Handler parses methodResponse XML back to JSON. The XML-RPC response is parsed, type-converted (XML-RPC types mapped to JSON types), and returned to the consumer as clean JSON. Faults are mapped to appropriate HTTP error responses.

Your XML-RPC server stays untouched. Keep your existing methods and endpoint. The handler handles all XML construction and parsing — consumers never see XML.

Demo API: US Congress and Presidents

This demo uses a US Congress and Presidents dataset — 12,790 officials from 1789 to the present day. Every member of the Senate and House of Representatives, every President and Vice President, with biographical data, party affiliation, state, term dates, and committee assignments. Methods include lookups by name, state, party, chamber, term year, and full-text search.

← Owner Guide

Code Example

Look up a president by name

# XML-RPC methodCall to look up a historical official
curl http://localhost:4006/xmlrpc \
  -H "Content-Type: text/xml" \
  -d '<?xml version="1.0"?>
<methodCall>
  <methodName>getOfficialByName</methodName>
  <params>
    <param>
      <value><string>Abraham Lincoln</string></value>
    </param>
  </params>
</methodCall>'

Response (XML)

<?xml version="1.0"?>
<methodResponse>
  <params>
    <param>
      <value>
        <struct>
          <member>
            <name>name</name>
            <value><string>Abraham Lincoln</string></value>
          </member>
          <member>
            <name>role</name>
            <value><string>President</string></value>
          </member>
          <member>
            <name>party</name>
            <value><string>Republican</string></value>
          </member>
          <member>
            <name>state</name>
            <value><string>Illinois</string></value>
          </member>
          <member>
            <name>termStart</name>
            <value><string>1861-03-04</string></value>
          </member>
          <member>
            <name>termEnd</name>
            <value><string>1865-04-15</string></value>
          </member>
        </struct>
      </value>
    </param>
  </params>
</methodResponse>

Through the Opsalis handler, the same call becomes simple JSON:

Same call via the Opsalis handler (JSON)

# Consumer sends JSON — handler builds the XML-RPC methodCall
curl http://localhost:3000/api/congress/getOfficialByName \
  -H "Content-Type: application/json" \
  -d '{ "name": "Abraham Lincoln" }'

XML-RPC Type Mapping

XML-RPC defines a small set of data types. The handler automatically converts between these and their JSON equivalents:

XML-RPC Type	JSON Equivalent	Example
`<string>`	string	`"hello"`
`<int>` / `<i4>`	number	`42`
`<double>`	number	`3.14`
`<boolean>`	boolean	`true`
`<dateTime.iso8601>`	string (ISO 8601)	`"2026-03-01T12:00:00Z"`
`<base64>`	string (base64)	`"SGVsbG8="`
`<array>`	array	`[1, 2, 3]`
`<struct>`	object	`{ "key": "value" }`

XML-RPC at a Glance

Concept	Details
Transport	HTTP POST
Data format	XML
Spec format	Method list (no formal IDL; introspection via system.listMethods)
Request structure	`<methodCall>` with `<methodName>` and `<params>`
Error handling	`<fault>` with faultCode and faultString
Best for	Legacy systems, CMS platforms, embedded devices, WordPress integrations