XMLRPC

XML-RPC is a remote procedure call protocol that uses XML to encode its calls and HTTP as the transport. It allows a client to invoke procedures on a remote server as if they were local methods, enabling cross-language interaction through a simple, platform-neutral format.

A request is an XML document containing a methodCall element with a methodName and a params list. Each param holds a value encoding a data item such as int, boolean, string, double, dateTime.iso8601, or base64, and may also represent composite types like struct (a mapping of member names to values) and array. A response uses methodResponse and contains either a single value or a fault, the latter represented as a value containing a struct with faultCode and faultString.

XML-RPC commonly uses HTTP POST with content-type text/xml. The protocol has two commonly referenced versions: 1.0, which defined the core encoding, and 2.0, which adds refinements but preserves the same encoding shape. Implementations and libraries exist across languages, including Python, Java, PHP, and C#, contributing to its early adoption in cross-platform services.

Limitations include verbose XML payloads, lack of built-in security or authentication mechanisms, and reliance on HTTP. While simple and lightweight to implement, XML-RPC has been largely supplanted in modern web services by REST and JSON-RPC, though it remains in legacy systems and some constrained environments.