WSDLs

Web Services Description Language (WSDL) is an XML-based language used to describe a web service's available operations, the messages involved, and how to access the service. A WSDL document serves as a machine-readable contract between a service provider and a client, enabling automatic generation of client stubs and server skeletons and guiding communication between applications.

A WSDL document typically consists of several parts. The definitions element sets the target namespace. The types section defines data types using XML Schema. Messages describe the data elements exchanged in an operation. Port types (or interfaces in newer versions) group a set of operations. Bindings specify how each operation is transmitted, including the protocol and data format. Services collect endpoints, and ports define a concrete URL where a binding is exposed. Each operation can have input, output, and optional fault messages.

The most common binding is SOAP over HTTP, but WSDL also supports other protocols and, in later versions, bindings that accommodate REST-style interactions. WSDL 1.1 is the widely implemented version historically tied to SOAP-based services. WSDL 2.0 introduced a revised model with broader binding options and improved interoperability, though adoption varied.

In practice, WSDLs describe the service contract in a machine-readable form so clients can discover, understand, and invoke the service without human intervention. They are central to tooling that generates client libraries and service proxies. As web services have evolved, RESTful approaches with OpenAPI have grown in popularity for describing non-SOAP services, while WSDL remains common for traditional SOAP-based interfaces.