WSDL

WSDL, the Web Services Description Language, is an XML grammar for describing what a SOAP web service does and how to call it. A WSDL document names the service’s operations, the messages they exchange, the data types those messages carry, and the network endpoints and protocol bindings where the service lives. In the SOAP era it was the machine-readable contract that tooling read to generate client stubs and server skeletons — the ancestor of what OpenAPI does for REST today.

• Operations and messages - Describes each callable operation and the request/response messages it exchanges, typed against XML Schema.
• Bindings and endpoints - Maps abstract operations onto concrete SOAP-over-HTTP bindings and the service URLs that host them.
• Tooling-first by design - Written to be consumed by code generators and IDEs, not by humans reading it directly, which made strongly-typed enterprise integration possible.
• XML all the way down - The document itself, the schemas it references, and the SOAP envelopes it describes are all XML, keeping it tightly coupled to that stack.

I still run into WSDL constantly when I map legacy enterprise landscapes, and translating a WSDL into an OpenAPI is one of the more common modernization jobs I see. It rarely disappears cleanly — the operations, types, and bindings carry business meaning that has to be carried forward, not discarded. For governance and the agentic turn, an old WSDL is often the only surviving written contract for a service, which makes it a starting point worth reverse-engineering rather than ignoring.