SOAP

SOAP (Simple Object Access Protocol) is a W3C messaging protocol for exchanging structured information between services as XML envelopes, carried over HTTP, SMTP, and other transports. It grew up alongside WSDL and the broader WS-* stack to give enterprises a formal, contract-first way to describe and invoke operations across systems. Long before REST and OpenAPI became the default, SOAP was how much of the enterprise did integration, and a great deal of that infrastructure is still running.

• XML envelope - Every message is a strict XML document with a header and a body, parseable and validatable against a schema.
• Transport independence - Designed to ride over HTTP, SMTP, and message queues rather than being tied to a single protocol.
• WS-* extensions - Built-in seams for WS-Security, WS-ReliableMessaging, and transactions that enterprises leaned on for guarantees REST left to convention.
• WSDL contracts - A machine-readable description of every operation, its inputs, and outputs, that predates the OpenAPI era of API description.

In the work I do today, SOAP shows up mostly as legacy that has to be inventoried, governed, and often translated. I have spent real time turning WSDL into OpenAPI so older services can be discovered, tested, and secured with modern tooling. SOAP is a reminder that “the API economy” did not start with REST, and that any honest map of an organization’s estate still has to account for the XML-and-envelope integrations quietly holding critical systems together.