SOAP

A mature, XML-based messaging protocol designed for high-security, transactional communication between distributed systems.

SOAP (Simple Object Access Protocol) remains the industry standard for enterprise-level integrations requiring strict contracts and ACID compliance. It relies on a formal WSDL (Web Services Description Language) to define operations, ensuring both client and server stay in sync. While REST dominates web APIs, SOAP excels in banking and legacy telecommunications by providing built-in error handling and WS-Security extensions. It operates primarily over HTTP or SMTP, wrapping data in a structured envelope to guarantee reliable delivery in complex environments.

https://www.w3.org/TR/soap/