SOAP
An older, XML-based, contract-heavy protocol for web services — verbose but strictly standardized, still seen in enterprise/finance.
SOAP (Simple Object Access Protocol) is a mature, XML-based protocol for web services, with a strict envelope format, a formal contract (WSDL), and built-in standards for security, transactions, and reliability. It predates REST and is heavier and more rigid, but its formality suits regulated enterprise integration.
Worked example: a bank or legacy enterprise API might expose SOAP endpoints where every request is a verbose XML envelope validated against a WSDL contract, with WS-Security handling signing and encryption at the message level. Gotcha: SOAP’s verbosity and tooling overhead are why REST and JSON largely replaced it for web and mobile APIs — but its rigid contracts and enterprise standards mean it persists in finance, telecom, and government, so “which protocol?” depends on whether you value strict formal contracts or lightweight simplicity.