SOAP APIs, which utilize the Simple Object Access Protocol, are a type of application programming interface designed for exchanging data using XML and following the Remote Procedure Call (RPC) pattern. Initially developed by Microsoft in 1998, SOAP APIs are distinct from other RPC APIs due to the SOAP specification rules and are versatile in transport protocol usage, although HTTP/HTTPS is most common. They rely on WSDL (Web Services Description Language) to describe available functions and support various authentication methods, including OAuth 2.0 and WS-Security for enhanced security. Despite the rise of REST and newer technologies like GraphQL, SOAP APIs remain prevalent, especially in scenarios requiring robust security and transaction reliability, adhering to ACID principles. SOAP's extensibility allows developers to integrate complex web services, but its inherent complexity often necessitates the use of libraries to simplify client development. While SOAP's dominance has waned, it continues to be a reliable choice for secure and dependable web integrations.