SOAP (Simple Object Access Protocol) – a protocol for exchanging structured information in web services.

Three layers:
- Envelope – encapsulates all the data in a message and identifies the XML document as a SOAP message.
- Header – contains additional information about the SOAP message. This information could be authentication credentials.
- Body – includes the details of the actual message that need to be sent from the web service to the calling application.
Key Features:
- Uses XML for message format.
- Follows strict standards, ensuring high security and reliability.
- Requires a WSDL file for service definitions.
Typical Use Cases:
- Financial services.
- Payment gateways.
- Applications requiring strong security and ACID compliance.