SOAP

SOAP (Simple Object Access Protocol) is an XML-based protocol for exchanging messages between applications running on different systems and in different programming languages. Unlike the REST style, SOAP is a strictly defined protocol with a formal contract, which makes it popular in enterprise, banking and telecommunications environments.

How SOAP works

Communication in SOAP is based on exchanging structured XML messages called envelopes. Every envelope has a fixed structure:

• Envelope — the root element wrapping the whole message,
• Header — optional metadata such as credentials or transaction details,
• Body — the actual request or response payload,
• Fault — a section describing any error.

A SOAP service is described by a WSDL file that precisely defines the available operations, data types and message structure — giving the client and server an unambiguous contract. SOAP is transport-independent: it most often runs over HTTP, but can also work through SMTP or other protocols. The WS-* family of standards adds advanced features such as signatures, encryption and transactions.

SOAP in practice

SOAP remains present wherever a formal contract, transactions and strong security matter — in banking, payment and insurance systems, and in integrations with public institutions. Many older but still-running enterprise APIs are built on SOAP. New, lightweight web integrations more often choose REST with the JSON format, yet knowing SOAP is essential when maintaining and connecting to existing enterprise-grade systems.