Simple Object Access Protocol (SOAP) is a message specification for exchanging information between systems and applications. When it comes to application programming interfaces (APIs), a SOAP API is developed in a more structured and formalized way. Think of SOAP as being like the national postal service: It provides a reliable and trusted way to send and receive messages between systems (and within enterprise applications). It is older, established, and dependable—but it can be slower than competing architectural styles like REST.

A background of SOAP APIs

SOAP was a standard that emerged in the late 1990s to give businesses the ability to move data around between corporate networks. It was introduced just as the web was maturing, and while it does use HTTP primarily as a transport for the messages being passed around, its architectural patterns are not as closely aligned with HTTP as REST; SOAP can also employ other protocols. While REST is more of a style, SOAP gives you much more guidance on the structure of the request and response, as well as the message content and how it will be encoded. Simply put, using SOAP when designing APIs focuses on the message, whereas using REST when designing APIs focuses on defining them as resources.

When to use SOAP APIs

1. Formal Contracts: If a strict contract between client and server is needed using WSDL (Web Services Description Language).
2. Advanced Security: For enterprise-level security like WS-Security, message-level encryption, and secure transactions.
3. ACID Compliance: When you need atomic, consistent, isolated, durable operations (like in banking/financial systems).
4. Stateful Operations: If the API needs to maintain state across requests (using sessions).
5. Standardized Protocols: When you need to follow strict standards (e.g., SOAP over HTTP, SMTP).
6. Legacy Systems: If you’re working with existing enterprise systems (like ERP, CRM) that already use SOAP.

Benefits of using SOAP APIs

• SOAP is language, transport, and even platform independent, whereas REST requires the use of HTTP.
• SOAP is very secure, which makes it perfect for systems that handle sensitive data, such as financial services and online banking applications.
• SOAP works well in distributed enterprise environments, instead of depending on direct point-to-point communication.
• SOAP has built-in error handling features, which makes it easy to understand what happened when a request fails.

SOAP API disadvantages

• SOAP does not support caching API calls.
• SOAP is much more complicated than REST, which can have performance implications.
• SOAP is much less adaptable than REST.
• SOAP is usually slower than REST.

SOAP API use cases

• Transfers at banks: Bank transfers require communication between different banks or bank branches, which may involve multiple calls to different web services. Security is also extremely important for this use case.
• Booking flights: Much like with bank transfers, different web services must be called to check availability and flight pricing information.
• Billing services: People who work in fields like telecommunication operations need to connect with numerous systems to generate billing information, which often includes sensitive data.
• Navigation companies: Shipping and transport companies need to combine information from lots of different sources to calculate the best routes possible.
• City management: SOAP APIs connect many city management processes to ensure the city is run properly. All of these processes— from traffic light management to sewage system operations—need to work in a predictable way.

REST vs SOAP APIs

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