What is SOAP API?

What is SOAP?

SOAP stands for 'Simple Object Access Protocol'. It is a way for structured data to be sent between computer networks so that online services can work. It uses XML (Extensible Markup Language) to encode messages and mostly depends on other application layer protocols, like HTTP and SMTP. This is also what it uses for message negotiation and delivery.

SOAP APIs are frequently employed in situations where security, reliability, and stringent message formatting are critical. In contrast to REST, which is centered on resources, SOAP is driven by protocols and emphasizes standards.

Key Features of SOAP APIs

1. Based on protocols: SOAP only works with a certain protocol, while REST can use multiple protocols, such as HTTP and HTTPS.
2. XML Messaging: SOAP formatting requests and replies with XML makes sure that both humans and machines can read the messages.
3. WS-Security: SOAP is suited for secure applications due to its built-in security standards.
4. ACID Compliance: SOAP ensures that all operations succeed or fail by supporting Atomicity, Consistency, Isolation, and Durability (ACID).
5. Strict Error - Handling: SOAP APIs give clear error messages, which makes them perfect for complicated deals that need full handling of errors.

SOAP Message Structure​

Every SOAP message consists of a few core components, which look something like this:

<soap:Envelope>
   <soap:Header>...</soap:Header>
   <soap:Body>...</soap:Body>
   <soap:Fault>...</soap:Fault>
</soap:Envelope>

Let’s walk through each piece:

• Envelope: The outer wrapper indicating that the message is a SOAP message.
• Header: Optional metadata section for info like security credentials or transaction IDs, similar to an address label on a package.
• Body: Contains the main data being sent, whether it's a request or a response.
• Fault: Error-reporting section included when something goes wrong, providing details about the issue.

A minimum SOAP message has an Envelope and Body, but adding a Header provides you a lot of flexibility for authentication and data routing across systems.

Example of SOAP request

<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:web="http://www.example.com/webservice">
  <soapenv:Header/>
  <soapenv:Body>
     <web:GetCityWeatherByZIP>
        <web:ZIP>10001</web:ZIP>
     </web:GetCityWeatherByZIP>
  </soapenv:Body>
</soapenv:Envelope>

SOAP vs. REST​: A Quick Comparison

Criteria	SOAP	REST
Protocol	Protocol-based (strict)	Resource-based (flexible)
Message Format	XML	JSON, XML, YAML, etc.
Security	WS-Security, built-in	Relies on external security (OAuth, etc.)
Transaction Support	Built-in support for ACID transactions	Limited transaction support
Use Case	High-security, enterprise applications	Scalable, stateless, general-purpose
Complexity	More complex, requires more overhead	Simpler, easier to implement

Both have their place. SOAP is often used in internal systems (think of big enterprise systems talking to each other), while REST is the darling of public APIs—think Twitter, Stripe, or other APIs that expose services to developers. For more details, read here.

WSDL: The SOAP Blueprint​

A feature that distinguishes SOAP from other API formats is its utilization of Web Services Description Language, or WSDL. A WSDL file functions similarly to a SOAP API contract or design. SOAP differs from other API formats because it utilizes WSDL (Web Services Description Language). A WSDL file functions as a contract or blueprint for your SOAP API. It defines exactly:

• What operations the API can perform (e.g., GetCustomerDetails, MakePayment).
• What data fields are expected for each operation.
• What types of responses you’ll get back.

This is very helpful because it takes away the need to guess how to use the API. The WSDL tells you everything you need to know ahead of time, so you can confidently code against it. You can also use WSDL to have client-side code that talks to the API automatically generated, which saves you a lot of work.

Getting started with SOAP service integration​

SOAP APIs may be more difficult to implement than REST APIs, but they provide more resilience and stability. Below is a detailed guide on how to incorporate one into your project.

1. Get the WSDL: The initial requirement for utilizing the desired API is to acquire the WSDL file.
2. Use a SOAP library: The majority of programming languages provide libraries to simplify handling SOAP operations. There are libraries for most programming languages that make working with SOAP easier. For example:
   • Zeep for Python
   • SOAP - A SOAP client and server for node.js.
3. Make your request: Once you’ve got your WSDL and SOAP library ready, the next step is constructing your request. This will usually involve filling out a SOAP body with the data fields the API expects.
4. Send and receive: Finally, send your request via HTTP POST. The response will come back in XML format, which your library should parse for you.

SOAP API Advantages

• Extensibility: XML format allows easy addition of new elements.
• Security: Robust features like WS-Security for sensitive data.
• Transaction Support: ACID compliance for multi-step transactions.
• Error Handling: Provides detailed error messages for easier debugging.

SOAP API Limitations

• Tight Coupling: Changes can require updates on both service and client sides.
• Overhead: Larger, more verbose messages increase latency and resource usage.
• Performance: Slower compared to lighter protocols like REST.
• Limited Flexibility: Restricted to XML format, unlike REST's support for multiple formats.

Best Use Cases for SOAP​

So, when should you use SOAP over REST or other API formats? Here’s where SOAP is used:

• Highly regulated industries: Finance, healthcare, and government are heavily controlled. These areas benefit from SOAP's rigidity and security.
• Legacy systems: Many older systems were developed to use SOAP and won't be refactored.
• Transactions: SOAP's error management and transaction functionality make it ideal for financial transactions and ACID-compliant processes.
• Non-HTTP messaging: SOAP can work over protocols other than HTTP, so if you’re dealing with SMTP, TCP, or other legacy communication methods, SOAP fits right in.

Conclusion

Despite REST’s dominance, SOAP persists in certain areas. The reason? Standardization and security.Soap APIs can restrict what's sent, how, and how it's processed. This makes it ideal for sensitive data businesses like finance and healthcare. SOAP is trustworthy for transaction-heavy contexts due to its error handling and state management features.