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What is SOAP API?

What is SOAP?​

SOAP, or Simple Object Access Protocol, is a messaging protocol that allows different software systems to communicate with each other over a network. Think of it as the post office of APIs. You package your data, address it properly, and SOAP ensures it’s delivered intact. It’s a protocol that’s been around for decades and is still widely used in legacy systems and high-security environments (e.g., banking, government). SOAP messages are written in XML, a format that’s both human-readable and machine-readable.

Unlike modern API formats that are lightweight and flexible, SOAP is very strict—every message has to follow a rigid structure. This structure ensures reliable communication between services but can also make it slower and more resource-intensive than newer approaches like REST.

SOAP vs. REST​

This is probably the question everyone has at some point: Should I use SOAP or REST? Here’s a quick comparison:

  • SOAP is a protocol that defines strict rules on how messages should be formatted and exchanged.
  • REST (Representational State Transfer) is an architectural style, meaning it doesn’t enforce strict rules. It’s flexible and works with many data formats (JSON, XML, plain text).

In short:

  • SOAP = structure, rules, security, XML.
  • REST = flexibility, simplicity, JSON (but can be XML too).

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. You can find out more details and technical comparison in this article.

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: This is the outer wrapper for the message. It tells the system, "Hey, this whole is a SOAP message!"
  • Header: Optional but useful. This is where you can add meta-information like security credentials, authentication tokens, or transaction IDs. Think of it like the address label on your package.
  • Body: The meat of the message. This contains the actual data you’re sending—whether it’s a request or a response.
  • Fault: This section is included when something goes wrong. It’s SOAP’s error-reporting mechanism. A minimal SOAP message will have just the Envelope and Body, but adding a Header gives you a ton of flexibility in handling things like authentication or routing data across multiple systems.

Example:

<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 Protocols and Formats​

SOAP messages are always written in XML, which is both a blessing and a curse. The upside is that XML is highly structured and well-understood. The downside is that it’s verbose—meaning SOAP messages are bigger and heavier compared to something like a RESTful JSON message.

As for how SOAP messages are transported, SOAP is protocol-agnostic. That means it can work over a variety of transport protocols:

  • HTTP/HTTPS: The most common choice. It’s reliable, and everyone’s firewall is configured for it.
  • TCP: For when you need low-level control over the connection.
  • SMTP/FTP: Yes, SOAP can even be sent via email or file transfer.
  • WSDL: The Blueprint for SOAP

One thing that sets SOAP apart from other API formats is the use of WSDL (Web Services Description Language). A WSDL file is like 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 super helpful because you don’t need to guess how to interact with the API. The WSDL gives you all the details in advance, so you can code against it with confidence. Plus, you can use WSDL to auto-generate client-side code that interacts with the API, taking a lot of manual effort off your plate.

Security and Reliability: Where SOAP Shines​

One of the biggest reasons SOAP is still used is its security features. SOAP has built-in support for WS-Security, which allows you to sign and encrypt your messages, ensuring they can’t be tampered with. You can also use SSL/TLS encryption for transport-level security. There are a few popular security protocols that SOAP works with:

  • HTTP Basic Auth: The simplest form of authentication (just a username and password).
  • WS-Security: Provides signing and encryption at the message level, giving you fine-grained control over who can read or alter the message.

In addition to security, SOAP is also reliable thanks to its ACID compliance. This makes it perfect for bank transactions, payment processing, and any situation where data integrity is critical. If something goes wrong during a transaction, the underlying implementation of the SOAP services ensures everything either rolls back or retries until it’s complete.

Getting started with SOAP service integration​

SOAP APIs can be more complex to implement than REST, but they offer robustness and reliability. Here’s a step-by-step on how to integrate one:

  1. Get the WSDL: The first thing you’ll need is the WSDL file for the API you’re trying to consume. This is your roadmap.
  2. Use a SOAP library: 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.

Best Use Cases for SOAP​

So, when should you use SOAP over REST or other API formats? Here’s when SOAP really shines:

  • Highly regulated industries: Think finance, healthcare, and government. SOAP’s rigid structure and built-in security features make it ideal for these sectors.
  • Legacy systems: Many older systems were built to work with SOAP, and they’re not going to be refactored anytime soon.
  • Transactions: If you’re dealing with financial transactions or any process that needs to be ACID-compliant, SOAP’s error handling and transaction support make it the right choice.
  • 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.

Advantages and Limitations​

Advantages:

  • Standardized: SOAP’s rigidity ensures you always know what to expect. It enforces consistent messaging formats across systems.
  • Security: With WS-Security and SSL/TLS, you can ensure your data is safe in transit and at rest.
  • Reliability: SOAP supports transactions and error handling, making it perfect for mission-critical applications.
  • Protocol flexibility: SOAP isn’t limited to HTTP. You can send SOAP messages over SMTP, TCP, or any other protocol.

Limitations:

  • Heavy payloads: SOAP’s reliance on XML means larger, more verbose messages compared to REST’s JSON.
  • Performance: Due to the large payloads, SOAP can be slower and more resource-intensive.
  • Steep learning curve: SOAP requires a good understanding of XML, WSDL, and the various transport protocols.

Why Does SOAP Still Exist?​

Despite REST’s dominance, SOAP persists in certain areas. The reason? Standardization and security. SOAP APIs can enforce stricter rules about what’s sent, how it’s sent, and how it’s processed. This makes it great for industries that deal with sensitive data (like finance and healthcare). Additionally, SOAP has built-in error handling and state management features, making it reliable for transaction-heavy environments.