What is SOAP API?

What is SOAP?

SOAP, also known as Simple Object Access Protocol, is a communication protocol enabling diverse software systems to interact on a network. You package your data, address it properly, and SOAP ensures it’s delivered intact. It is a long-standing protocol that is still commonly used in older systems and secure environments such as banking and government. SOAP messages become readable by both humans and machines when they are written in XML.

SOAP is known for its strictness, requiring every message to adhere to a rigid structure, unlike the flexible and lightweight modern API formats. 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

Probably the most common question everyone has at some point is: Use SOAP or REST? Here is a brief comparison:

• SOAP is a protocol that establishes precise guidelines for formatting and exchanging messages.
• REST (Representational State Transfer) is an architectural style that does not impose strict regulations. It is adaptable and compatible with various data formats like JSON, XML, and 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. For more details, read here.

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 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 only have 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

Always writing SOAP messages in XML is a blessing and a curse. One benefit is that XML is well-structured and understood. Verbose SOAP communications are larger and heavier than RESTful JSON ones.

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. (we guess, no one uses this)

WSDL: The SOAP Blueprint

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.

Security and Reliability: Where SOAP Shines

The security features of SOAP are a big reason why it is still used. Because WS-Security is built into SOAP, you can sign and secure your messages to make sure they can't be changed. For protection at the transport level, you can also use SSL/TLS encryption.

There are a few popular security protocols that SOAP works with:

• HTTP Basic Auth: Simple authentication (username and password).
• WS-Security: handles message-level signature and encryption, providing you fine-grained control over message reads and changes.

Due to ACID compliance, SOAP is secure and trustworthy. 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 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.

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: 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.

Advantages and Limitations

Advantages:

• Standardized: SOAP’s rigidity ensures you always know what to expect. It enforces consistent messaging formats across systems.
• Security: Utilizing WS-Security and SSL/TLS ensures the protection of your data during transmission and storage.
• Dependability: SOAP is ideal for crucial projects as it has the capability to manage transactions and handle errors effectively.
• 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 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.