What Is a SOAP API? Complete Beginner Guide

SOAP APIs have been around for more than two decades, but they are still widely used in banking, healthcare, telecom, travel, insurance, and enterprise software.

If you are learning APIs, you will probably hear more about REST and GraphQL. However, many large organizations still rely on SOAP because it is secure, standardized, and works well for complex systems.

What Is a SOAP API?

SOAP stands for Simple Object Access Protocol.

It is a protocol used for communication between applications over a network. A SOAP API allows one system to send a request to another system using XML, and receive a structured XML response.

Unlike REST APIs, which usually exchange JSON, SOAP APIs always use XML and follow a strict format.

For example:

1. A banking app can use a SOAP API to transfer money.
2. A travel website can use a SOAP API to check flight availability.
3. A healthcare system can use a SOAP API to fetch patient records securely.

SOAP was originally created so different systems, written in different programming languages and running on different platforms, could communicate reliably.

Why Is SOAP Still Used in 2026?

Even though newer API styles exist, SOAP remains common because many enterprises need:

1. Strong security
2. Formal contracts between systems
3. Reliable messaging
4. Built-in error handling
5. Compatibility with older enterprise software

SOAP is especially popular in:

1. Banking and financial services
2. Healthcare
3. Government systems
4. Telecom companies
5. Enterprise ERP and CRM software
6. Payment gateways

If an organization has been running critical systems for years, there is a good chance it still uses SOAP.

How Does a SOAP API Work?

SOAP APIs work using a request-response model.

1. A client sends a SOAP request.
2. The request is wrapped in an XML structure called a SOAP Envelope.
3. The server processes the request.
4. The server returns a SOAP response in XML.

A SOAP message usually travels over HTTP or HTTPS, although SOAP can also work with SMTP or other protocols.

Structure of a SOAP Message

Every SOAP message has the same basic structure:

<soap:Envelope>

  <soap:Header>

    ... optional metadata ...

  </soap:Header>

  <soap:Body>

    ... actual request or response ...

  </soap:Body>

</soap:Envelope>

The SOAP message contains three main parts:

1. Envelope

The Envelope is the outer wrapper of the SOAP message. It tells the receiver that this is a SOAP request.

2. Header

The Header is optional. It contains extra information such as:

1. Authentication tokens
2. Security settings
3. Session IDs
4. Routing information

3. Body

The Body contains the actual data being sent.

For example, if you want to get the weather for a city, the Body may contain the city name.

What Is WSDL?

WSDL stands for Web Services Description Language.

A WSDL file describes everything about a SOAP API, including:

1. Available operations
2. Request parameters
3. Response format
4. Endpoint URL
5. Authentication requirements

You can think of WSDL as a contract between the client and the server.

For example, a WSDL file may tell you:

1. There is an operation called GetCustomer
   
2. It requires a CustomerID
   
3. It returns customer details

Because SOAP APIs are contract-based, many tools can automatically generate requests directly from the WSDL file.

Example:

<definitions>

  <service name="CustomerService">

    <port name="CustomerPort">

      <operation name="GetCustomer" />

    </port>

  </service>

</definitions>

SOAP Request vs WSDL

Many beginners get confused between SOAP and WSDL.

In simple words:

1. WSDL tells you what to send.
2. SOAP is what you actually send.

Common SOAP Terminology

Here are a few SOAP-related terms you may see:

What Is a SOAP Fault?

When something goes wrong, a SOAP API returns a SOAP Fault.

Example:

<soap:Fault>

  <faultcode>soap:Client</faultcode>

  <faultstring>Invalid Customer ID</faultstring>

</soap:Fault>

SOAP faults are useful because they provide a structured way to handle errors.

Common SOAP fault types include:

1. Client: The request is invalid.
2. Server: The server failed.
3. VersionMismatch: Wrong SOAP version.
4. MustUnderstand: Required header is missing.

SOAP Versions

There are two major versions of SOAP:

The main difference is the namespace and content type used.

SOAP 1.1:

Content-Type: text/xml

SOAP 1.2:

Content-Type: application/soap+xml

SOAP vs REST

SOAP and REST are often compared because both are used to build APIs.

REST is usually easier for beginners because it is simpler and more lightweight.

SOAP is better when:

1. You need enterprise-level security
2. The API contract must be very strict
3. The system requires guaranteed delivery
4. You are working with older enterprise systems

Advantages of SOAP APIs

SOAP APIs have several benefits:

1. Strong Security

SOAP supports advanced security standards like WS-Security, encryption, and digital signatures.

2. Formal Contract

Because of WSDL, both the client and server know exactly what to expect.

3. Reliable Messaging

SOAP can guarantee message delivery, which is important in financial and healthcare systems.

4. Platform Independent

SOAP works between systems written in Java, .NET, PHP, Python, and many other languages.

5. Better for Enterprise Workflows

SOAP is useful when multiple systems need to communicate in a very controlled way.

Disadvantages of SOAP APIs

SOAP also has some drawbacks:

1. XML Is Verbose

SOAP messages are usually much larger than JSON messages.

2. Harder to Learn

SOAP includes many concepts such as WSDL, namespaces, headers, and SOAPAction.

3. Slower Performance

Because XML is larger and more complex to parse, SOAP APIs are generally slower than REST APIs.

4. More Boilerplate

Creating a SOAP request manually takes more effort than sending a simple REST request.

How to Test a SOAP API

1. Get the SOAP service endpoint URL or WSDL URL from your API provider.
2. Open the Requestly and click the Import button. Under Choose WSDL Source, either paste your WSDL URL (e.g., https://example.com/service?wsdl) or upload a .xml / .wsdl file directly.
3. Select the appropriate SOAP version from the dropdown. You can also choose to organize the imported requests into a collection for easier management.
4. Confirm the import. Requestly will automatically parse the WSDL definition and pre-fill the request body, headers, and endpoint for each available operation — no manual setup needed.
5. If building a request manually instead, click + New, select a standard HTTP request, set the method to POST, and enter your SOAP service URL. Then go to the Body tab, select raw → XML, and paste your SOAP envelope. Make sure it includes the required Envelope and Body elements along with the correct namespaces.
6. Fill in any required parameters within the XML body, then click Send. Inspect the XML response in the response panel, or review any SOAP faults returned by the server.