TABLE OF CONTENTS
3. Main Components of SOAP API
5. WSDL (Web Services Description Language)
9. SOAP API vs REST API (Optional Comparative Section)
12. Types of SOAP API Bindings
14. Common SOAP API Request Methods
15. Limitations of SOAP API's Compared to REST API's
1. Overview
This article provides a clear overview of SOAP (Simple Object Access Protocol) APIs, including their features, components, and how they work. SOAP is a protocol designed for exchanging structured information in web services. It enables communication between applications over the internet using XML-based messaging.
Note: To perform API calls using SOAP, the request and data must be in XML format. Make sure to select XML in the body section and choose XML from the format dropdown menu.
2. Key Features of SOAP API
- Protocol-Based: Utilizes XML for structuring request and response messages.
- Transport Independent: Can operate over various protocols such as HTTP, SMTP, and TCP (including custom low-level implementations, though TCP is less common for web APIs).
- Strict Standards: Employs WSDL (Web Services Description Language) to describe available services and their operations.
- Platform & Language Independent: Supports interoperability across different programming languages and platforms.
- Built-in Error Handling: Provides standardized error handling mechanisms within the protocol.
3. Main Components of SOAP API
- Envelope: Defines the start and end of the SOAP message. It is the root element that encapsulates the entire message structure.
- Header (Optional): Contains metadata such as authentication, transaction handling, or message routing information.
- Body: Holds the actual message content or data intended for the recipient.
- Fault: Provides information about errors that occurred during the processing of the message.
- XSD (XML Schema Definition):Describes the structure and constraints of XML documents used in SOAP messages. Ensures that the XML content adheres to predefined formats and rules.
4. How SOAP API Works
- Client Request: The client sends an XML-formatted request to the server over a transport protocol (typically HTTP).
- Server Response: The server processes the request and returns an XML-formatted response.
- WSDL (Web Services Description Language): Defines the available operations and the structure of request and response messages, ensuring both client and server understand the communication format.
5. WSDL (Web Services Description Language)
- Purpose: Defines the interface of a SOAP web service, allowing clients to understand how to interact with it.
- What It Describes:
- Available operations (functions or methods)
- Data types used in messages
- Structure of request and response messages
- Communication protocols and endpoints
- Key Elements:
- <definitions> – Root element of the WSDL document
- <types> – Defines data types used by the web service
- <message> – Defines the messages used in the communication
- <portType> – Describes the operations provided by the service
- <binding> – Specifies the protocol and data format for each operation
- <service> – Specifies the endpoint (URL) of the web service
6. Security in SOAP API
- WS-Security Standard: SOAP uses the WS-Security standard to ensure message-level security.
- Security Features:
- Message encryption
- Digital signatures
- Username tokens for authentication
- Timestamping and security tokens
- Transport-Level Security: Can be combined with HTTPS to provide an additional layer of transport-level security.
7. Advantages of SOAP API
- High Security & Reliability: Built-in standards like WS-Security ensure secure and reliable message delivery.
- Standardized & Extensible: Based on well-defined protocols and easily extended to support additional features.
- Supports Complex Operations: Ideal for handling complex, multi-step transactions and operations.
- Enterprise-Ready: Well-suited for enterprise-level applications requiring strict standards and robust functionality.
8. Disadvantages of SOAP API
- More Complex than REST: Requires strict standards and additional setup, making it harder to implement.
- Slower Performance: XML-based messaging adds overhead, making communication more verbose and slower compared to lightweight formats like JSON.
- Harder to Test and Debug: XML structure and strict schema validation can complicate testing and troubleshooting.
- Tightly Coupled to WSDL: Strong dependency on WSDL makes changes to the service interface more complex.
9. SOAP API vs REST API (Optional Comparative Section)
| Feature | SOAP | REST |
| Protocol | Strict | Flexible |
| Data Format | XML only | JSON, XML, etc. |
| Performance | Slower | Faster |
| Use Case | Enterprise apps | Web/mobile apps |
10. Use Cases
- Banking and Financial Services: Requires high security, reliability, and transactional support.
- Telecommunication: Supports complex operations and standardized messaging across systems.
- Enterprise Applications: Ideal for large-scale, mission-critical systems with strict compliance requirements.
- Legacy System Integration: Works well with older systems that rely on SOAP-based protocols.
11. Tools for SOAP API
- SOAP UI: A dedicated tool for testing and debugging SOAP web services.
- Postman: Primarily used for REST APIs, but also supports sending and testing SOAP requests.
- WSDL Generator: Tools or frameworks used to automatically generate WSDL files from service definitions.
- Java Libraries:
- Apache Axis
- Apache CXF
- JAX-WS (Java API for XML Web Services)
- These libraries are commonly used for building and consuming SOAP services in Java applications.
12. Types of SOAP API Bindings
SOAP can use different bindings (protocols) for transport:
| Binding Type | Description |
| SOAP over HTTP | Most commonly used. |
| SOAP over HTTPS | Adds encryption and security. |
| SOAP over SMTP | Used for sending SOAP messages via email (rare). |
| SOAP over TCP | Used in closed networks for efficiency. |
13. SOAP Action
SOAPAction HTTP Header: Defines the intent of the SOAP request.
Helps the server determine which operation to invoke.
Example: SOAPAction: http://example.com/GetUserDetails
14. Common SOAP API Request Methods
| HTTP Method | Usage |
| POST | Most SOAP requests use POST with an XML body. |
| GET | Can be used to fetch WSDL or simple queries (less common). |
| OPTIONS / HEAD | Used for metadata or service discovery (rare). |
15. Limitations of SOAP API's Compared to REST API's
Even though SOAP is powerful and secure, it has several limitations when compared to REST APIs, especially for modern web and mobile applications.
| Aspect | SOAP Limitation | How REST is Better |
| Data Format | SOAP uses only XML (verbose and slower to parse). | REST supports JSON, XML, HTML, plain text, etc., making it lightweight and faster. |
| Message Size | SOAP messages are large and complex due to XML and envelope structure. | REST messages are smaller and transmit data efficiently. |
| Performance | Processing XML and following strict schemas makes SOAP slower. | REST uses lightweight JSON and stateless communication, making it faster. |
| Complexity | Requires WSDL, strict structure, and namespaces. | REST is simple — uses URLs and HTTP methods (GET, POST, PUT, DELETE). |
| Browser Compatibility | SOAP is not natively supported by browsers. | REST can be easily used in browsers with AJAX / Fetch API. |
| Caching | SOAP does not support caching easily. | REST supports HTTP caching (GET requests can be cached). |
| Ease of Use | Difficult to test or debug due to XML structure and WSDL dependency. | REST can be tested easily in Postman, curl, or directly from browser. |
| Flexibility | Tightly coupled with the service contract (WSDL). | REST is loosely coupled — any change in endpoint or data doesn’t require schema regeneration. |
| Error Handling | SOAP errors use <Fault> elements which are verbose and less intuitive. | REST uses standard HTTP status codes (200, 400, 404, 500, etc.). |
| Overhead | Extra XML tags and namespace declarations cause more bandwidth usage. | REST’s lightweight JSON reduces network overhead. |
16. Execution Level
- Environment variables are supported.
- To store a variable, you can use either Data::variable or simply variable to assign the value.
- Data generation is also supported.
- Data set is supported.
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