Application Programming Interfaces (APIs) allow different software systems to communicate with each other. There are two main architectural styles used for APIs – SOAP and REST.
What is SOAP?
SOAP stands for Simple Object Access Protocol. It is a protocol specification for exchanging structured information between systems over HTTP. SOAP uses XML for its message format and relies on remote procedure calls (RPC) to make requests between systems.
Some key characteristics of SOAP APIs:
- Uses XML for messaging format
- Follows strict SOAP standards and specifications
- Uses WSDL (Web Services Description Language) to describe API interface
- Supports RPC-style invocation through SOAP interface
- Can be used with a variety of transport protocols like HTTP, SMTP, TCP, etc.
SOAP defines a standard communication protocol specification for web services interfaces. It has built-in mechanisms for security, XML data types, etc. SOAP APIs require more bandwidth and resources than REST due to the overhead of XML messages. They have excellent support for transactions and ACID (Atomicity, Consistency, Isolation, Durability) properties in distributed systems.
Example SOAP Request
POST /InStock HTTP/1.1
Host: www.example.org
Content-Type: application/soap+xml; charset=utf-8
Content-Length: 299
<?xml version="1.0"?>
<soap12:Envelope xmlns:soap12="http://www.w3.org/2003/05/soap-envelope">
<soap12:Header>
</soap12:Header>
<soap12:Body>
<m:GetStockPrice xmlns:m="http://www.example.org">
<m:StockName>IBM</m:StockName>
</m:GetStockPrice>
</soap12:Body>
</soap12:Envelope>
What is REST API?
REST (Representational State Transfer) is an architectural style for building distributed web services APIs. REST APIs use HTTP protocol for communication between systems.
Key characteristics of REST APIs:
- Uses HTTP methods like GET, POST, PUT, DELETE for CRUD operations
- Stateless – no client session data stored on server
- Cacheable for enhancing performance
- Uniform interface between components
- Use JSON or XML for message formatting
REST APIs follow a resource-oriented architecture where each component is a resource that can be referenced with a global identifier (URI). REST usesHTTP status codes to indicate API response status and errors.
REST APIs allow building scalable web services that can be consumed by a wide range of clients. They are lightweight and more suitable for internet-scale applications. REST APIs are easy to understand, test and implement.
Example REST API Request
GET /stocks/IBM HTTP/1.1
Host: www.example.org
Accept: application/json
HTTP/1.1 200 OK
Content-Type: application/json
{
"stock" : "IBM",
"price" : 121.19
}
Key Differences between SOAP and REST APIs
Here are some key differences between SOAP and REST APIs:
| Parameter | SOAP | REST |
|---|---|---|
| Architectural Style | Procedure-oriented | Resource-oriented |
| Protocol | SOAP | HTTP |
| Message Format | XML | JSON, XML |
| Caching | Not cacheable | Cacheable |
| Statefulness | Stateful | Stateless |
| Bandwidth Usage | High | Low |
Some key points on the differences:
- SOAP is procedure-oriented, while REST focuses on resources
- SOAP uses XML for messaging while REST can use lighter JSON
- SOAP isn’t cacheable while REST API responses can be cached
- SOAP maintains state whereas REST APIs are stateless
SOAP API Advantages
Here are some of the key advantages of using SOAP APIs:
- Strict standards and specifications – SOAP is based on a strongly-defined standards and specifications. This makes integration less ambiguous.
- Language, platform, and transport independent – SOAP allows for communication between different languages, platforms and network protocols.
- Built-in error handling – SOAP has built-in error handling through SOAP fault elements in the response.
- Extensive security capabilities – SOAP defines security protocols like WS-Security for end-to-end security.
- Support for transactions – SOAP enables managing distributed transactions through protocols like WS-Coordination.
Due to these advantages, SOAP excels in enterprise scenarios that require tight integration, security, transactions and standards-based messaging. However, the additional capabilities come at the cost of more complexity compared to REST.
REST API Advantages
Here are some of the key advantages of using REST APIs:
- Lightweight and fast – No extensive processing required, which improves speed and efficiency.
- Scalability – Stateless nature of REST improves scalability across distributed systems.
- Caching – REST responses can be cached to improve performance.
- Loose coupling – Reduced dependency between client/server improve maintainability.
- Ease of testing – REST interfaces are easier to test as calls are made over standard HTTP.
Due to these advantages, REST works very well for public-facing APIs and web services exposed to a wide range of consumers. The simplicity and scalability of REST make it the go-to choice for web APIs.
When to use SOAP or REST?
The choice between a SOAP API and REST API depends on the specific use case and requirements of an application. Here are some general guidelines on when to use which type of API:
- Use SOAP if you need end-to-end security features
- Use SOAP if you require formal contracts between systems
- Use SOAP if you need transactions and coordination protocols
- Use SOAP for legacy enterprise integration requiring tight coupling
- Use REST for public-facing APIs requiring scalability
- Use REST if you need to implement caching for performance
- Use REST for web APIs consumed by a wide range of devices
- Use REST for simpler distributed architectures
Many modern web APIs prefer REST architecture for its scalability. However, SOAP suits extremely formal inter-system communication well. The maturity of an enterprise architecture often influences the choice too.
Can SOAP and REST work together?
Yes, SOAP and REST can co-exist and work together in an enterprise architecture. A microservices-based landscape allows flexible intermixing of SOAP and REST APIs.
For example, internal microservices communicating sensitive data might use SOAP APIs. At the same time, public-facing APIs can be exposed over REST interfaces. Orchestration engines like Apache Camel can route messages between different styles of APIs.
Some common integration patterns include:
- Exposing a SOAP-based web service as a REST API using an integration layer
- Consuming a REST API as a SOAP web service using a transformer
- Managing communication between SOAP and REST APIs using an Enterprise Service Bus (ESB)
A well-designed enterprise architecture allows leveraging the strengths of both SOAP and REST within a single ecosystem. SOAP provides strong standards-based integration while REST enables scalable internet-friendly services.
Conclusion
SOAP and REST represent two different architectural approaches to designing web APIs and web services interfaces.
SOAP is a robust protocol for enterprise systems integration. It has extensive capabilities like security, transactions, etc. But SOAP is relatively more complex.
REST provides a simpler, lightweight and scalable style for building web APIs. It shines for public internet-based services.
Most modern web APIs adopt a REST approach for its simplicity and scalable nature. However, SOAP still suits complex enterprise integration scenarios that require formal contracts and security.
The growth of microservices and enterprise integration patterns allows SOAP and REST to co-exist and complement each other. The choice depends on the specific use case, requirements and maturity of the systems involved.