Application Programming Interfaces (APIs) have become an integral part of software development. They allow different applications to communicate with each other by exposing data and functionality. There are two main architectural styles used for web APIs – REST (Representational State Transfer) and SOAP (Simple Object Access Protocol). REST has gained immense popularity over the last decade and is now the preferred choice for public web APIs. But why is REST more popular than SOAP? Let’s explore some of the key differences between the two architectures.
What is REST?
REST stands for Representational State Transfer. It relies on a stateless client-server protocol (typically HTTP) where each request contains all the information required by the server to carry out the operation. The server does not store any state about the client session. REST uses HTTP methods explicitly to define the requested operation e.g. GET, POST, PUT, DELETE. Data is represented in easy to understand formats like JSON and XML.
Some key principles of REST APIs:
- Statelessness – No client session data is stored on the server
- Cacheability – REST responses should indicate if they are cacheable
- Client-Server separation – Client app and server app evolve independently
- Uniform interface – All components look the same from outside
What is SOAP?
SOAP stands for Simple Object Access Protocol. It relies on XML messaging over HTTP to make requests and receive responses. SOAP defines a rigid messaging framework specifying the message structure, error handling, security etc.
Some key highlights of SOAP APIs:
- SOAP is a protocol. REST is an architectural style.
- SOAP uses XML for messaging. REST uses plain XML or JSON.
- SOAP requires more bandwidth and resource than REST.
- SOAP cannot make use of HTTP caching features. REST supports caching.
- SOAP supports ACID (transactions, validation) better than REST.
Why is REST more popular?
There are several factors that have contributed to the immense popularity and adoption of REST APIs over SOAP APIs, especially for public facing web APIs. Let’s go over the key ones:
Simplicity
REST has a very simple and lightweight architecture. There is no strict specification like SOAP so it provides a lot of implementation flexibility. The resources are accessed using simple URIs and common HTTP methods are used to manipulate them. SOAP on the other hand requires method parameters and return types to be strictly defined in the WSDL contract.
Easy to use and test
REST APIs are easy to understand, use and test. They can be tested using a simple tool like Postman or cURL. No need to deal with complex XML schemas and SOAP envelopes. JSON is concise and easy for humans and machines to quickly parse and understand. The simplicity also results in faster development times.
Scalability
The stateless nature of REST makes it very scalable for internet level loads. The server does not have to maintain any client state between requests. SOAP web services have to maintain state as part of the protocol itself. SOAP also tends to be more resource intensive due to XML messaging.
Performance
REST has better performance since it uses lighter weight protocols and requires less bandwidth. SOAP has significant overhead due to XML serialization and deserialization. REST relies on plain old XML or JSON for data representation. JSON is very lightweight especially for mobile internet usage.
Caching
REST leverages HTTP caching features through headers like ETags, Cache-Control etc. This improves performance and reduces calls to the backend server. SOAP does not include caching semantics and requires requests to always hit the backend server.
Slim implementation
A REST API may only expose a dozen URIs and methods for the full API functionality. SOAP web services require a lot more code to handle envelopes, headers, namespaces, faults etc. This simplification results in smaller codebase and lean implementations.
When to use SOAP?
While REST may be a better choice for public API design, SOAP still has its merits in certain use cases:
- Internal enterprise systems within firewall (intranet)
- operations that require ACID compliancy (transactions, validation)
- formal contracts required between systems
- legacy systems designed around SOAP
- stateless nature of REST may not fit the problem space
So for internal enterprise apps where advanced messaging features may be required, SOAP could be a better fit. But for public internet APIs consumed by a wide range of clients, REST clearly is the preferred choice.
Conclusion
REST has gained popularity over SOAP due to its simplicity, lightweight nature, performance, scalability and seamless integration with HTTP features. It has become the de-facto standard for building public web APIs that need to be consumed by a diverse set of clients. However, SOAP still suits certain use cases where formal contracts and ACID compliancy is required. For public facing APIs, REST dominates due to its flexibility, ubiquity and performance.