Application Programming Interfaces (APIs) allow different software systems to communicate with each other. There are two main architectural styles used for APIs – REST (Representational State Transfer) and SOAP (Simple Object Access Protocol). In recent years, REST has emerged as the preferred approach for building APIs, outpacing SOAP in popularity and usage. This article will explore the key differences between REST and SOAP and explain why REST APIs are generally considered superior.
What is REST?
REST is an architectural style for building distributed systems based on hypermedia. It leverages HTTP protocol and its verbs to implement create, read, update, and delete operations. REST uses URL paths to represent resources and HTTP methods like GET, POST, PUT, PATCH, and DELETE to operate on these resources.
Some key principles of REST include:
- Client-server separation – Client and server act independently
- Statelessness – No client context passed from request to request
- Cacheability – REST responses must indicate if they are cacheable
- Layered system – Client is not aware of server stack composition
- Uniform interface – Simplifies architecture using standards like HTTP, JSON, URI
- Code on demand (optional) – Ability to send executable code to client
REST uses a resource-oriented approach. Resources are manipulated using HTTP methods and identified using URIs. REST is focused on exposing data. JSON is the most common response format used with REST though XML can also be used.
What is SOAP?
SOAP is a protocol for exchanging information between systems using XML. It provides a basic messaging framework on top of transport protocols like HTTP and SMTP. The SOAP specification defines an XML format for message envelopes and encoding rules for data types.
Some key highlights of SOAP include:
- XML-based messaging protocol
- Transport protocol independent – Can leverage HTTP, SMTP, TCP, etc.
- Defines message structure and encoding rules
- Supports RPC style communication
- Advocates use of WS-* standards
SOAP focuses on exposing operations. RPC calls are made using SOAP and XML is used for request and response messages. SOAP APIs tend to be more rigid and verbose due to the XML envelope structure.
Key Differences Between REST and SOAP
Now that we have examined REST and SOAP independently, let’s explore how they differ from each other:
| Criteria | REST | SOAP |
|---|---|---|
| Architectural Style | Representational State Transfer (REST) | Service Oriented Architecture (SOA) |
| Protocol | HTTP | HTTP, SMTP, TCP, etc. |
| Message Format | JSON (Typically) | XML |
| Focus | Exposing data | Exposing operations |
| Resource Identification | URL Paths | endpoints |
| Caching | Cacheable | Not cacheable |
| Envelope | No envelope | XML envelope required |
| Strictness | Loose, flexible | Strict standard compliance |
As the table summarizes, REST and SOAP have very distinct characteristics. REST leverages simpler HTTP protocol while SOAP can use multiple transport protocols. JSON is preferred for REST while XML is native for SOAP. REST exposes resources through URL paths whereas SOAP exposes operations as service endpoints. REST has better cache support and is less strict about standards compliance compared to SOAP.
Why is REST preferred over SOAP?
Now that we have contrasted REST and SOAP, let’s examine why REST has emerged as the dominant API style.
Lightweight and Simple
REST APIs are lightweight, simple, and flexible. They leverage HTTP, JSON, URL paths and verbs to expose resources. SOAP uses XML for messages, requires more bandwidth, and introduces complexity with WSDLs and XSDs. REST is simple to understand and implement. SOAP has a steeper learning curve.
High Performance
The performance of REST APIs is better compared to SOAP. The XML parsing required by SOAP can be resource intensive and cause latency. The JSON payloads used by REST are lightweight. REST allows caching which improves performance. SOAP does not define caching mechanisms.
Scalability
REST APIs scale better because of their stateless nature. Any server can respond to requests for any client since no session info is stored locally. SOAP APIs are harder to scale as maintaining session state is more cumbersome.
Flexibility
REST offers more flexibility compared to SOAP. REST permits different data formats like JSON, YAML, XML, etc. SOAP only allows XML. REST allows the use of HTTP verbs in a flexible manner. With SOAP, all operations are invoked using POST requests.
Separation of Client and Server
REST enforces a clear separation between the client and the server. The client is not concerned with the server stack. SOAP blurs this separation by exposing the underlying logic and forcing client dependencies on WSDLs and XSDs.
Visibility and Monitoring
Since REST leverages plain HTTP, it is easy to monitor, log, and analyze REST API traffic. Existing tools can be utilized for this purpose. With SOAP, additional tools and instrumentation is required since it may use non-HTTP protocols.
Ease of Use for Web Developers
For web developers, REST APIs feel like a natural fit since all verbs and constructs map to the HTTP protocol which is intrinsically understood. SOAP feels more alien as it mandates dealing with XML, WSDLs, etc. REST has better alignment with the ethos of the web.
When is SOAP a Better Choice?
While REST may be better suited for public-facing APIs, SOAP does have some advantages in certain use cases:
- Strict validation – SOAP’s XML schemas provide strict validation
- Standardized – SOAP has standardized (WS-*) extensions for security, transactions, etc.
- ACID compliance – SOAP supports transactions natively
- Legacy systems – SOAP better interfaces with legacy systems
- Internal APIs – SOAP remains useful for internal service orchestration
Thus, for internal APIs that require strict standards compliance, SOAP may be preferable. SOAP’s support for transactions also makes it suitable for use cases demanding ACID properties.
Can REST and SOAP Co-exist?
As we have seen, both REST and SOAP have their relative strengths and weaknesses. REST is great for public APIs but SOAP can still be a valid choice for internal use cases.
Fortunately, REST and SOAP can co-exist together to serve different purposes. For example, a RESTful API can be created for exposing data to public developer channels. Internally, SOAP APIs can be used for orchestrating services that require strict contracts and ACID properties.
Microservices architectures frequently leverage this model. The API gateway exposes a REST interface while the microservices use SOAP/RPC interfaces to communicate with each other internally.
Conclusion
While SOAP ushered in the era of web services, REST has since taken over as the API style of choice. The lightweight nature, superior performance, separation of concerns and alignment with web architecture make REST preferable over SOAP in most cases.
However, SOAP still retains an advantage in specific use cases like internal APIs that demand rigorous contracts and ACID compliance. Thus, both REST and SOAP can complement each other by being leveraged for what they are best suited for.