Web services allow different software systems to communicate with each other over the internet. There are two major architectures for web services – SOAP and REST. SOAP stands for Simple Object Access Protocol and REST stands for Representational State Transfer. Both have their own advantages and disadvantages.
SOAP has been around longer, originating in the 1990s. It defines a XML-based messaging protocol for exchanging structured data between systems. REST is a newer, lightweight alternative that emerged in the 2000s. It relies on simple HTTP methods like GET, POST, PUT and DELETE to access resources.
So when should you use SOAP vs REST? Here’s a quick overview of how they compare:
SOAP
- Defines XML-based messaging protocol
- Strict contract between client and server
- Supports ACID transactions
- More overhead (XML, WS-Security)
- Built-in error handling
- More rigid structure and governance
REST
- Uses simple HTTP methods to access resources
- Loose coupling between client and server
- Stateless and cacheable
- Lightweight, less overhead
- Flexible, less governance
- Client handles error checking
Let’s dig deeper into the key differences and use cases for each architecture.
SOAP Web Services
SOAP stands for Simple Object Access Protocol. It is a messaging protocol that allows programs built in various languages to communicate with each other.
The SOAP specification defines an XML format for sending messages between systems. These SOAP messages consist of three parts:
- Envelope – root container for the message
- Header – optional metadata
- Body – XML payload with request/response
SOAP relies onapplication layer protocols, most commonly HTTP, to transfer messages. The XML syntax provides a standardized packaging format that is programming language and platform independent.
Here are some key characteristics of SOAP web services:
Strict Contract
SOAP uses WSDL (Web Services Description Language) to define a formal contract describing the interface between the client and server. This includes:
- Endpoints
- Operations
- Input/output parameters
The WS-Security standard also defines options for adding security to SOAP messages such as encryption and digital signatures.
Having a strict contract allows the client and server to be tightly coupled. The interface is clearly defined, making integration easier. However, changes require updating the WSDL contract.
Built-in Error Handling
SOAP includes built-in mechanisms for handling errors through SOAP fault messages. If an error occurs, the server returns a SOAP fault rather than the expected output. The client application can then act on this fault message appropriately.
Supports ACID Transactions
SOAP enables transactions through features like WS-Coordination and WS-AtomicTransactions. ACID stands for Atomicity, Consistency, Isolation and Durability – properties that guarantee database transactions are processed reliably.
SOAP transactions provide “all-or-nothing” execution to ensure database integrity. Either the entire transaction succeeds or fails.
XML Messaging Format
SOAP defines an XML format for encapsulating messages. XML provides a standardized way to encode data that can be read by any application. It adds overhead compared to simpler JSON text, but enables rich features like namespaces, extensions, validation and tooling support.
The XML format allows SOAP to be platform and language agnostic. Clients and servers can be implemented in any preferred language.
Transport Independence
SOAP messaging is independent of the underlying transport protocol. While HTTP is most commonly used, it can also be paired with other protocols like SMTP, JMS and MQ. This provides flexibility to use different protocols in different scenarios.
When to Use SOAP
Here are some common use cases where SOAP excels:
- Enterprise integration – SOAP is widely adopted for connecting enterprise systems due to its robustness and security.
- Transactions – ACID transactions require SOAP to coordinate state across multiple systems.
- Formal contracts – SOAP is ideal when service interfaces need to be strictly defined.
- Legacy systems – Integrating with older technologies often requires SOAP payloads.
SOAP is well-suited for systems that need tight coupling, transactional integrity, security features and formal contracts.
REST Web Services
REST (Representational State Transfer) is an architectural style for building distributed systems based on hypermedia. It relies on standard HTTP methods and status codes to access resources rather than a custom messaging format.
Here are some key principles of REST:
Client-Server Architecture
REST systems have a separation of concerns between a client and server. Clients are not concerned with data storage and can focus on UI, while servers handle data access and processing. This improves portability across platforms.
Statelessness
REST interactions are stateless between requests. HTTP is an inherently stateless protocol – each request contains all information needed to process it without relying on previous interactions. This improves scalability and reliability.
Cacheability
REST responses define themselves as cacheable or non-cacheable. Web caches can reuse cached responses to improve performance. Responses state if they should be cached to prevent clients using stale data.
Uniform Interface
REST defines four interface constraints – identification of resources, manipulation through representations, self-descriptive messages, and use of hypermedia. This applies generic architectural principles rather than mandating a specific interface definition.
Layered System
REST allows you to use a layered system architecture to place load balancers, firewalls, caching and other components between clients and servers. This improves scalability, security and performance.
HTTP Methods
REST leverages standard HTTP methods to perform operations on resources:
- GET – Read a resource
- POST – Create a resource
- PUT – Update/replace a resource
- DELETE – Delete a resource
This provides a simple, unified interface for accessing resources.
When to Use REST
Here are some common use cases where REST excels:
- Public APIs – REST is commonly used for public APIs due to its simplicity and scalability.
- Web services – REST’s use of HTTP makes it a natural fit for web services.
- Mobile applications – REST Lightweight, cacheable responses work well for mobile.
- API gateways – REST is easy to integrate with API gateways like Kong and Apigee.
REST is well-suited for systems with looser coupling, that need to scale, have variable payloads and connect over the internet.
Comparing SOAP and REST
Now that we’ve covered SOAP and REST independently, let’s compare them across a few categories:
Contract
SOAP defines a formal contract using WSDL. REST has no formal contract – just an exchange of resources using URLs, headers and HTTP methods.
SOAP contracts allow official governance of interface changes. With REST, changes can happen without coordination since there’s no formal contract.
Data Format
SOAP only allows XML payloads. REST can use simpler formats like JSON or plain text. This makes REST more lightweight.
However, XML provides better tooling support for features like validation and namespaces. JSON parsers are available for most platforms today.
| Property | SOAP | REST |
|---|---|---|
| Contract | Formal (WSDL) | No formal contract |
| Data Format | XML only | XML, JSON, plain text |
| Caching | Limited support | Built-in support |
| Transactions | Supported (ACID) | Not natively supported |
Caching
REST supports built-in HTTP caching mechanisms. Responses explicitly state cache policies. SOAP has limited support for caching through protocols like WS-Transfer.
Transactions
SOAP supports ACID transactions through protocols like WS-Coordination. REST interactions are independent requests with no native transactional support.
Security
SOAP includes built-in support for security through WS-Security.
REST uses standard HTTP security like SSL, OAuth and API keys. Third-party API gateways add enhanced security.
Tooling
SOAP tooling like proxies and code generation is well-established.
REST relies more on standarn HTTP tooling. API modeling tools like Swagger help design and document REST APIs.
Performance
SOAP’s XML messages require more bandwidth and parsing. REST with JSON is more lightweight.
However, SOAP is faster once the XML parsing overhead is complete. It has lower latency due to connection reuse.
Here is a summary comparison:
| SOAP | REST | |
|---|---|---|
| Message Size | Large XML | Light JSON |
| Parse Time | High XML overhead | Minimal JSON parsing |
| Latency | Low due to connection reuse | Slightly higher |
| Caching | Limited support | Built-in support |
In terms of pure speed, SOAP has lower latency but REST has lower overall time due to simple messaging. Caching improves REST performance.
Conclusion
In summary:
- SOAP is a robust protocol for enterprise integration requiring transactional integrity, ACID compliance and formal contracts.
- REST provides a simpler, lightweight architecture focused on performance, scalability and web use cases.
- There are still valid use cases for both approaches today.
- Understand your specific integration needs to choose the right approach.
When connecting systems over the public internet, REST is typically the right choice due to its focus on scalability and web technologies. For private enterprise integration where strict governance is required, SOAP is well-suited.
As a rule of thumb, public APIs are predominantly REST while internal enterprise systems often use SOAP. However, there are always exceptions, and many organizations use a mix of both approaches.
This article provided a high-level overview of how SOAP and REST compare. Both technologies continue to evolve – for example, REST extensions provide some transactional capabilities. The right choice depends on analyzing the specific requirements around contract needs, data formats, security policies and other integration goals.