SOAP vs. REST: What API Testers and Developers Need to Know

When designing or working with APIs, developers often face a common question: Should I use SOAP or REST? SOAP and REST are two of the most common methods for building web APIs. While both enable communication between systems, they differ significantly in structure, behavior, and how they fit into modern software architecture.

This guide outlines how each approach works, when each is most appropriate, and how to evaluate them based on your project's goals.

What is the difference between SOAP and REST APIs?

Let's start with the basics. Both SOAP and REST are approaches to accessing web services, but they follow fundamentally different methods for how systems communicate.

• SOAP (Simple Object Access Protocol) is a protocol with strict standards and structure. It relies on XML for message formatting and is heavily used in enterprise environments where security, reliability, and transactional integrity are key.
• REST (Representational State Transfer) is an architectural style. RESTful APIs leverage standard HTTP methods and typically use lightweight message formats like JSON, making them easier to develop and faster to process.

While SOAP is older and standardized, REST is modern, flexible, and dominant in public APIs today.

Understanding SOAP APIs

How SOAP works

SOAP is a protocol that provides a highly structured way to exchange information between systems. Every SOAP message is formatted in XML and wrapped in a standardized envelope that includes both the request details and any required metadata. Communication is defined by a WSDL, or Web Services Description Language, which acts as a contract between sender and receiver.

Although SOAP is often used over HTTP, it can also operate on other network protocols such as SMTP or TCP. This transport flexibility allows it to fit into a variety of infrastructure environments.

Key features of SOAP

• XML messaging: Every message must be formatted in XML, which allows for strict validation and typing
• Contract-based: Requires a WSDL file that defines all operations, inputs, and outputs
• Transport agnostic: Can run over HTTP, SMTP, TCP, and others
• Built-in capabilities: Includes support for security (WS-Security), transaction handling, and error reporting

Common use cases for SOAP

SOAP is typically used in environments where data accuracy, integrity, and security are critical. Examples include:

• Financial services that require guaranteed message delivery
• Healthcare systems that must meet compliance and validation standards
• Enterprise service buses that integrate multiple internal systems
• Any system that relies on formal interfaces and long-term contract stability

Understanding REST APIs

How REST works

REST is a design style built on top of standard web protocols. RESTful APIs expose data as resources, each identified by a unique URL. Interactions with those resources happen through HTTP methods such as GET, POST, PUT, and DELETE. Unlike SOAP, REST does not require a contract file or enforce strict message structures.

REST typically uses JSON to format its messages, although XML or other formats can also be used. Because REST is stateless, each request must include all the information necessary to complete the action as no session memory is retained between calls.

Key features of REST

• HTTP-driven: Uses common methods (GET, POST, PUT, DELETE) to manipulate resources
• Lightweight format: JSON is the most common format, making messages smaller and easier to process
• Stateless communication: Each request is self-contained and independent
• Flexible structure: No required contract or schema, allowing rapid iteration

Common use cases for REST

REST is widely used in applications that prioritize speed, flexibility, and scalability. It is especially common in:

• Public APIs that need to be easy to adopt (e.g., Twitter, GitHub, Stripe)
• Mobile applications that require fast, efficient communication
• Web-based applications with evolving requirements
• Cloud-native platforms and microservices architectures

SOAP vs REST: side-by-side comparison

The table below summarizes the key differences between SOAP and REST:

How to choose between SOAP and REST APIs

Choosing the right approach depends on the nature of the API, who will use it, and how critical reliability and consistency are to its function.

SOAP is a better choice when:

• The API must handle sensitive transactions, such as in banking or insurance
• Compliance requirements dictate the use of XML and schema validation
• The system needs to support complex workflows with built-in reliability and rollback

REST is typically more appropriate when:

• The API is intended for widespread use, including by third-party developers
• Speed, simplicity, and responsiveness are priorities
• The development cycle requires flexibility and frequent changes
• The client is a mobile or web application with limited processing resources

Other considerations include the technical skill set of the team, the level of control over both ends of the API, and existing infrastructure. Organizations may even use both SOAP and REST in different layers of the same system depending on purpose and audience.

Summary takeaways for technical teams

SOAP and REST solve the same problem, enabling communication between systems, but offer different strengths. SOAP is designed for environments where structure, reliability, and contract enforcement are essential. REST is optimized for flexibility, performance, and modern application development.

Selecting the right design style requires balancing business priorities with technical constraints. For stable, high-assurance integrations, SOAP offers robust guarantees. For fast-moving development and broad compatibility, REST is often the better fit.

By understanding both models, teams can make informed decisions that lead to APIs that are easier to maintain, better aligned with user expectations, and more adaptable to future needs.