API Contracts and Documentation Standards

In the world of REST APIs, contracts serve as formal agreements defining how API providers and consumers interact. A well-defined API contract is essential for successful integration, reducing development time, and ensuring reliability.

Unlike SOAP APIs, which had built-in documentation and schema validation from the beginning via WSDL (Web Services Description Language), REST APIs initially lacked standardized documentation approaches. This gap led to various proprietary solutions with inconsistent results.

Today, we have several established standards for documenting REST APIs, with OpenAPI (formerly known as Swagger) being the dominant specification. Let’s explore the evolution and characteristics of the main API contract types.

Contract Types Evolution

Swagger emerged as a solution-seeking approach to REST API documentation and is effectively a compilation of several open source ideas. The ability to automatically generate clients and servers based on complete API documentation helped it gain widespread adoption. In 2016, Swagger was renamed to OpenAPI when it became an official API standard under the Linux Foundation.

The name Swagger has survived to this day, but now only represents the tooling ecosystem and no longer the API specification standard itself. Modern developments like Scalar are building upon these foundations to provide even more interactive and user-friendly documentation experiences.

Comparison of API Contract Types

Below is a comprehensive comparison of the main API contract types used for REST APIs:

Pros and Cons of API Contract Types

OpenAPI 3.x

Pros:

• Industry standard with the widest adoption and tooling support
• Comprehensive specification capabilities for complex APIs
• Excellent for automation, testing, and code generation
• Strong support in most programming languages and platforms
• Backing from major industry organizations

Cons:

• More verbose than some alternatives
• Steeper learning curve for beginners
• YAML/JSON format can be error-prone without specialized editors
• Requires additional tooling for optimal documentation display

Swagger 2.0

Pros:

• Established ecosystem with mature tooling
• Well-understood by developers
• Good compatibility with older systems
• Simpler than OpenAPI 3.x for basic use cases

Cons:

• Legacy specification with limitations
• Lacks newer features of OpenAPI 3.x
• Less expressive for complex API scenarios
• Gradually being phased out in favor of OpenAPI 3.x

Scalar

Pros:

• Modern, highly interactive user experience
• Enhanced visualization of API documentation
• Excellent developer experience for API consumers
• Integrated authentication flow testing
• Built-in light/dark themes and customization options
• Mobile-friendly responsive design

Cons:

• Newer technology with less widespread adoption
• Relies on OpenAPI for the underlying specification
• Requires additional setup beyond basic OpenAPI
• Limited custom tooling compared to OpenAPI/Swagger

RAML (RESTful API Modeling Language)

Pros:

• Human-friendly YAML-based format
• Strong type system and reusability features
• Good for designing APIs before implementation
• Built with API-first approach in mind

Cons:

• Smaller community and ecosystem than OpenAPI
• Less integration with major cloud platforms
• Fewer code generation options
• Less standardization across the industry

API Blueprint

Pros:

• Markdown-based, making it very human-readable
• Simple to learn and start writing
• Good for smaller projects and rapid prototyping
• Documentation-first approach

Cons:

• Limited tooling compared to OpenAPI
• Less expressiveness for complex validation rules
• Smaller ecosystem and community support
• Not as suitable for large enterprise APIs

Choosing the Right Contract Type

When selecting an API contract type, consider:

1. Team expertise: Which format are your developers already familiar with?
2. Ecosystem requirements: What integrations and tools do you need?
3. Project complexity: How complex is your API and its validation rules?
4. Documentation goals: Who will consume your documentation and how?
5. Longevity: Which standard is likely to be supported long-term?

For most .NET developers working with REST APIs today, OpenAPI 3.x is the recommended standard due to its industry adoption, tooling support, and native integration with ASP.NET Core. For enhanced documentation experiences, consider Scalar as a complementary solution built on top of OpenAPI.