OAS2

OAS2, or OpenAPI Specification version 2, is a widely adopted standard for describing RESTful APIs in a machine-readable format. Originally known as the Swagger Specification, it was developed to improve API documentation and facilitate better communication between API developers and consumers. The specification provides a structured way to define endpoints, parameters, request/response models, and authentication mechanisms, enabling tools to automatically generate client libraries, server stubs, and interactive documentation.

The OAS2 specification is defined using a JSON or YAML schema, allowing developers to describe APIs in a consistent and human-readable manner. Key components include paths, operations (GET, POST, etc.), parameters, schemas for request/response bodies, and security definitions. It also supports versioning, enabling developers to manage API changes systematically. Tools like Swagger UI leverage OAS2 files to generate interactive API documentation, while client libraries can be auto-generated from the same specification.

Adoption of OAS2 has been widespread due to its simplicity and compatibility with existing workflows. However, it has been succeeded by OpenAPI Specification version 3.0 (OAS3), which introduces improvements such as better support for asynchronous APIs, improved security definitions, and enhanced data type handling. Despite this, OAS2 remains relevant for legacy systems and projects that rely on its established tooling ecosystem.

The specification is maintained by the OpenAPI Initiative, a Linux Foundation project aimed at standardizing API descriptions. While OAS3 is now the preferred version, OAS2 continues to serve as a foundational reference for API documentation best practices.