The OpenAPI Specification, originally known as the Swagger Specification, is a specification for machine-readable interface files for describing, producing, consuming, and visualizing RESTful web services. Originally part of the Swagger framework, it became a separate project in 2016, overseen by the OpenAPI Initiative, an open-source collaboration project of the Linux Foundation. Swagger and some other tools can generate code, documentation and test cases given an interface file.
Swagger development began in early 2010 by Tony Tam, who was working at online dictionary company Wordnik. In March 2015, SmartBear Software acquired the open-source Swagger API specification from Reverb Technologies, Wordnik's parent company.
In November 2015, SmartBear announced that it was creating a new organization called the OpenAPI Initiative under the sponsorship of the Linux Foundation. Other founding member companies included 3Scale, Apigee, Capital One, Google, IBM, Intuit, Microsoft, PayPal, and Restlet. SmartBear donated the Swagger specification to the new group. RAML and API Blueprint were also under consideration by the group.
In September 2016, the API World conference presented an API Infrastructure award to SmartBear for its ongoing work on Swagger.
In July 2017, the OpenAPI Initiative released version 3.0.0 of its specification. MuleSoft, the main contributor to the alternative RESTful API Modeling Language (RAML), joined the OAS and open-sourced their API Modeling Framework tool, which can generate OAS documents from RAML input.
|3.0.3||2020-02-20||Patch release of the OpenAPI Specification 3.0.3|
|3.0.2||2018-10-08||Patch release of the OpenAPI Specification 3.0.2|
|3.0.1||2017-12-06||Patch release of the OpenAPI Specification 3.0.1|
|3.0.0||2017-07-26||Release of the OpenAPI Specification 3.0.0|
|2.0||2014-09-08||Release of Swagger 2.0|
|1.2||2014-03-14||Initial release of the formal document|
|1.1||2012-08-22||Release of Swagger 1.1|
|1.0||2011-08-10||First release of the Swagger Specification|
Applications implemented based on OpenAPI interface files can automatically generate documentation of methods, parameters and models. This helps keep the documentation, client libraries, and source code in sync.
The OpenAPI Specification is language-agnostic. With OpenAPI's declarative resource specification, clients can understand and consume services without knowledge of server implementation or access to the server code.
Tools that work with OpenAPI
The OpenAPI Initiative maintains a list of implementations for version 3.0 of the specification. SmartBear still brands its OpenAPI tools with the Swagger moniker. The Swagger UI framework allows both developers and non-developers to interact with the API in a sandbox UI that gives insight into how the API responds to parameters and options. Swagger can handle both JSON and XML.
Swagger Codegen contains a template-driven engine to generate documentation, API clients and server stubs in different languages by parsing the OpenAPI definition. In July, 2018, William Cheng, the top contributor to Swagger Codegen, and over 40 other contributors to Swagger Codegen forked the code into a project named OpenAPI Generator under the OpenAPI Tools organization. 
- Representational State Transfer
- Overview of RESTful API Description Languages including RAML, WADL, WSDL, and others.
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- OpenAPI Initiative (OAI) website
- Swagger website
- OpenAPI Specification on GitHub
- Directory of OpenAPI definitions
- OpenAPI Editor: A rich UI Eclipse OpenAPI (OAS) editor and studio to design, develop and test OAS3/OpenAPI
- OpenAPI for Electronic Data Interchange (EDI)
- OpenAPI editor by Remain API Studio. A rich OpenAPI editor and a full OAS3 specs-compliant development environment