AsyncAPI and OpenAPI are different ways of defining application programming interfaces (APIs), also commonly known as API specs.
Both API specs serve a crucial role in defining and governing distributed computing systems, but AsyncAPI and OpenAPI are used for different styles of integration. Application programming interfaces, or simply “APIs”, are a key part of modern programming that make it easier to exchange information between applications. APIs tell you what information an application expects to receive, and what information an application sends, without needing to know what the internal details of an application. As long as the API doesn’t change, different teams can invoke the application’s functionality without worrying about the messy details underneath. Because of this, separate teams can work independently on implementations.
APIs have been around for a while. For instance, the painful Simple Object Access Protocol (SOAP) used APIs in the early 2000s, but they really started getting interesting when representational state transfer (REST) came along. REST, which used the ubiquitous HTTP protocol, was lightweight and fun to work with. Did REST solve all the problems of SOAP by being lightweight, fun, and easy? Not quite, because it was a little too lightweight in some cases. There was no great way to tell people in your company or a partner what needed to be in REST requests, and what they could expect as a reply. Whether the request needed a PUT or a POST was a constant source of confusion, and without a standard way of describing REST APIs, you had to resort to emailing.
Something needed to happen. Swagger and WADL to the rescue! The goal of these competing standards was to standardize a way of documenting what a REST API looked like that was both readable by humans (somewhat, anyway) and could programmatically validate incoming requests and generate code.