OpenAPI: One Standard – Many Possibilities

Well-documented application programming interfaces (often just called API) are a prerequisite for developing programs or applications. The same applies to applications that are called up and used via a web browser, for example.  In this BloQ article, we would like to take a closer look at a standard for describing APIs: the OpenAPI specification.

In order for organizations to operate efficiently, it’s not just the people and departments that need to work well together. In most cases, organizations use a variety of different software solutions to help them optimally map their own processes. In principle, this approach makes a lot of sense; after all, you always want to use the best, specialized software for your particular department. However, it is also clear that these software solutions must harmonize with each other in order to ensure a uniform database without redundancies as well as flawless processes. In the quality world, too, companies are not infrequently confronted with this challenge. It is then the task of the software provider to ensure a connection to enterprise software in the form of a data interface.

However, such data interfaces should not be confused with programming interfaces (APIs), which are the subject of this BloQ article. Developers of software programs and applications depend on these APIs, which are provided by software systems. But that’s not the end of the story: We all encounter such programming interfaces in our everyday lives – without being immediately aware of them. For example, when we work with our computer and want to retrieve information or execute functions, APIs ensure that the system understands and fulfills our request. They basically act as intermediaries between us, the users, and the information we need.

APIs are used in a wide range of application scenarios; if we are working in a web environment, for example on online stores, they are used in the form of so-called web APIs. Examples of the use of web APIs include the integration of various payment options, rating systems and other services. But what role does the OpenAPI specification play exactly?

“The OpenAPI Specification (OAS) defines a standard, programming language-agnostic interface description for HTTP APIs, which allows both humans and computers to discover and understand the capabilities of a service […]. Similar to what interface descriptions have done for lower-level programming, the OpenAPI Specification removes guesswork in calling a service.” (On the OpenAPI specification v. 3.1.0) What is first described in a rather complicated way in this quote is intended to do just the opposite: the OpenAPI specification provides a standard, an open and vendor-neutral description format through which (especially REST-compliant, more on that later) APIs can be described, tested and documented.

A brief history detour: The OpenAPI specification was once part of the Swagger software project, but then became an independent project in 2016, managed by the OpenAPI Initiative. Well-known companies such as Google, Microsoft and SAP are members of the initiative. In the meantime, the standard is in version 3.1.0.

A challenge in the development of an API is often to get the implementation, documentation as well as testing under one roof – without having to accept longer processing times. OpenAPI makes it possible to describe a programming interface in a uniform way. Among other things, this creates more efficient collaboration between back-end and front-end developers, who can develop, implement and test APIs independently of each other, since code components are created by both teams.

In addition, the development and documentation of an API is not exactly trivial. The common standard provided by the OpenAPI specification greatly simplifies processes. This is not least due to the fact that both the documentation of the interface and the distribution of it can be done much more straightforwardly without a common programming language.

The OpenAPI standard defines the structure for various objects. These objects (for example, Info Object, Paths Object, or Server Object) represent a summary of properties that can be used to build an API. In practice, OpenAPI is then used primarily to describe REST APIs (Representational State Transfer). Programming interfaces based on the REST scheme give developers quick and easy access to data, e.g. on the current weather or traffic situation, which they can implement in their own applications. The structured data is easier to process, but documentation is needed to show what the endpoints are called and what data they provide. The OpenAPI specification gives developers a standardized basis for REST APIs, which not only simplifies development but also minimizes implementation effort. The API definition primarily uses the two data formats JSON and YAML.

Thanks to the many advantages mentioned, OpenAPI has established itself as a description standard for programming interfaces in recent years and is used across the board. No wonder, because for companies that develop their own applications as well as for those that can continue to use these applications without any problems thanks to OpenAPI, the specification represents a win-win situation.