What are OpenAPIs? The OpenAPI specification and 'open' APIs

OpenAPI is the reference standard for describing RESTful APIs that allow access to third parties, so-called 'open' APIs.

  • Author: Alessandra Caraffa
  • //
  • Date: 01/06/2022
  • //
  • Reading time: 3 min

The term OpenAPI refers to a standard, the OpenAPI specification (OAS), which allows the structure and operation of an API, or Application Programming Interface, to be described according to a shared model.

OpenAPIs can also refer to those application programming interfaces (APIs) that allow third parties access to their own data and tools, what with different shades of meaning are called in Italy 'open' or public APIs, which are accessible with minimal restrictions related more often to the practical use of APIs than to copyright or the like.

Open APIs in the API driven market

The introduction of a specification that would provide a standard for RESTful APIs was long overdue, and led to the explosion of the API market in a few years, which is now one of the leading markets in the software industry.

Indeed, what could be the ultimate sense of exposing one's data, if not to enable the widest possible use of it? One only has to think of the emerging universe of the API driven market to understand the role of the OpenAPI standard in the creation of new products and functionalities.

According to Market Data Forecast, the global API market will be worth $8.41 billion in 2027, with an estimated growth of +34% compared to the scenario provided by 2021 data. And even if not all APIs adhere to the OpenAPI standard, it is equally true that it is thanks to the standardisation of definitions, structures and processes that it has been possible to implement APIs practically everywhere.

While as early as 2020 90 per cent of developers claimed to use third-party APIs (data: SlashData), we know that today almost all the applications we use on a daily basis communicate with other applications via APIs. In this sense, the introduction of an open standard specification such as OpenAPI has provided a fast, shared and secure direction to the developer community and a zero point to the exponential growth of the API market.

What is OpenAPI?

The OpenAPI specification (OAS) defines a standard interface for RESTful APIs that allows humans and software applications to understand the functionality of the service without having to interact with the source code or original documentation.

The OpenAPI standard is independent of the programming language, and allows an API to be described by means of a document in YAML or JSON format that meets certain shared requirements.

The OAS standard - now at version 3.0 - can be used to describe a RESTful API in such a way as to generate stubs, i.e. functional portions of code, or documentation from a kind of lingua franca that makes interactions between APIs easier and the development of new applications much faster - assuming a central role in the evolution of the API-driven market.

The OpenAPI specification is now in a repository on GitHub and is based on open data available to the whole community of developers: thanks to the work of the OpenAPI Initiative, in 2017 the 'common language' of APIs became a resource available to the whole community.

Swagger, the specification and the OpenAPIs

On 5 April 2016, Amazon announced that its API Gateway would henceforth support Swagger 2.0 definitions. Just a few months earlier, the Swagger 2.0 standard had been donated to the Open API Initiative, an organisation set up ad hoc by the fathers of the Linux Foundation in association with leading IT companies such as Google, IBM, Microsoft and PayPal.

The OpenAPI Initiative was set up with the aim of "creating an open governance model around the Swagger Specification" capable of supporting and speeding up the adoption and dissemination of Swagger as the basic framework for describing RESTful APIs.

Swagger was the name of the specification until the publication of its version 3.0 in 2017: since then we refer to the specification as OpenAPI, while calling Swagger some of the most well-known tools for OAS implementation - editors, code generators, libraries and API validation tools that can generate new definitions from an existing API.

All this happened while the world of software applications was going through a momentous change, with the transition from a monolithic approach to the so-called microservices architecture. The vast majority of everyday applications, from Netflix to GPS navigation apps, now consist of a complex network of microservices and third-party APIs, elements integrated on various levels that allow companies to expose data and tools to the outside world, and to use or implement others' resources in the same way.

The benefits of OpenAPI

OpenAPI is a standard supported and adopted by pivotal companies in the IT industry, and can be seen as the fruit of the valuable experience of those who first developed, tested and implemented thousands of APIs over the years. The authority of such a source of truth (source of truth, for developers) is undoubtedly among the main reasons for large and small companies to adopt the OpenAPI 3.0 standard.

But there are also several practical reasons behind the enormous popularity of the OAS model: it is the REST specification that supports multiple programming languages, and can generate stubs in the required language. The OpenAPI specification also provides privileged access to all the tools of the Swagger ecosystem, including those that can automatically generate documentation, tests and mock servers.

The other advantages are essentially related to the open nature of code and data: the extensive community of developers working with the Open API shares language, tools, resources and knowledge, and constitutes an asset of rare value.

OpenAPI: revolution for digital payments

APIs are one of the key tools of the digital transformation: in line with the best expectations of Industry 4.0, the architecture put in place by the Open API universe is an excellent example of an asset multiplier, capable of aggregating and distributing data and services in ever new ways, giving rise to a whole market of products.

In this sense, digital payments have undergone a real revolution in terms of digital transformation: the PSD2 regulation, which will become effective in EU countries in 2019, in fact requires all electronic payment transactions and related data to be provided according to the Open Banking standard, which among other things provides for the use of API services that are open and accessible to third parties.

This is an innovation that opens up a further market, that of services that will be able to use the data exposed by platforms to create increasingly specific products and applications. According to data from the Innovative Payment Observatory of the Politecnico di Milano, 7 out of 10 banking institutions are adapting to the measure without exploring its potential. On the other hand, an interesting reality of new players is emerging, 55% of them young innovative companies, which through open APIs offer additional services for consumers and products increasingly targeted to the needs of users.


Access reliable, high quality data!

Discover over 380 services

in the Marketplace Openapi.com


What are OpenAPIs? The OpenAPI specification and 'open' APIs

Share on: