Building a modern Web API has become the newest challenge for developer relations. In the five years since it began building APIs, MasterCard has learned a number of lessons, many of which it has included in its Open API Declaration, released today.The Declaration is filled with promises from MasterCard to developers using its APIs. Sebastien Taveau, chief developer evangelist at MasterCard, said that building APIs requires openness and dialogue with the developers who will be consuming it.(Related: MasterCard on its data-management APIs)“Anyone who is trying to enter the world of APIs has to make it very clear and make it very transparent about what are the intentions. What are you going to do for the developers? For the community?” said Taveau. He added that these external developers need to be kept at the forefront of whatever the business is trying to achieve.To that end, the MasterCard Open API Declaration includes a quote from the company’s CEO, Ajay Banga, saying that MasterCard is “bringing together developers and entrepreneurs using MasterCard APIs to create new applications to drive a new generation of commerce through our products, solutions and services.” Taveau said that one of the driving reasons for the release of this declaration comes from a fundamental change inside MasterCard. “MasterCard is a tech company now,” he said. “We provide one of the only global infrastructures that allows quick deployment at the world level for any kind of commerce application.“[But] no one can do it alone. The most successful applications you find today are a mix of technologies. They come from multiple companies. The developers are the brains for creating this patchwork. They gather what they think will bring the best user experience. You can’t do it alone, and if you are one of the best technologies to actually help implement one of these applications, you have to pay it forward.”To that end, the Open API Declaration contains promises such as “When a developer requests a technology feature or reports a problem, the Open API Team won’t just listen. We’ll act.”The Declaration also states that, for MasterCard’s APIs, “Ease of use is our highest priority. We will achieve this by collaborating closely with developer communities to provide perspective and ideas for creating new tools, enhancing existing ones, and continuously evolving our platform as technology evolves and advances.”The Declaration also offers a road map for the MasterCard APIs and tool sets out to 2020. It can be found on MasterCard’s Developer Zone Website.