An API, whether designed for private internal use or public use, needs much careful consideration during development to be useful. It requires to not only implement the desired functionality, but it must also be reliable, well designed, and easy to use.
The RestCase blog post titled REST APIs: From Idea to Release outlines the creation of a “minimum viable product” (MVP). This is the minimum implementation of a product with enough features to release to early users for testing. Many companies will opt to create an MVP the easy way, building functionality into the initial release of an API without building in the necessary reliability and usability. A proper MVP should provide both the minimum functionality as well as the reliability and usability of a final product. However it can be difficult to build a good MVP for an API considering the applications that depend on them. If applications are developed that depend on an early MVP of your API, it can be hard to update and improve on it without breaking those applications. A common downfall of APIs is a constant release of updates that break integration with existing applications.
Taking the time to test your API can be a big investment, but it is an important one. Getting feedback from your core users about your API early in its development cycle can help shape the API to their needs. If you commit yourself to a design or an implementation before testing, it could be too late or too expensive to change it based on feedback. This testing/feedback cycle sacrifices speed for quality, but ensures a functional and usable API based on what your users need.
One useful testing cycle for API development is slowly opening up new versions for testing to specific groups. Start by organizing a developer focus group, which opens up your API to developers that will actually use it. After, follow up by publicly mocking up the feedback from the developer focus group, and use that to start a beta or invite only testing group for real-world testing. After building a beta/invite community and updating based on their feedback, it’s finally time to release your new or updated API.