This is a good questions. Why should I worry about API First? Why should it be a first class citizen in our architecture. The reason is simple: You need to be able to respond to change quickly and rapidly develop new solutions to fix your problems. With the other "first" approaches the process is very long because you are always creating custom stuff or having to weave together the APIs at the point of integration and the lack of consistency and versioning can lead to chaos.
We have all been on projects where the solution architecture was created and then someone said oh you need to provide blah data to this other system and oh there is another system that needs this other data often happening later in the project when resources are tight and you have to piece together an API quickly to solve the issue. In the worst case you may even have different people work on different parts of the API. Then when it comes time to work with these other applications that need your data, they look at your API and scratch their head. When do I need to pass the API an auth token and when is it an API Key and when do I pass a json and when do I pass a series of string parameters? Even stranger is when you have one API that is REST and returning XML and the other is JSON based. I have seen that from the same vendor for the same application. It is scary.
For this reason, sitting down and designing a basic API framework from the start is important. Even if the API initially returns a small amount of data or maybe nothing at all, the basic mechanisms of authentication, how data is passed to the API, how it is returned, how the API is versioned is worked out. Then as you build out the API you add new interfaces or version interfaces that change still supporting the older ones. This makes people integrating with you very happy.
Now back to the initial reason: "You need to be able to respond to change quickly and rapidly develop new solutions." By making your API a first class citizen and documenting it well it means that someone building a new solution can quickly look at your documentation and determine which APIs they need to use and once they have used one API they will know how to work with others because of the consistent design. If they need new functionality, you can extend the API for them without affecting other solutions.
Now this only happens with proper API design. In the next few articles we will describe some of the best practices that will help you do this.
