API (Application Programming Interface) is an interface for the application programming. It defines and describes interactions with different functions and parts of software system. A typical example is a mobile application - server application connection. In the case of mobile applications, HTTP communication using REST with JSON data format is used. We do like to make our own APIs in Ackee but sometimes we use our clients’ APIs too. If an API is not suitable for a mobile application, we add an application middleware too.