The base concept behind API Core framework is that it handles URLs conforming with conventions of the REST architecture as the queries of a relational query language. This makes possible to build REST APIs supporting the unlimited deepness of relations between data models, just by specifying an entity-relation model for almost any kind of data. Thus the channel agnosticism of the solutions provides support for not just HTTP, but other communication channels and protocols, like GraphQL, or WebSocket. This document provides a brief overview of similar API frameworks whose inspired the development of API Core, states the positive and negative aspects of each and introduces the architecture of API Core. Also provides an in-depth demonstration of the inner workings of the query processor component and provides a detailed overview of the new developments required to support the microservice architecture in the latest version of the system, including support for Swagger metadata, the external edges and the extended schema system, which now includes support for type information. Besides these, the document introduces API composition and the brand new API Gateway. Finally gives an in practice introduction to using API Core via a simple demo application and provides a glimpse into the future of the framework.