Qi4j is a very flexible system, and many of the critical implementation details are in so called Extensions. They extend the Core Runtime with functionality that we want to provide multiple and independent implementations of, such as persistence, query and indexing.
Extensions have access to most of the Core Runtime internals through well defined APIs, which also reside in the Core SPI available to extensions.
All the Model, Resolution and Binding of the entire Structure, i.e. Application, Layer, Module, Composite, Fragment, are available through the API. Also the Query, Indexing and Persistence SPIs are under the Extension SPI umbrella.
