In previous versions, the driver relied solely on Java visibility rules: everything was either private or part of the public API. This made it hard to cleanly organize the code, and things ended up all together in one monolithic package; it also created a dilemma between providing useful hooks for advanced users, or keeping them hidden to limit the API surface.
Starting with 4.0, we adopt a package naming convention to address those issues:
com.datastax.oss.driver.api is part of the “official” public API of the driver, that can be used by regular client applications to execute queries. It follows semantic versioning and binary compatibility is guaranteed across minor and patch versions.com.datastax.oss.driver.internal is the “internal” API, primarily used by driver components to communicate with each other. It also exposes hooks for expert tweaking or framework implementors. We'll do our best to also keep that API stable, but full compatibility is not strictly guaranteed.