RethinkDB Horizon is an open-source developer platform for building realtime, scalable web apps. It is built on top of RethinkDB, and allows app developers to get started with building modern, engaging apps without writing any backend code.

Horizon consists of three components:

Horizon server: a middleware server that connects to/is built on top of RethinkDB, and exposes a simple API/protocol to front-end applications.
Horizon client: a JavaScript client library that wraps Horizon server’s protocol in a convenient API for front-end developers.
Horizon CLI: a command line tool, hz, aiding in scaffolding, development, and deployment.