NATS vs Radar: What are the differences?
Developers describe NATS as "Lightweight publish-subscribe & distributed queueing messaging system". Unlike traditional enterprise messaging systems, NATS has an always-on dial tone that does whatever it takes to remain available. This forms a great base for building modern, reliable, and scalable cloud and distributed systems. On the other hand, Radar is detailed as "High level API and backend for writing web apps that use push messaging". Radar is built on top of engine.io, the next-generation backend for socket.io. It uses Redis for backend storage, though the assumption is that this is only for storing currently active data.
NATS and Radar can be primarily classified as "Realtime Backend / API" tools.
Radar is an open source tool with 209 GitHub stars and 35 GitHub forks. Here's a link to Radar's open source repository on GitHub.