What are some alternatives?
When comparing NATS and PubNub, you can also consider the following products
Firebase - Firebase is a cloud service designed to power real-time, collaborative applications for mobile and web.
Socket.io - Realtime application framework (Node.JS server)
Pusher - Pusher is a hosted API for quickly, easily and securely adding scalable realtime functionality via WebSockets to web and mobile apps.
SignalR - SignalR is a server-side software system designed for writing scalable Internet applications, notably web servers.
Apache Kafka - Apache Kafka is an open-source message broker project developed by the Apache Software Foundation written in Scala.
deployd - API development tool for Web and Mobile developers.