Ably is described as '’s APIs provide developers with the infrastructure to power realtime functionality at any scale. Instead of building and operating infrastructure, engineers can focus on shipping the features and functionality that matter to their customers' and is an website in the Development category. There are nine alternatives to Ably, not only websites but also apps for a variety of platforms, including Mac, Windows, Linux and Android. The best alternative is Firebase, which is free. Other great sites and apps similar to Ably are Socket.io (Free, Open Source), Etebase (Free, Open Source), Supabase (Free, Open Source) and Pubnub (Freemium).
Etebase makes it easy to build end-to-end encrypted applications by taking care of the encryption and its related challenges. Think Firebase but encrypted in a way that only your users can access their data.
deepstream.io is a clusterable realtime server that handles persistent tcp and websocket connections. It offers data-sync, pub/sub and request/response. It's available for Linux, Mac and Windows and comes with clients in JS, Java and many others.
Lightstreamer is a server for delivering real-time messages to browser-based and mobile applications, using WebSockets and HTTP. Features include dynamic throttling, bandwidth and frequency control, delta delivery, and total control over security and the quality of service.