It's a good introduction, and it's a good document to introduce the problems induced by WebSockets (which Ably can come in an solve at scale).

I recently wrote about the Woes of Websocket: http://www.adama-lang.org/blog/woe-of-websocket with an errata based on HN feedback: http://www.adama-lang.org/blog/more-websocket-woe

The depth of this topic is very interesting, and I'm excited as I'm building some of the final pieces for my SaaS (which could compete with Ably).