I have an application whose primary function works in real time, through websockets or long polling.
However, most of the site is written in a RESTful fashion, which
I learned a little lesson (the hard way). I made a number crunching application that runs on Ubuntu AWS EC2 cloud services (uses powerful GPUs), and I wanted to make a front-end for it just to watch its progress in realtime. Due to the fact that it needed realtime data, it was obvious that I needed websockets to push the updates.
It started with a proof of concept, and worked great. But then when we wanted to make it available to the public, we had to add user session, so we needed login features. And no matter how you look at it, the websocket has to know which user it deals with, so we took the shortcut of using the websockets to authenticate the users. It seemed obvious, and it was convenient.
We actually had to spend quiet some time to make the connections reliable. We started out with some cheap websocket tutorials, but discovered that our implementation was not able to automatically reconnect when the connection was broken. That all improved when we switched to socket-io. Socket-io is a must !
Having said all that, to be honest, I think we missed out on some great socket-io features. Socket-io has a lot more to offer, and I am sure, if you take it in account in your initial design, you can get more out of it. In contrast, we just replaced the old websockets with the websocket functionality of socket-io, and that was it. (no rooms, no channels, ...) A redesign could have made everything more powerful. But we didn't have time for that. That's something to remember for our next project.
Next we started to store more and more data (user history, invoices, transactions, ...). We stored all of it in an AWS dynamodb database, and AGAIN, we used socket-io to communicate the CRUD operations from the front-end to the backend. I think we took a wrong turn there. It was a mistake.
Having said all that, we are going live next week. We got there in time, everything works. And it's fast, but will it scale ?