Hello there!
Well, most of the code goes into session handling and keeping track of the user, then keeping track of which servers have which users and thus get which messages. I've done this before, and it's a similar situation if you have many application servers, transferring sessions between them is first. Second, is message passing from the reddis(in this case) to each individual server that happens to have a pertinant user who should get the message.
Anyway I have set something this like up at least once or twice, and I even did it using reddis at least once!
We could talk about how detailed the architecture should be, and probably a bit more about your situation. This will determine the message passing technique to best use bandwidth and more.
So, why do you need multiple servers in the first place?
What is the current business need, or is this proof of concept to be used in something else like a proposal or tutorial website?
Thanks & I look forward to working more with you!