How to create real-time web applications?

Question

PHP

Good afternoon. Interested in how to create (in general terms) real-time web applications.
Interested in technology, approach.
If, for example, using node.js, sockets - for the purpose of the chat, it's clear, but how to add events when the user is online \ offline, how to combine \ implement it? Two sockets open, or all in one? What if there are many such real-time events?
PS So far, the transition and development of such applications is difficult.

Answer 1

I can't give you a specific solution, but something.
Real-time Web Application with Socket.IO, Node.js, ...
together with autobahn.ws/js
socket 1, why more.
All of course depends on the direction of the application. so I can't give specific advice.
client went offline? save data locally, since there are already various storage methods. got a connection? restore connections, and all that is needed (subscriptions, tokens)
send a portion of data to the server.
everything seems to be not difficult.
"What if there are many such real-time events?" - and there are not enough of them. nothing. with proper architecture and scalability, it is possible to achieve processing of very many events

Answer 2

1. For each client, a separate socket is created (that is, for n clients on the server, we have n sockets open and 1, the server accepting new connections), which can be stored in a common heap.
2. events are available to you when the client disconnects, etc. Once this has happened, you can set user statuses.