Keeping sync between two devices accessing the same account (different/same session) - mysql

Question

Kind of curious as I'm aiming for a stateless setup, how some people go about coding/setting up their session handling when many devices accessing a single account occurs.
I work with Node.JS currently but the pseudo is appreciated,
This is how my sessions look currently, ID is a unique value. (Redis stored JSON by KEY)
{"cookie": {
"originalMaxAge": null,
"expires": null,
"secure": true,
"httpOnly": true,
"domain": "",
"path": "/",
"sameSite": "strict"
},
"SameSite": "7e5b3108-2939-4b4b-afdc-39ed5dbd00d0",
"loggedin": 1,
"validated": 1,
"username": "Tester12345",
"displayself": 1,
"avatar": "{ \"folder\": \"ad566c0b-aeac-4db8-9f54-36529c99ef15/\", \"filetype\": \".png\" }",
"admin": 0,
"backgroundcolor": "#ffffff",
"namebackgroundcolor": "#000000",
"messagetextcolor": "#5d1414"}
I have no issues with this setup until I have a user logged in twice different devices and one decides to adjust their colors or avatar; one session is up to date and the other is completely lost.
I do my best when possibly to call out to database to ensure the information is up to date when it's most important but curious for this small slip up what I should be doing? I'd hate to call for database each request to get this information but think most do this any-how?
I could set up in my mind a hundred different ways to go about this but was hoping maybe someone who has dealt with this has some excellent ideas about this. I'd like to just be efficient and not make my databases work as hard if they don't need to, but I know session handling makes the call each request so trying to determine a final thought.
Open to all ideas, and my example above is a JSON insert into Redis; I'm open to changing to MySQL or another store.

Answer 1

One way to notify devices and keep them up-to-date about changes made elsewhere is with a webSocket or socket.io connection from device to the server. When the device logs in as a particular user and then makes a corresponding webSocket or socket.io connection to the server, the server keeps track of what user that connection belongs to. The connection stays connected for the duration of the user's presence.
Then, if a client changes something (let's use a background color as an example), and tells the server to update its state to that new color, the server can look in its list of connections to see if there are any other connections for this same account. If so, the server sends that other connection a message indicating what change has been made. That client will then receive that notification and can update their view immediately. This whole thing can happen in milliseconds without any polling by the client.
If you aren't familiar with socket.io, it is a layer on top of webSocket that offers some additional features.
In socket.io, you can add each device that connects on behalf of a specific account to a socket.io room that has a unique name derived from the account (often an email address or username). Upon login:
// join this newly connected socket to a room with the name
// of the account it belongs to
socket.join(accountName);
Then, you can easily broadcast to all devices connected to that room with one simple socket.io API call:
// send a message to all currently connected devices using this account
io.emit(accountName, msg);
When socket.io connections are disconnected, they are automatically removed from any rooms that they have been placed in.
A room is a lightweight collection of currently connected sockets so it works quite well for a use like this.