Client to Client Communication

Basically there are 2 ways to communicate between clients and both the ways uses Pub/Sub System.

The first way does not include any server coding (all your logic is saved in front-end application). You subscribe , watch and publish from the front-end application.

socket.on('connect', () => {
    // Subscribe to the channel (can be any channels you want)
    channel = socket.subscribe('any-channel-name')
    
    // Listen on messages in that channel
    channel.watch((message) => {
        // Execute code when receive any messages on the channel 
    })

    // Publish any message you want
    channel.publish('any-message-you-want')
})

Second way is publishing messages from the server, and not from the front-end application.

// Client
socket.on('connect', () => {
   socket.send('event-name', 'my message')
})

// Server
socketServer.on('connection', (socket) => {
   socket.on('event-name', (message) => {
       socketServer.publish('The Channel you want to publish', message)
   })
})

as you can see in second example all logic of publish moved to the server, where you can make many things before publishing, also second method gives you ability to publish messages even if client is not subscribed to the channel.

The last part i wanted to mention about is specific client communication, like what if you want to send message to a specific client, and not to the group. That is pretty easy too, you just have to create unique channel (can be user_id) for each client and use onsubscribe middleware (check server documentation) on the server to allow only this particular client to subscribe to this unique channel.

Then you can use second method to send message to the client from another client.

// Client
socket.on('connect', () => {
   socket.send('send-to-friend', {receiverId: 'xyz', message: 'Hello'})
})

// Server
socketServer.on('connection', (socket) => {
   socket.on('send-to-friend', (message) => {
       socketServer.publish(message.receiverId, message.message)
   })
})

How you get receiverId on front-end you have to figure that out by your self as there many different ways of doing it.