Join other sockets to a dynamic room in multi instance

Hi, new to socket.io here. I find it very easy to build simple chat app following tutorial, but having difficulties as I progress to build more complex features.

I want to create a group chat app like whatsapp or facebook messenger. I can do this with room feature and broadcasting messages to the room when all users are connected and joined to the same room. Where I’m stuck is when I start a new group chat.

Suppose, A wants to create a new group chat with B and C. So A starts with a message like this. (assume B, C are also connected to socket.io server)

{ 
  id: 'groupchat-123', // randomly generated id
  receipients: ['B', 'C'],
  message: 'Hey',
  sender: 'A'
}

The outcome I want is, B & C receives this message on the client with no user interaction.

On server side, A will do socket.join(‘groupchat-123’). But B, and C haven’t join to the same room yet, hence broadcasting won’t do anything to B and C. How do I make B and C join the room before broadcasting? How’d I achieve this if I have multiple node instances?

[sibe7691]

I dont suppose you have some code to share?

[arthurpark]

Got it. So I’ve made it work by creating predefined channel(“join”) from server and let subClient to subscribe to that channel. Then I’m publishing to that channel(“join”) when I need to force other sockets to join a specific room. Thanks for your help!