socket.io, how to attach state to a socket room

Question

I was wondering if there was any way of attaching state to individual rooms, can that be done?

Say I create a room room_name:

socket.join("room_name")

How can I assign an object, array, variable(s) to that specific room? I want to do something like:

io.adapter.rooms["room_name"].max = maxPeople

Where I give the room room_name a state variable max, and max is assigned the value of the variable maxPeople that is say type int. Max stores a variable for the maximum amount of people allowed to join that specific room. Other rooms can/could be assigned different max values.

Answer 1

Well, there is an object (internal to socket.io) that represents a room. It is stored in the adapter. You can see the basic definition of the object here in the source. so, if you reached into the adapter and got the room object for a given name, you could add data to it and it would stay there as long as the room didn't get removed.

But, that's a bit dangerous for a couple reasons:

1. If there's only one connection in the room and that user disconnects and reconnects (like say because they navigate to a new page), the room object may get destroyed and then recreated and your data would be lost.

2. Direct access to the Room object and the list of rooms is not in the public socket.io interface (as far as I know). The adapter is a replaceable thing and when you're doing things like using the redis adapter with clustering it may work differently (in fact this probably does because the list of rooms is centralized into a redis database). The non-public interface is also subject to change in future versions of socket.io (and socket.io has been known to rearrange some internals from time to time).

So, if this were my code, I'd just create my own data structure to keep room specific info.

When you add someone to a room, you make sure your own room object exists and is initialized properly with the desired data. It would probably work well to use a Map object with room name as key and your own Room object as value. When you remove someone from a room, you can clean up your own data structure if the room is empty.

You could even make your own room object be the central API you use for joining or leaving a room and it would then maintain its own data structure and then also call socket.io to to the join or leave there. This would centralize the maintenance of your own data structure when anyone joins or leaves a room. This would also allow you to pre-create your room objects with their own properties before there are any users in them (if you needed/wanted to do that) which is something socket.io will not do.