Rethinkdb pattern for dynamic pubsubs through socketio - meteor

Question

Coming from the Meteor world & I'm curious how to replicate the cached pubsub/observers functionality. For a basic example, let's say I have a todolist where each todo has a userId and I want to keep todos private to each userId (but a userId could exist on multiple connected devices, eg phone + desktop). I imagine I have to create some publish function that verifies the userId by the socketId from the sent request, then create a socket namespace specific to that query (since the query could include more than a userId constraint). Then, register an emitter that only sends the changes to those socketIds that are verified to listen to the given namespace. Am I close? All my research just returns basic things like publishing to all connected users based on keywords. Any links to reading material would be great! Here's a first attempt with the missing logic in comments...
export function sendTodosByUserId(io, userId) {
//How to auth? By linking a client socketId to a user in a lookup table?
connect()
.then(conn => {
r
.table('todos')
.filter(todos => todos("userId").eq(userId))
.changes().run(conn, (err, cursor) => {
cursor.each((err, change) => {
//Do I emit a unique message? namespace? How do I handle 2 clients using the same userId?
io.emit('TODO_CHANGE', change);
});
});
});
}

Answer 1

You could implement some way of mapping new sockets to the correct user. For example, you could put the userId in a Express session and store the user's socket ids in a simple object.
var userSockets = {};
io.sockets.on('connection', function(socket) {
var userId = socket.handshake.session.userId;
if(userSockets[userId]) {
userSockets[userId].push(socket.id);
} else {
userSockets[userId] = [socket.id];
sendTodosByUserId(io, userId)
}
socket.on('disconnect', function() {
var i = userSockets[userId].indexOf(socket.id);
userSockets[userId].splice(i);
if(userSockets[userId].length === 0) {
delete userSockets[userId];
}
});
});
In your sendTodosByUsedId, you would just loop over the socket array belonging to the user, and emit to every socket.
var sockets = userSockets[userId];
for(var i = 0; i < sockets.length; ++i) {
io.to(sockets[i]).emit('TODO_CHANGE', change);
}
Note that this will not work if you have multiple nodes the user can be connected to. Then you might have to store the userSockets-object in e.g. Redis.
Alternatively, you could just have your users join a room named e.g. user:<userId>, and emit to this on every todo-change.
io.sockets.on('connection', function(socket) {
var userId = socket.handshake.session.userId;
socket.join('user:' + userId);
socket.on('disconnect', function() {
console.log("Rooms are left automatically on disconnect");
}
});
On todo change:
io.to('user:' + userId).emit('TODO_CHANGE', change);