How does memory work across Heroku dynos?

Question

Let's say I have a dynamically created array in node, and I am running it on a heroku dyno:

var connections = [];
var wm = new WeakMap();

io.sockets.on('connection', function(socket) {

    socket.once('disconnect', function() {
        connections.splice(connections.indexOf(socket), 1);
        socket.disconnect();
        console.log("Disconnected: %s sockets remaining.", connections.length);
    });

    socket.on('idAndApi', (data) => {
        wm.set(socket, data);
    });

    connections.push(socket);
    console.log("Connected: %s sockets connected.", connections.length);
});

Now, it is possible the connections array and the wm weakmap could be created across other dynos.

Will heroku be able to span those variables across each dyno? Or do I need to use some other data-structure (such as a database) to hold the information?

Answer 1

Dynos are independent, and node processes should be stateless (this is true in any environment, Heroku or otherwise).

Store state externally in a place that it can be managed and shared to best suit your app. Redis and Postgres are two of the most common systems for node.

Since you're using socket.io, you should be sure to follow their guide for spanning multiple nodes:

http://socket.io/docs/using-multiple-nodes/