I'm building a Node.js app where I was thinking of using MongoDB but I'm reconsidering it after reading the comments here.

The database's only job is to keep persistence between server restarts. On server start I fetch the a collection from the database and populate a variable with it, which I use for any direct access.

When a user updates something it will update the variable as well as updating the collection in the database.

Something like this:

    var foo = readFromDB();

    socket.on('connection', function() {
        this.emit(foo);
    });

    socket.on('update', function(data) {
        foo[data.id] = data.val;

        updateDB(data.id, data.val);
    });

(In my real application each collection has sub-arrays etc and not just key-value like in the example.)

What should I use instead of MongoDB for this? Or is my approach fundamentally flawed?