I suppose if anything it is the customer subscribes.  But actually, in practice, it doesn&#x27;t really feel quite like this because in FP you just have a bunch of functions and there isn&#x27;t the concept of things &#x27;owning&#x27; other things like you have in more OO based language.  What actually happens is that you add a (sort of) listener to the subscription that fires a message in a particular update function, which manipulates some part of the model.