It doesn&#x27;t, it differs from polling <i>each product</i>, since it provides a single resource for all of them.<p>An alternative would be for the Inventory system to allow the Email system to register a callback URL (aka Webhook) with it that gets called when the stock updates - allowing for push events while maintaining a decoupled architecture -, but that is only useful if the stock only gets updated sporadically, as you should still have a resource that can be polled, in case the Email system dies and then needs to check what happened when it was offline.