How can my web service know of any changes similar to subscriptions?

CloudKit Webservices is great!


What I haven't discovered yet is the ability for my web server to subscribe (also known as web hooks). The reason for this is that I really like to focus on Apple products and build great apps above on CloudKit, however the success of my application also depends on platform indepence. As my group of friends is seperated amongst Android and iOS users I like to use the rich push notification (iOS10) and database features of CloudKit for Apple and send a mail and present a web interface for Android users. To be able to send that mail or android push notification I need to know when certain changes are happening in the database so I don't have to constantly query for them myself. The thing that makes most sence is subscriptions which arive at my web server, but I haven't figured it out yet. Perhaps someone can shine a light on the matter.