How to save user PushSubscriptions?

[pwFoo]

I have client side code which create a notification api subscription and send it to the server (PW, process module). Now I think about how to save the subscription?

{
	endpoint: "https://fcm.googleapis.com/fcm/send/<DEVICE_ID>", 
	key: <KEY>, 
	token: <AUTH_TOKEN>, 
	expirationTime: null
}

URL and DEVICE_ID is different dependent on browser (android, chrome, firefox, edge).

 

How and where (in the page tree) should I save multiple user subscriptions?

If a unsubscribe is executed the subscription have to be removed from the database / user. So I need a identifier to delete the subscription.

[pwFoo]

Same with subscribed content. How to save content subscriptions with PW?