How do you design Telegram's feature where you get a notification each time a contact of yours joins the service? what does it look like in terms of a scalable backend architecture?
Scenario: Two users, where both have a common contact, join Telegram. Then, that contact joins Telegram too, the system should then notify both users that their contact has joined Telegram.
First thing that comes to my mind is to have a DB design similar to what is described here: https://stackoverflow.com/a/38111303/12250254 and a similar one https://stackoverflow.com/a/11189651/12250254
It should scale pretty well
So basically when another user joins Telegram an empty conversation (i.e. dialog / chat) is created and treated as an empty chat between two users. Besides that it bubbles up above all existing chats in order to draw user's attention as if it were a regular incoming message.
In terms of notifications if we speak about web I would recommend you to check Web Push Notifications. Although I believe it's much less effective than native app notifications, like we have in Telegram Desktop or Telegram mobile apps.
