How to send notification according to user's interest? : Many have you have noticed that sometimes we gets notifications according to our needs and Interest. For Example: Suppose you are using any Hotel Booking App, You entered the city and checked some hotels but didn't booked any hotel and closed the App. You will notice, after few hours or days they will send you notifications according to what I checked on their App(Like some offers or Planning to visit "xyz" city?) How they send such notifications? What they use for last searched cities/hotels of each user and send notification accordingly??   