I am creating a mobile app and when users want to add another user, I need to check if the user in their address book actually has the app installed or not. Kind of like the big chat apps where it only shows you users from your address book/contact list that have the app installed.
How is this achieved? When you register does it upload all of your contacts to the server and then check against that list to see if that number also registered? I wouldn't think so because these apps are so fast when loading users you can message that it doesn't look like there is any call to the server, it is instant.
The part I don't understand is that if on my phone I have saved the persons number as 0111234567 that works when I call them or message then in an instant messenger.
But that format isn't what they would sign up with in the app, it would be something like +27 111234567
So, when searching the database how can it compare those 2 which aren't the same?