title: I want to sync user contacts to firebase firestore in one go
body: <p>I am building chat application somewhat like whatsapp. I want to show registered app users list from user's device contact list while creating new group. Now in order to do that I have to compare each and every contact number with firebase firestore users. And any normal user can have more than 500 contacts in device. And moreover firestore has limitation to for querying the db so I can not compare more than one number at a time, the whole process takes almost 6-7 minutes as well as each read operation costs financially.</p>
  
  <p>How can I overcome with this situation, or what is the better way to deal with this particular scenario?</p>