User profile synchronization service issues

user profile service instance run on WFE while syncnoration service on the app servers.

Thanks for your reply I will follow up this links and update the thread accordingly.... However one more query on User profile service... If I go to the below navigation I can see user profile service and user profile synchronization service available both on app and wfe server... I would like to know what is the recommended way to start these services... I mean User profile service and user profile synchronization service should be run on which server, app or wfe??? By the way I am using sharepoint 2013. From central admin > System settings > Server in the farm ( user profile service and ups synchronization service available both in app and wfe server)