Question $4$: Consider the following scenario where user A and B are trying to update the same object in the application using dual writes.

This application stores three copies of the data in database, cache and index separately, assuming the two users are posting the update requests at the same time, and the application are trying to access three places simultaneously.

a$.$ Due to the problem of data synchronization, those requests may introduce data inconsistence to the application, specify all the cases that values could stored in different places.