4. a. Consider a database with objects X and Y and assume that there are two transactions T1 and T2. Transaction T1 reads objects X and Y and then writes object X. Transaction T2 reads objects X and Y and then writes objects X and Y. Give three example histories over transactions T1 and T2 that result in

a write-read conflict

a read-write conflict

a write-write conflict

Show that each serial history involving transactions T1 and T2 preserves the consistency requirement of the database

b. Specify a recovery strategy which would never require undoing or re-doing side-effects (both undoing and redoing are not required). How would such a recovery strategy be implemented?