2.

Consider the following sequences of actions, listed in the order they are submitted to the DBMS:

Sequence 1: T1:R(X), T2:W(Y), T3:W(Y), T3:W(X), T1:W(Y), T1:Commit, T2:Commit, T3:Commit

Sequence 2: T1:R(Y), T2:W(Y), T2:W(X), T3:W(Y), T1:W(X), T1:Commit, T2:Commit, T3:Commit

Describe how the concurrency control mechanism handles each sequence and each of the

following concurrency control mechanisms.

Assume that the timestamp of transaction Ti is i. For the lock-based concurrency control

mechanism, add lock and unlock requests to the previous sequence of actions as per the locking

protocol. The DBMS processes actions in the order shown. If a transaction is blocked, assume

that all its actions are queued until it is resumed; the DBMS continues with the next action

(according to the listed sequence) of an unblocked transaction.

Question :

a. Strict 2PL with timestamps used for deadlock prevention.

b. Strict 2PL with deadlock detection. (Draw the waits-for graph in case of

deadlock.)

c. Timestamp concurrency control with buffering of reads and writes (to

ensure recoverability) and the Thomas Write Rule.