Question

Time Stamp Ordering Concurrency Protocol

Below are several sequences of events, including start events, where sti means that
transaction Ti starts. These sequences represent real-time, and the timestamp
scheduler will allocate timestamps to transactions in the order of their starts. Tell
what happens as each executes. {Assume the Timestamp for TS(T1)=1 and
TS(T2)=2 and RTS=WTS=0
a) st1; st2; r1(A), r2 (B); w2 (A); W1(B);
b) st1; r1(A); st2; w2 (B ); r2 (A); w1(B);

Answer 1

The sequences demonstrate the Time Stamp Ordering Concurrency Protocol in action, with transactions being successfully executed or aborted based on their timestamps and the read/write timestamps of the objects being accessed to avoid conflicts.

Step-by-step explanation:

The question pertains to the Time Stamp Ordering Concurrency Protocol, which is used in databases to manage concurrent transactions in a way that prevents conflicts and ensures data integrity. Let's dissect the sequences given:

• Transaction T1 starts (st1) with timestamp 1.
• T2 starts (st2) with timestamp 2 after T1.
• T1 reads A (r1(A)).
• T2 reads B (r2(B)).
• T2 attempts to write to A (w2(A)); however, since the write timestamp (WTS) of A is initially 0, T2's write operation will proceed and the WTS of A will be updated to 2.
• T1 attempts to write to B (w1(B)); similarly, this operation will proceed and the WTS of B will be updated to 1.

The second sequence is as follows:

• T1 starts and reads A.
• While T1 is active, T2 starts.
• T2 writes to B (w2(B)). Since the WTS of B is 0, this operation will proceed, and the WTS will be updated to 2.
• T2 then attempts to read A (r2(A)), which will succeed since the read timestamp (RTS) of A (which is initially 0) is less than T2's timestamp.
• Finally, T1 attempts to write to B (w1(B)). However, since T1's timestamp (1) is less than the WTS of B (which was updated to 2 by T2), this write operation will be rejected by the concurrency control, preventing a write-write conflict.