Multiple
choices questions in DBMS, Concurrency control mechanism, Why incomplete transactions should not visible to
other transactions in RDBMS? What is cascading rollback? Why cascading rollback is not possible under strict 2PL? How cascading rollback is avoided by rigorous two phase locking protocol?
DBMS MCQ - Concurrency control protocol avoids cascading rollback
1. Which of the following concurrency control protocol can avoid cascading rollback?
a) Simple 2 Phase Locking protocol
b) Strict 2 Phase Locking protocol
c) Rigorous 2 Phase Locking protocol
d) Timestamp ordering protocol
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Answer: (b) Strict 2 Phase Locking protocol and (c) Rigorous 2 Phase Locking protocol Transactions that use either strict or rigorous 2PL protocols delay releasing the EXCLUSIVE locks on their data items until the transaction commits. Hence, other concurrent transactions cannot consume the data items that were modified by the transactions under strict or rigorous 2PL until the later release the locks. What is cascading rollback and how is it caused?If a transaction consumes the data item that was written by an uncommitted transaction, it may lead to cascading rollback, in case if the uncommitted transaction is decided to abort.Example: Suppose T1 and T2 are two concurrent transactions. If T1 writes a data item A and T1 not yet committed. Now, T2 reads the value of A which was just modified by T1. This is uncommitted read / dirty read. At this time, if for some reason T1 decides to rollback or abort. This forces T2 to rollback because, T2 used the data produced by T1.
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