Database Management System: ACID Properties
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Questions and Answers

What is the main purpose of the two-phase locking protocol?

  • To ensure serializability in concurrent transactions (correct)
  • To ensure atomicity in database transactions
  • To ensure durability in database transactions
  • To ensure isolation in database transactions

    • What is the primary difference between strict 2PL and rigorous 2PL?

  • The concurrency level supported by these protocols
  • The order in which locks are acquired and released
  • The type of locks released after a transaction commits or aborts (correct)
  • The type of transactions that can use these protocols

    • What is the consequence of a deadlock in a database system?

  • Transactions will abort and restart
  • Transactions will wait indefinitely for a resource (correct)
  • The database will shut down
  • Transactions will be rolled back

    • What is the primary cause of starvation in a database system?

    <p>Transaction scheduling algorithm</p> Signup and view all the answers

    What is the main goal of concurrency control mechanisms?

    <p>To ensure data consistency</p> Signup and view all the answers

    What is the primary benefit of using timestamp ordering in concurrency control?

    <p>Increased concurrency</p> Signup and view all the answers

    What is the main difference between conservative 2PL and basic 2PL?

    <p>The timing of lock acquisition</p> Signup and view all the answers

    What is the main goal of the expanding phase in two-phase locking?

    <p>To acquire new locks</p> Signup and view all the answers

    What is the primary benefit of using two-phase locking?

    <p>Guaranteed serializability</p> Signup and view all the answers

    What is the main difference between a deadlock and starvation?

    <p>A deadlock occurs due to resource unavailability, while starvation occurs due to scheduling</p> Signup and view all the answers

    Study Notes

    Desirable Properties of Transactions

    • Atomicity: a transaction is performed in its entirety or not at all
    • Consistency preservation: takes database from one consistent state to another
    • Isolation: not interfered with by other transactions
    • Durability or permanency: changes must persist in the database

    Transaction States

    The Lost Update Problem

    • If T1 transfers 5 seat reservations from X to Y and T2 reserves 4 seats on X, the final result should be X = 79, but in the interleaving of operations, X = 84 because the update in T1 that removed the five seats from X was lost

    The Temporary Update Problem

    The Incorrect Summary Problem

    The Unrepeatable Read Problem

    • Transaction T reads the same item twice, value is changed by another transaction T between the two reads, T receives different values for the two reads of the same item

    Concurrency Control Based on Timestamp Ordering

    • Strict Timestamp Ordering:
      • If TS(T) > read_TS(X), then delay T until the transaction T’ that wrote or read X has terminated
      • If TS(T) > write_TS(X), then delay T until the transaction T’ that wrote or read X has terminated
    • Thomas’s Write Rule:
      • If read_TS(X) > TS(T) then abort and roll-back T and reject the operation
      • If write_TS(X) > TS(T), then just ignore the write operation and continue execution
      • Else, execute write_item(X) of T and set write_TS(X) to TS(T)

    Guaranteeing Serializability by Two-Phase Locking

    • Two-phase locking protocol:
      • Expanding (growing) phase: new locks can be acquired but none can be released, lock conversion upgrades must be done during this phase
      • Shrinking phase: existing locks can be released but none can be acquired, downgrades must be done during this phase
    • The two phases must be mutually exclusive

    Variations of Two-Phase Locking

    • Basic 2PL: technique described above
    • Conservative (static) 2PL: requires a transaction to lock all the items it accesses before the transaction begins
    • Strict 2PL: transaction does not release exclusive locks until after it commits or aborts
    • Rigorous 2PL: transaction does not release any locks until after it commits or aborts

    Deadlock and Starvation

    • Deadlock: occurs when each transaction T in a set is waiting for some item locked by some other transaction T’
    • Starvation: occurs if a transaction cannot proceed for an indefinite period of time while other transactions continue normally
    • Solution: first-come-first-served queue

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    Description

    Learn about the desirable properties of transactions in a database management system, including atomicity, consistency preservation, isolation, and durability. Understanding these ACID properties is crucial for ensuring data integrity.

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