Database Locking Mechanisms
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Database Locking Mechanisms

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Questions and Answers

Which statement best describes row-level locking?

  • It allows transactions to access different rows on the same page without restriction. (correct)
  • It allows concurrent transactions to access the same row simultaneously.
  • It locks entire pages, preventing access to any row on the locked page.
  • It requires minimal overhead compared to other locking methods.

    • What is a characteristic of binary locks?

  • They automatically escalate to page-level locks.
  • They require complex management to track multiple states.
  • They prevent other transactions from accessing locked items. (correct)
  • They can have multiple values beyond locked and unlocked.

    • Which locking protocol involves changing a lock's type based on the session's needs?

  • Simple locking
  • Graph based locking
  • Row-level locking
  • Two phase locking (correct)

    • What is the primary function of the lock_item operation?

    <p>To request access to an item for reading or writing.</p> Signup and view all the answers

    In which scenario would a transaction be forced to wait when using binary locks?

    <p>When the lock state is 1, meaning the item is locked by another transaction.</p> Signup and view all the answers

    What distinguishes page-level locks from row-level locks?

    <p>Page-level locks restrict access to an entire disk page.</p> Signup and view all the answers

    Which type of locking provides the least amount of concurrency and flexibility?

    <p>Page-level locking</p> Signup and view all the answers

    What is a significant management challenge of row-level locking?

    <p>It involves high overhead due to many locks for each row.</p> Signup and view all the answers

    What is the primary purpose of acquiring a lock before accessing a data item during transaction execution?

    <p>To ensure the isolation property of concurrently executing transactions</p> Signup and view all the answers

    What does lock granularity refer to in the context of locking mechanisms?

    <p>The portions of database objects that are locked</p> Signup and view all the answers

    Which type of lock prevents all other transactions from accessing any rows within a table until the lock is released?

    <p>Table-level lock</p> Signup and view all the answers

    What are the drawbacks of database level locking?

    <p>It can slow down data access and block transactions from using different tables</p> Signup and view all the answers

    Which statement best describes the advantage of using row-level locking?

    <p>It prevents other transactions from accessing only specific rows, improving concurrency</p> Signup and view all the answers

    What is a shared lock primarily used for in transaction management?

    <p>To allow multiple transactions to read a data item simultaneously</p> Signup and view all the answers

    Which locking protocol would be most effective in minimizing transaction wait times in a multiuser environment?

    <p>Row-level locking</p> Signup and view all the answers

    What is the main disadvantage of using binary locks?

    <p>They increase the likelihood of deadlocks</p> Signup and view all the answers

    What is the primary difference between a shared lock and an exclusive lock?

    <p>A shared lock allows multiple transactions to read simultaneously, whereas an exclusive lock allows only one transaction to write.</p> Signup and view all the answers

    Which statement is true regarding the upgrading of a lock from shared to exclusive?

    <p>The transaction must hold the shared lock exclusively or wait.</p> Signup and view all the answers

    What condition must be met for a transaction schedule to be guaranteed serializable?

    <p>Every transaction in the schedule must follow the two-phase locking protocol.</p> Signup and view all the answers

    In the context of locking, what does the shrinking phase signify?

    <p>It allows existing locks to be released but prohibits acquiring new locks.</p> Signup and view all the answers

    Which of the following can lead to deadlocks in transactions?

    <p>Multiple transactions requesting locks in a circular dependency.</p> Signup and view all the answers

    What is the purpose of locks in transaction management?

    <p>To prevent data inconsistencies during concurrent transactions.</p> Signup and view all the answers

    Which locking operation allows a transaction to read an item while other transactions can also read it?

    <p>read_lock(X)</p> Signup and view all the answers

    What is the consequence of improperly implementing locking protocols in transactions?

    <p>The transaction schedule may not be serializable, leading to inconsistencies.</p> Signup and view all the answers

    Study Notes

    Lock Granularity

    • Disk Page Lock: Locks an entire page on a disk.
    • Row Level Lock: Locks a single row, allowing concurrent access to different rows on the same page.
    • Modern DBMS automatically changes a lock from row-level to page-level when a session requests multiple locks on the same page, improving efficiency.

    Types of Locking Protocols

    • Simple Locking: A basic locking mechanism where an object is either locked (1) or unlocked (0).
    • Two Phase Locking: More robust protocol with two phases:
      • Growing Phase: Acquiring new locks but not releasing any.
      • Shrinking Phase: Releasing existing locks but not acquiring any new ones.
    • Graph Based Locking: A more complex approach where locks are represented as nodes in a graph, used in advanced scenarios.
    • Binary Locks: A type of lock with only two states: locked (1) or unlocked (0). They enable transactions to acquire locks for exclusive use.
    • Time stamp techniques: Alternatives to locking, using timestamps to ensure data accuracy.
    • Multi-version techniques: Maintain multiple versions of data to allow concurrent access without conflicts.

    Locking the Data Item

    • Prevents data inconsistency during concurrent transactions.
    • Ensures data used by a transaction is not accessed by another transaction until the first one completes.
    • Each data item in the database is associated with a lock, which is acquired before data access and released after completion.

    Lock Granularity

    • Indicates the portion of a database object locked.
    • Database Level Locking: Locks the entire database, preventing access to any tables. It is unsuitable for multiuser DBMS due to slow data access time.
    • Table Level Locking: Locks the entire table, preventing access to any rows. It is better than database level locking but can cause traffic jams with multiple transactions waiting.

    Shared/Exclusive Locks (Read/Write Locks)

    • Shared Lock: Allows multiple transactions to read the same data concurrently.
    • Exclusive Lock: Provides exclusive write access to a data item, blocking other transactions from accessing it.
    • Upgrading: Converting a shared lock to an exclusive lock.
    • Downgrading: Converting an exclusive lock to a shared lock.

    Preventing Data Inconsistencies

    • Locks prevent data inconsistency, but can introduce problems:
      • Non-serializable transaction schedules.
      • Deadlocks, where transactions perpetually wait for each other to unlock data.

    Two-Phase Locking (2PL)

    • Ensures transaction schedules are serializable.
    • Divides transactions into two phases: growing (acquiring locks) and shrinking (releasing locks).
    • All locking operations occur before the first unlock, guaranteeing serializability.

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    Related Documents

    Chapter 4.pdf

    Description

    Explore the various locking mechanisms used in databases, including disk page locks and row-level locks. This quiz covers locking protocols such as simple locking, two-phase locking, and graph-based locking, providing a comprehensive understanding of how concurrent access is managed. Test your knowledge on these essential concepts in modern Database Management Systems.

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