Two-Phase Locking Techniques
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Questions and Answers

What is the primary purpose of concurrency control protocols?

  • To increase transaction execution speed
  • To guarantee serializability (correct)
  • To reduce network latency
  • To manage memory allocation

    • Which of the following accurately describes binary locks?

  • They are only applicable to read operations.
  • They have three states: locked, unlocked, and pending.
  • They can be in either locked (1) or unlocked (0) states. (correct)
  • They allow multiple transactions to access data simultaneously.

    • What operation must a transaction perform to access an item in two-phase locking?

  • lock_item(X) (correct)
  • change_item(X)
  • access_item(X)
  • read_item(X)

    • What distinguishes shared locks from exclusive locks in a two-phase locking system?

    <p>Shared locks allow only read access while exclusive locks allow both read and write access.</p> Signup and view all the answers

    What is the role of a lock manager in concurrency control?

    <p>To track and control access to locks</p> Signup and view all the answers

    Why might binary locking be considered too restrictive for database items?

    <p>Only one transaction can access an item at any time.</p> Signup and view all the answers

    Which operation must be performed to release a lock on a data item?

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

    What do multiversion concurrency control protocols utilize?

    <p>Multiple versions of a data item</p> Signup and view all the answers

    What is the primary purpose of generating timestamps in transaction management?

    <p>To maintain the order of transaction submissions</p> Signup and view all the answers

    Which statement is true regarding concurrency control techniques based on timestamps?

    <p>They do not use locks, thus preventing deadlocks.</p> Signup and view all the answers

    What happens in the basic timestamp ordering algorithm when conflicting operations are detected?

    <p>The later operation is rejected by aborting the transaction that issued it.</p> Signup and view all the answers

    Which mechanism allows interleaving of transaction operations while still maintaining order?

    <p>Timestamp ordering</p> Signup and view all the answers

    What is the role of read_TS(X) and write_TS(X) in timestamp-based concurrency control?

    <p>They represent the timestamps for read and write operations on a database item.</p> Signup and view all the answers

    Which statement accurately describes Thomas’s write rule?

    <p>It rejects fewer write operations than the basic TO algorithm.</p> Signup and view all the answers

    What does the strict timestamp ordering algorithm guarantee?

    <p>It guarantees schedules are both strict and conflict serializable.</p> Signup and view all the answers

    What is a potential issue that can arise with the basic timestamp ordering algorithm?

    <p>Starvation of certain transactions</p> Signup and view all the answers

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

    <p>To guarantee that transactions are serializable</p> Signup and view all the answers

    During which phase of two-phase locking can new locks be acquired but none can be released?

    <p>Expanding (growing) phase</p> Signup and view all the answers

    What must occur for a transaction to convert a lock from one state to another?

    <p>The transaction must already hold the lock</p> Signup and view all the answers

    Which variation of two-phase locking requires a transaction to lock all items before starting?

    <p>Conservative (static) 2PL</p> Signup and view all the answers

    Which type of locking allows transactions to hold exclusive locks until they commit or abort?

    <p>Strict 2PL</p> Signup and view all the answers

    What is a limitation of the two-phase locking protocol?

    <p>It can limit the concurrency of transactions</p> Signup and view all the answers

    What is the correct action during a downgrading procedure in lock management?

    <p>Issue a read_lock after a write_lock operation</p> Signup and view all the answers

    What happens if a schedule does not follow the two-phase locking protocol?

    <p>The schedule can lead to nonserializable transactions</p> Signup and view all the answers

    What does granularity of data items refer to?

    <p>The size of data items</p> Signup and view all the answers

    How does the size of data items affect concurrency?

    <p>Larger data items lower the degree of concurrency</p> Signup and view all the answers

    What is the purpose of intention locks in multiple granularity locking?

    <p>To indicate the locking intention at different levels</p> Signup and view all the answers

    Which intention lock type indicates that shared locks will be requested on a descendant node?

    <p>Intention-shared (IS)</p> Signup and view all the answers

    In the context of multiple granularity locking, what does an exclusive lock typically allow?

    <p>Write access by the current transaction only</p> Signup and view all the answers

    What potential issue can arise from holding locks on index pages?

    <p>Transaction blocking</p> Signup and view all the answers

    What is a conservative approach to locking in indexes?

    <p>Locking the root node in exclusive mode and then accessing child nodes</p> Signup and view all the answers

    Which lock type indicates that exclusive locks will be requested on a descendant node while the current node is locked in shared mode?

    <p>Shared-intention-exclusive (SIX)</p> Signup and view all the answers

    What is one advantage of using multiversion concurrency control techniques?

    <p>They can accept read operations that would otherwise be rejected.</p> Signup and view all the answers

    Which of the following statements accurately describes multiversion two-phase locking?

    <p>It utilizes three locking modes: read, write, and certify.</p> Signup and view all the answers

    In optimistic concurrency control, when are updates applied to the database?

    <p>Once the transaction is validated and finished.</p> Signup and view all the answers

    What characteristic defines snapshot isolation concurrency control?

    <p>Transactions see data items based on committed values at the time the snapshot is created.</p> Signup and view all the answers

    How does multiversion concurrency control maintain serializability?

    <p>By keeping multiple versions of data and permitting reads from older versions.</p> Signup and view all the answers

    What is the primary purpose of the validation phase in optimistic concurrency control?

    <p>To confirm that the transaction’s updates do not violate serializability.</p> Signup and view all the answers

    What additional storage requirement is associated with multiversion concurrency control?

    <p>All versions of the items being accessed must be stored.</p> Signup and view all the answers

    What is one key difference between snapshot isolation and traditional locking mechanisms?

    <p>Snapshot isolation does not see updates occurring after a transaction starts.</p> Signup and view all the answers

    Study Notes

    Two-Phase Locking Techniques for Concurrency Control

    • Lock: Used in the database to identify the status of items and operations
    • Binary locks: Two states for locks: Locked (1), indicating an item cannot be accessed, and Unlocked (0), indicating the item is available for access.
    • Lock table: A table specifying items with locks, managed by the lock manager subsystem.
    • Shared/exclusive or read/write locks: Allows multiple transactions to read the data simultaneously but only one transaction to write to the data.
    • Lock conversion: Allows a transaction to change its lock from one state to another, such as upgrading from read to write or downgrading from write to read.
    • Guaranteeing Serializability by Two-Phase Locking: The two-phase locking protocol ensures serializability by dividing transaction operations into two phases: an expanding phase where locks are acquired and a shrinking phase where locks are released.
    • Variations of Two-Phase Locking: Three variations of the two-phase locking protocol:
      • Basic 2PL: The standard two-phase locking technique.
      • Conservative (static) 2PL: Requires all items to be locked before the transaction starts.
      • Strict 2PL: Transactions hold exclusive locks until the transaction commits or aborts.

    Concurrency Control Based on Timestamp Ordering

    • Timestamp: Each transaction is assigned a unique identifier, the timestamp, typically generated in the order the transactions were submitted.
    • Concurrency control techniques based on timestamps: These techniques do not rely on locks, minimizing the possibility of deadlocks.
    • Timestamp Ordering (TO) algorithm: This algorithm enforces serial order of conflicting operations through timestamps.
    • Strict TO algorithm: This algorithm ensures that schedules are conflict serializable and follow strict constraints.

    Multiversion Concurrency Control Techniques

    • The system maintains multiple versions of a data item, allowing read operations to access older versions and achieve higher concurrency.
    • Multiversion techniques based on timestamp ordering: Each version has timestamps associated with its read and write operations.
    • Multiversion two-phase locking using certify locks: Adds a "certify" lock mode to read/write locking, allowing transactions to be validated after they have acquired a lock.

    Validation (Optimistic) Techniques and Snapshot Isolation Concurrency Control

    • Optimistic techniques: Also known as validation or certification techniques, they perform validation checks after a transaction is finished to ensure serializability.
    • Concurrency Control Based on Snapshot Isolation: This technique creates a snapshot of the database when a transaction starts and uses this snapshot for all the transaction operations.
    • Serializable Snapshot Isolation (SSI): A variation of snapshot isolation that ensures that transactions are serializable.

    Granularity of Data Items and Multiple Granularity Locking

    • Granularity: The size of the data items to be locked.
    • Multiple Granularity Level Locking (MGL): Allows transactions to acquire locks at different levels, providing flexibility for managing access to data.
    • Intention locks (IS, IX, SIX): Indicate the intention of a transaction to acquire locks at lower levels of the hierarchy, such as shared (IS) or exclusive (IX) locks.

    Using Locks for Concurrency Control in Indexes

    • Concurrency Control Using Locks in Indexes: The two-phase locking protocol can be extended to indexes, such as B-trees and B+ trees.
    • Conservative approach: This approach locks the root node exclusively and accesses the appropriate child node as needed.

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    Chapter21.pdf

    Description

    Dive into the concepts of two-phase locking techniques used in concurrency control for databases. This quiz covers essential topics like lock types, lock conversion, and the significance of serializability. Test your understanding of how locking mechanisms ensure data integrity during transactions.

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