Concurrency Control Techniques Overview
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What is the primary purpose of concurrency control protocols in database management?

  • To encrypt data transactions
  • To backup data automatically
  • To enhance system performance
  • To guarantee serializability (correct)

    • What are binary locks used for in concurrency control?

  • To determine if an item can be accessed or not (correct)
  • To secure only read access to data items
  • To enhance the speed of data retrieval
  • To lock multiple data items simultaneously

    • What is a unique identifier assigned to each transaction in concurrency control?

  • Lock table
  • Timestamp (correct)
  • Version control
  • Lock manager

    • Which locking protocol requires an exclusive lock for writing operations?

    <p>Shared/exclusive locking</p> Signup and view all the answers

    What operation does a transaction issue to obtain access to a data item?

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

    What role does the lock manager subsystem play in locking mechanisms?

    <p>It controls and tracks access to locks</p> Signup and view all the answers

    How many transactions can hold a lock on a data item at one time in a two-phase locking protocol?

    <p>Only one transaction at a time</p> Signup and view all the answers

    Which of the following statements is true regarding read operations under shared/exclusive locks?

    <p>Multiple read operations can occur on the same item</p> Signup and view all the answers

    What type of lock must be obtained before performing a deletion operation on an existing data item?

    <p>Write lock</p> Signup and view all the answers

    Which concurrency control technique allows for shared locks when requesting a page?

    <p>B-link tree approach</p> Signup and view all the answers

    What is the phantom problem in concurrency control?

    <p>Insertion of records that satisfy accessed conditions by another transaction</p> Signup and view all the answers

    Which of the following describes latches in the context of concurrency control?

    <p>Temporary locks that do not follow usual protocols</p> Signup and view all the answers

    What is a key feature of the interactive transactions described in the concurrency control context?

    <p>User inputs depend on uncommitted transactions</p> Signup and view all the answers

    What is NOT included in the summary of concurrency control techniques?

    <p>Lock-free methods</p> Signup and view all the answers

    In the optimistic locking approach described, which type of lock is held exclusively on the leaf node?

    <p>Exclusive lock</p> Signup and view all the answers

    When should the output of a transaction be postponed until it is committed?

    <p>When user input is based on another uncommitted transaction's output</p> Signup and view all the answers

    What is the primary function of timestamps in concurrency control?

    <p>To enforce equivalent serial order on transactions</p> Signup and view all the answers

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

    <p>The transaction that issues the later operation is aborted</p> Signup and view all the answers

    How are timestamps generated to avoid conflicts?

    <p>Through a counter that increments with each assignment</p> Signup and view all the answers

    What is a characteristic of strict timestamp ordering?

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

    What issue may occur in the basic timestamp ordering algorithm apart from conflicts?

    <p>Starvation</p> Signup and view all the answers

    In the context of timestamped transactions, what does the term 'read_TS(X)' refer to?

    <p>The timestamp for the last read operation on item X</p> Signup and view all the answers

    Which statement accurately describes Thomas's write rule?

    <p>It allows some non-serializable schedules</p> Signup and view all the answers

    Which is an advantage of using timestamp-based concurrency control techniques?

    <p>They prevent deadlocks entirely</p> Signup and view all the answers

    What characterizes rigorous two-phase locking (2PL)?

    <p>No locks are released until after a transaction commits or aborts.</p> Signup and view all the answers

    What is a deadlock in the context of transactions?

    <p>A situation where all transactions are waiting for a resource held by another.</p> Signup and view all the answers

    Which protocol ensures all needed locks are acquired before a transaction begins?

    <p>Deadlock prevention protocols</p> Signup and view all the answers

    How does the 'wait-die' timestamp protocol function?

    <p>Older transactions take priority, resulting in younger ones being aborted.</p> Signup and view all the answers

    What does the wait-for graph help determine?

    <p>Which transactions are currently in a deadlock state.</p> Signup and view all the answers

    Which of the following best describes starvation?

    <p>Transactions cannot proceed indefinitely while others complete.</p> Signup and view all the answers

    What is the aim of implementing a cautious waiting algorithm?

    <p>To avoid deadlock situations.</p> Signup and view all the answers

    What happens in a timeout protocol?

    <p>Transactions are aborted if they take too long to acquire a lock.</p> Signup and view all the answers

    What characterizes the expanding phase of the two-phase locking protocol?

    <p>New locks can be acquired but none can be released.</p> Signup and view all the answers

    What is the purpose of lock conversion in two-phase locking?

    <p>To change the state of a lock held by a transaction.</p> Signup and view all the answers

    In which phase of two-phase locking can downgrading be performed?

    <p>Only during the shrinking phase.</p> Signup and view all the answers

    Which statement is true regarding conservative (static) two-phase locking?

    <p>It helps in preventing deadlocks by requiring predeclaration of read and write sets.</p> Signup and view all the answers

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

    <p>It may prohibit some serializable schedules.</p> Signup and view all the answers

    What is true about strict two-phase locking?

    <p>Exclusive locks must be held until after a transaction commits or aborts.</p> Signup and view all the answers

    What operation typically occurs first in lock upgrading?

    <p>Issuing a read_lock operation.</p> Signup and view all the answers

    In two-phase locking, what happens during the shrinking phase?

    <p>Existing locks can be released, but no new locks can be acquired.</p> Signup and view all the answers

    Study Notes

    Concurrency Control Techniques

    • Concurrency control protocols are essential for guaranteeing serializability in database systems.
    • Two-phase locking protocols ensure data integrity by preventing concurrent access to data items through locking mechanisms.
    • Timestamp-based concurrency control assigns unique identifiers to each transaction and utilizes these timestamps for order enforcement and conflict resolution.
    • Multiversion concurrency control techniques maintain multiple versions of data items to manage concurrent access.
    • Validation or certification of transactions verifies the consistency of data modifications made by individual transactions.

    Two-Phase Locking

    • A lock is a variable associated with a data item that controls access to that item.
    • Binary locks have two states: locked (1) and unlocked (0). When locked, no access is allowed. When unlocked, the item is available for access.
    • A transaction requests access to a data item by issuing a lock_item(X) operation.
    • Shared/exclusive locks (read/write locks) allow multiple transactions to read the same data item simultaneously but only one transaction to write to it at a time.
    • Lock conversion allows a transaction with an existing lock to convert it into a different type of lock. Upgrading from read_lock to write_lock is possible, as well as downgrading from write_lock to read_lock.

    Guaranteeing Serializability

    • The two-phase locking protocol ensures serializability by enforcing a specific order for locking and unlocking operations.
    • It consists of two phases:
      • Expanding phase: New locks can be acquired, but existing locks cannot be released.
      • Shrinking phase: Existing locks can be released, but no new locks can be acquired.
    • If every transaction in a schedule adheres to the two-phase locking protocol, the schedule is guaranteed to be serializable.
    • While two-phase locking enhances concurrency, it may impose limitations on the degree of concurrency permitted.

    Variations of Two-Phase Locking

    • Different variations of two-phase locking exist, offering trade-offs between concurrency and deadlock prevention:
      • Basic 2PL: The fundamental two-phase locking model.
      • Conservative (static) 2PL: Requires that a transaction lock all the items it needs before it begins, leading to deadlock-free behavior.
      • Strict 2PL: Exclusive locks are not released until the transaction commits or aborts.
      • Rigorous 2PL: All locks are held until the transaction commits or aborts, ensuring strictness and synchronization.

    Deadlock and Starvation

    • Deadlock occurs when transactions in a set are waiting for resources locked by other transactions, creating a circular dependency resulting in a stalemate.
    • Deadlock prevention protocols aim to avoid deadlock by ensuring that transactions lock resources in a specific order or lock all necessary resources before starting.
    • Timestamp-based protocols (wait-die and wound-wait) utilize timestamps for resolving conflicts and preventing deadlock.
    • No waiting algorithms abort transactions that cannot acquire a lock immediately.
    • Cautious waiting algorithms are deadlock-free by controlling the order of lock requests.
    • Deadlock detection involves identifying deadlock states within the system.
    • Victim selection determines which transaction is aborted in a deadlock situation.
    • Starvation happens when a transaction is indefinitely delayed while other transactions proceed, potentially requiring solutions like first-come-first-served queues.

    Concurrency Control Based on Timestamp Ordering

    • Timestamps are unique identifiers assigned to transactions, typically based on the order of submission or the system clock.
    • Timestamp ordering (TO) concurrency control enforces serializability without using locks, avoiding deadlock.
    • Database items are assigned two timestamps: read_TS(X) and write_TS(X) to track access history.
    • The basic TO algorithm rejects conflicting operations by aborting the transaction with the later timestamp.
    • The strict TO algorithm guarantees strict and conflict serializability.
    • Thomas’s write rule is a modification of the basic TO algorithm that rejects fewer write operations, relaxing the strictness of conflict serializability.

    Concurrency Control and Indexes

    • Indexes are frequently used to speed up data retrieval, and locking strategies are critical for ensuring their integrity and consistency during concurrent access.
    • Locking can be applied at different granularities, ranging from individual nodes to entire pages or index structures.
    • Pessimistic locking in indexes involves acquiring locks before attempting to access data. This ensures exclusive access, but potentially limits concurrency.
    • Optimistic locking assumes that conflicts are rare and only acquires locks when necessary, balancing concurrency and potential for conflicts.
    • B-link tree approaches utilize linked sibling nodes to reduce the need for locking, allowing more concurrent access by sharing locks across sibling nodes.

    Other Concurrency Control Issues

    • Insertion and deletion operations require specific locking mechanisms to maintain data integrity.
    • The phantom problem arises when a new record is inserted that satisfies the conditions of an existing transaction, creating a conflict that is not recognized by the concurrency control protocol.
    • Interactive transactions present challenges as user interactions can introduce unpredictable dependencies between transactions, requiring mechanisms to manage the order of output updates.
    • Latches are short-duration locks used for specific, limited access to resources, potentially deviating from traditional concurrency control protocols.

    Summary

    • Concurrency control techniques are crucial for ensuring data consistency and managing concurrent access in database systems.
    • Techniques like two-phase locking, timestamp-based ordering, multiversion protocols, and snapshot isolation have varying advantages and disadvantages in terms of concurrency, deadlock prevention, and complexity.
    • Locking can be applied at different granularities, impacting performance and concurrency.
    • Indexes require specific locking strategies to ensure their integrity and efficiency.
    • The phantom problem and challenges with interactive transactions highlight the need for careful consideration of concurrency control mechanisms in real-world database applications.

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    Related Documents

    Chapter21.pdf

    Description

    Explore essential concurrency control techniques used in database systems, including two-phase locking, timestamp-based methods, and multiversion control. Understand how these protocols ensure data integrity and consistency through effective management of concurrent access.

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