Database Deadlocks and Concurrency Control
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Database Deadlocks and Concurrency Control

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Questions and Answers

What is a disadvantage of using time stamping methods in concurrency control?

  • Decreases database resource demand
  • Increases processing overhead (correct)
  • Requires only one additional stamp field
  • Reduces memory needs

    • In the Wait/Die scheme, what happens to an older transaction if it cannot get the required locks?

  • It dies and is rescheduled (correct)
  • It continues with its operations
  • It is immediately canceled
  • It forces the younger transaction to release the locks

    • What is a key feature of the Wound/Wait scheme?

  • All transactions are allowed to access the locks simultaneously
  • Older transactions must wait for the younger ones to complete
  • Older transactions can request locks and preempt younger transactions (correct)
  • Younger transactions can preempt older transactions

    • What impact do time stamping methods have on a database’s memory requirements?

    <p>They require more memory due to additional fields</p> Signup and view all the answers

    Which of the following accurately describes the Wait/Die strategy?

    <p>Younger transactions are rolled back if older transactions cannot obtain locks</p> Signup and view all the answers

    What causes a deadlock in transaction management?

    <p>When two transactions wait indefinitely for each other to unlock data.</p> Signup and view all the answers

    Which term is commonly used to refer to a deadlock?

    <p>Deadly embrace</p> Signup and view all the answers

    Which method is NOT a control technique for managing deadlocks?

    <p>Deadlock termination</p> Signup and view all the answers

    What is the primary function of time stamping in transaction management?

    <p>To assign a unique global time stamp to ensure order.</p> Signup and view all the answers

    Which property of time stamping ensures that no two transactions have the same time stamp value?

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

    Under which condition would deadlock control methods be selected?

    <p>Depending on the database environment.</p> Signup and view all the answers

    Which time stamping property ensures that the values always increase over time?

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

    What can be considered a classification of deadlock control methods?

    <p>Prevention, detection, and avoidance</p> Signup and view all the answers

    What does atomicity ensure in a transaction?

    <p>All operations of a transaction must be completed or none at all.</p> Signup and view all the answers

    Which property guarantees that once transactions are committed, they cannot be undone?

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

    What is the consequence of an improper or incomplete transaction?

    <p>It can corrupt the database integrity.</p> Signup and view all the answers

    Which of the following provides consistency in a database?

    <p>Database remains in a consistent state after transactions.</p> Signup and view all the answers

    What does the isolation property of transactions imply?

    <p>Transactions can run without interferences from others.</p> Signup and view all the answers

    Which property ensures consistent results from concurrent transactions?

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

    How can users define constraints in a database?

    <p>Through enforceable business rules.</p> Signup and view all the answers

    What is not a characteristic of transactions in a database?

    <p>Transactions must be reversible in all cases.</p> Signup and view all the answers

    What is the first phase in optimistic concurrency control?

    <p>Read phase</p> Signup and view all the answers

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

    <p>To ensure changes do not affect database integrity</p> Signup and view all the answers

    Which transaction isolation level allows reading uncommitted data?

    <p>Dirty read</p> Signup and view all the answers

    What characterizes a nonrepeatable read?

    <p>Reading a row and getting different results from the same query</p> Signup and view all the answers

    During which phase are changes applied permanently to the database?

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

    What does the term 'phantom read' refer to?

    <p>Executing a query and encountering new matching rows later</p> Signup and view all the answers

    How are transaction isolation levels defined?

    <p>By the type of 'reads' allowed</p> Signup and view all the answers

    Which of the following is NOT a phase in optimistic concurrency control?

    <p>Isolation phase</p> Signup and view all the answers

    What happens if a transaction was committed before the last check point?

    <p>Nothing needs to be done.</p> Signup and view all the answers

    What is the purpose of ROLLBACK in a transaction?

    <p>To restore the database to a previous state.</p> Signup and view all the answers

    Which property ensures that transactions are completed fully or not at all?

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

    In the context of database transactions, what does a lock provide?

    <p>Unique access to a data item by one transaction.</p> Signup and view all the answers

    What is the role of the transaction log in recovery?

    <p>It helps to redo or rollback transactions after a failure.</p> Signup and view all the answers

    What does the two-phase locking protocol guarantee?

    <p>Serializability of transaction schedules.</p> Signup and view all the answers

    Which of the following is NOT a property of transactions?

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

    What must happen if a transaction had a ROLLBACK operation after the last check point?

    <p>The DBMS uses the transaction log to undo operations.</p> Signup and view all the answers

    Study Notes

    Deadlocks

    • Occur when two transactions wait indefinitely for each other to unlock data
    • Also known as deadly embrace
    • Control techniques include deadlock prevention, deadlock detection, and deadlock avoidance
    • Choice of deadlock control method depends on the database environment

    Concurrency Control With Time Stamping Methods

    • Assigns a global, unique time stamp to each transaction
    • Produces an explicit order in which transactions are submitted to the DBMS
    • Properties include uniqueness (no equal time stamps) and monotonicity (time stamp values always increase)
    • Disadvantages include:
      • Two additional timestamp fields needed for each value stored
      • Increased memory needs
      • Increased processing overhead
      • Demands many system resources

    Wait/Die and Wound/Wait Schemes

    • Wait/Die: an older transaction waits for a younger transaction to finish before requesting locks. If not, the newer transaction is rescheduled.
    • Wound/Wait: an older transaction can request the lock, preempt the younger transaction, and reschedule it. If not, the younger transaction waits.

    Concurrency Control with Optimistic Methods

    • Concurrency control approach assumes that conflicts between transactions are rare
    • Read Phase:
      • Transaction reads database
      • Executes computations
      • Updates private copy of database values
    • Validation Phase:
      • Transaction validated to ensure changes do not affect database integrity or consistency
    • Write Phase:
      • Changes permanently applied to the database

    ANSI Levels of Transaction Isolation

    • Defines transaction management based on transaction isolation levels
    • Isolation levels denote the degree to which transaction data is protected from other concurrent transactions
    • Described by the type of "reads" a transaction allows or not
    • Types of "reads":
      • Dirty Read: transaction reads data not yet committed
      • Nonrepeatable Read: transaction reads a row at time t1 and then at time t2 gets different results (original row may have been updated or deleted)
      • Phantom Read: transaction executes a query at time t1 then at time t2 gets additional rows that satisfy the query

    Transaction Recovery

    • Recovery process steps:
      • Identify the last checkpoint in the transaction log
      • If committed before the checkpoint, no action needed
      • If committed after the checkpoint, the transaction log is used to redo the transaction
      • If a ROLLBACK occurred after the last checkpoint, the DBMS uses the transaction log records to ROLLBACK or undo the operations by using the “before” values in the transaction log

    Summary

    • A transaction is a sequence of database operations that access the database
    • Transactions have four primary properties: atomicity, consistency, isolation, and durability
    • SQL supports transactions using COMMIT (saving changes to disk) and ROLLBACK (restoring the previous database state)
    • Concurrency control manages the simultaneous execution of transactions
    • A lock guarantees unique access to a data item by a transaction
    • Serializability is guaranteed through two-phase locking

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    Chp10.pdf

    Description

    Explore the concepts of deadlocks, concurrency control methods, and their impact on database management systems. This quiz covers techniques like wait/die and wound/wait schemes, as well as timestamping methods. Test your understanding of how these mechanisms help manage simultaneous transactions effectively.

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