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Questions and Answers
What is a disadvantage of using time stamping methods in concurrency control?
In the Wait/Die scheme, what happens to an older transaction if it cannot get the required locks?
What is a key feature of the Wound/Wait scheme?
What impact do time stamping methods have on a database’s memory requirements?
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Which of the following accurately describes the Wait/Die strategy?
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What causes a deadlock in transaction management?
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Which term is commonly used to refer to a deadlock?
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Which method is NOT a control technique for managing deadlocks?
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What is the primary function of time stamping in transaction management?
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Which property of time stamping ensures that no two transactions have the same time stamp value?
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Under which condition would deadlock control methods be selected?
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Which time stamping property ensures that the values always increase over time?
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What can be considered a classification of deadlock control methods?
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What does atomicity ensure in a transaction?
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Which property guarantees that once transactions are committed, they cannot be undone?
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What is the consequence of an improper or incomplete transaction?
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Which of the following provides consistency in a database?
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What does the isolation property of transactions imply?
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Which property ensures consistent results from concurrent transactions?
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How can users define constraints in a database?
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What is not a characteristic of transactions in a database?
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What is the first phase in optimistic concurrency control?
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What is the purpose of the validation phase in optimistic concurrency control?
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Which transaction isolation level allows reading uncommitted data?
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What characterizes a nonrepeatable read?
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During which phase are changes applied permanently to the database?
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What does the term 'phantom read' refer to?
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How are transaction isolation levels defined?
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Which of the following is NOT a phase in optimistic concurrency control?
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What happens if a transaction was committed before the last check point?
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What is the purpose of ROLLBACK in a transaction?
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Which property ensures that transactions are completed fully or not at all?
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In the context of database transactions, what does a lock provide?
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What is the role of the transaction log in recovery?
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What does the two-phase locking protocol guarantee?
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Which of the following is NOT a property of transactions?
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What must happen if a transaction had a ROLLBACK operation after the last check point?
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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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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.
