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