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Questions and Answers
Which statement best describes row-level locking?
Which statement best describes row-level locking?
- It allows transactions to access different rows on the same page without restriction. (correct)
- It allows concurrent transactions to access the same row simultaneously.
- It locks entire pages, preventing access to any row on the locked page.
- It requires minimal overhead compared to other locking methods.
What is a characteristic of binary locks?
What is a characteristic of binary locks?
- They automatically escalate to page-level locks.
- They require complex management to track multiple states.
- They prevent other transactions from accessing locked items. (correct)
- They can have multiple values beyond locked and unlocked.
Which locking protocol involves changing a lock's type based on the session's needs?
Which locking protocol involves changing a lock's type based on the session's needs?
- Simple locking
- Graph based locking
- Row-level locking
- Two phase locking (correct)
What is the primary function of the lock_item operation?
What is the primary function of the lock_item operation?
In which scenario would a transaction be forced to wait when using binary locks?
In which scenario would a transaction be forced to wait when using binary locks?
What distinguishes page-level locks from row-level locks?
What distinguishes page-level locks from row-level locks?
Which type of locking provides the least amount of concurrency and flexibility?
Which type of locking provides the least amount of concurrency and flexibility?
What is a significant management challenge of row-level locking?
What is a significant management challenge of row-level locking?
What is the primary purpose of acquiring a lock before accessing a data item during transaction execution?
What is the primary purpose of acquiring a lock before accessing a data item during transaction execution?
What does lock granularity refer to in the context of locking mechanisms?
What does lock granularity refer to in the context of locking mechanisms?
Which type of lock prevents all other transactions from accessing any rows within a table until the lock is released?
Which type of lock prevents all other transactions from accessing any rows within a table until the lock is released?
What are the drawbacks of database level locking?
What are the drawbacks of database level locking?
Which statement best describes the advantage of using row-level locking?
Which statement best describes the advantage of using row-level locking?
What is a shared lock primarily used for in transaction management?
What is a shared lock primarily used for in transaction management?
Which locking protocol would be most effective in minimizing transaction wait times in a multiuser environment?
Which locking protocol would be most effective in minimizing transaction wait times in a multiuser environment?
What is the main disadvantage of using binary locks?
What is the main disadvantage of using binary locks?
What is the primary difference between a shared lock and an exclusive lock?
What is the primary difference between a shared lock and an exclusive lock?
Which statement is true regarding the upgrading of a lock from shared to exclusive?
Which statement is true regarding the upgrading of a lock from shared to exclusive?
What condition must be met for a transaction schedule to be guaranteed serializable?
What condition must be met for a transaction schedule to be guaranteed serializable?
In the context of locking, what does the shrinking phase signify?
In the context of locking, what does the shrinking phase signify?
Which of the following can lead to deadlocks in transactions?
Which of the following can lead to deadlocks in transactions?
What is the purpose of locks in transaction management?
What is the purpose of locks in transaction management?
Which locking operation allows a transaction to read an item while other transactions can also read it?
Which locking operation allows a transaction to read an item while other transactions can also read it?
What is the consequence of improperly implementing locking protocols in transactions?
What is the consequence of improperly implementing locking protocols in transactions?
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Study Notes
Lock Granularity
- Disk Page Lock: Locks an entire page on a disk.
- Row Level Lock: Locks a single row, allowing concurrent access to different rows on the same page.
- Modern DBMS automatically changes a lock from row-level to page-level when a session requests multiple locks on the same page, improving efficiency.
Types of Locking Protocols
- Simple Locking: A basic locking mechanism where an object is either locked (1) or unlocked (0).
- Two Phase Locking: More robust protocol with two phases:
- Growing Phase: Acquiring new locks but not releasing any.
- Shrinking Phase: Releasing existing locks but not acquiring any new ones.
- Graph Based Locking: A more complex approach where locks are represented as nodes in a graph, used in advanced scenarios.
- Binary Locks: A type of lock with only two states: locked (1) or unlocked (0). They enable transactions to acquire locks for exclusive use.
- Time stamp techniques: Alternatives to locking, using timestamps to ensure data accuracy.
- Multi-version techniques: Maintain multiple versions of data to allow concurrent access without conflicts.
Locking the Data Item
- Prevents data inconsistency during concurrent transactions.
- Ensures data used by a transaction is not accessed by another transaction until the first one completes.
- Each data item in the database is associated with a lock, which is acquired before data access and released after completion.
Lock Granularity
- Indicates the portion of a database object locked.
- Database Level Locking: Locks the entire database, preventing access to any tables. It is unsuitable for multiuser DBMS due to slow data access time.
- Table Level Locking: Locks the entire table, preventing access to any rows. It is better than database level locking but can cause traffic jams with multiple transactions waiting.
Shared/Exclusive Locks (Read/Write Locks)
- Shared Lock: Allows multiple transactions to read the same data concurrently.
- Exclusive Lock: Provides exclusive write access to a data item, blocking other transactions from accessing it.
- Upgrading: Converting a shared lock to an exclusive lock.
- Downgrading: Converting an exclusive lock to a shared lock.
Preventing Data Inconsistencies
- Locks prevent data inconsistency, but can introduce problems:
- Non-serializable transaction schedules.
- Deadlocks, where transactions perpetually wait for each other to unlock data.
Two-Phase Locking (2PL)
- Ensures transaction schedules are serializable.
- Divides transactions into two phases: growing (acquiring locks) and shrinking (releasing locks).
- All locking operations occur before the first unlock, guaranteeing serializability.
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