What are the different lock modes used in the locking system? Discuss.
The correct answer and explanation is:
In a database locking system, various lock modes are used to manage concurrent access to data and maintain data consistency. The primary lock modes include:
1. Shared Lock (S)
Allows concurrent transactions to read (but not modify) a data item. Multiple transactions can hold shared locks on the same item simultaneously.
2. Exclusive Lock (X)
Grants a transaction both read and write access to a data item. Only one transaction can hold an exclusive lock on a data item at any time.
3. Update Lock (U)
A hybrid between shared and exclusive locks. Used to prevent deadlocks during an update. A transaction can read the data, but it must upgrade to an exclusive lock to modify it.
4. Intent Locks (IS, IX, SIX)
Used in multi-granularity locking to indicate a transaction’s intention to acquire a lock at a finer level.
- Intent Shared (IS): Intention to acquire shared locks on lower-level resources.
- Intent Exclusive (IX): Intention to acquire exclusive locks on lower-level resources.
- Shared with Intent Exclusive (SIX): Holds a shared lock on the object and intends to acquire exclusive locks on lower-level items.
Explanation (Approx. 300 Words)
Lock modes in a database management system are vital for ensuring consistency and isolation in concurrent transaction processing. When multiple users access the database simultaneously, locks help prevent conflicts, such as lost updates or dirty reads.
The shared lock permits multiple transactions to read a resource simultaneously, ensuring no transaction modifies it during the read. This is efficient for read-heavy systems. Conversely, the exclusive lock is stricter—it allows the transaction to both read and write a resource, but no other transaction can access it in any way until the lock is released.
The update lock is especially useful in preventing deadlocks. Without it, two transactions might each obtain a shared lock and then wait indefinitely to upgrade to exclusive locks. The update lock ensures only one transaction can prepare to write, reducing the chance of circular waiting.
Intent locks are used in hierarchical locking structures. They signal a transaction’s intention to lock lower-level resources before it actually does so. For instance, before locking a row in a table, a transaction might place an Intent Shared (IS) or Intent Exclusive (IX) lock on the table. This prevents other transactions from acquiring a conflicting lock on the entire table while lower-level locks are active.
By using these different lock modes and combining them wisely, the database ensures optimal concurrency while maintaining data integrity and preventing deadlocks and conflicts during transaction execution.