Locks can be classified based on size, duration and mode.
In DB2, locks can be classified based on various criteria. The most common classification is based on the scope and duration of the lock. Here are the main types of locks in DB2:
- Shared Locks (S): These locks are used for read operations. Multiple transactions can hold shared locks on the same resource concurrently, allowing for multiple readers.
- Exclusive Locks (X): These locks are used for write operations. Only one transaction can hold an exclusive lock on a resource at a time, preventing other transactions from accessing the resource simultaneously.
- Intention Locks (I): These locks indicate the intention of a transaction to acquire a certain type of lock on a resource. Intention locks help in coordinating and preventing conflicts between transactions.
- Intent Share Locks (IS): Indicates the intention to acquire a shared lock on a resource.
- Intent Exclusive Locks (IX): Indicates the intention to acquire an exclusive lock on a resource.
- Update Locks (U): A combination of shared and exclusive locks, used to indicate an intention to update a resource. It allows other transactions to hold shared locks but prevents them from acquiring exclusive locks.
- Next-Key Locks (N): Used in index scans to prevent phantom reads. It locks the current index entry and the “gap” to the next index entry.
- Read Stability Locks (RS): Provides a level of isolation for read operations, preventing other transactions from acquiring write locks on the same resource until the transaction holding the read stability lock completes.
- Cursor Stability Locks (CS): Similar to read stability locks but also allows the transaction holding the lock to reposition the cursor within a result set.
These locks play a crucial role in ensuring data consistency and isolation in a multi-user environment. The choice of lock type depends on the specific requirements of the application and the desired balance between concurrency and data integrity.