lockingtransactions

Locking transactions are a fundamental mechanism used in database management systems to ensure data consistency and integrity when multiple users or processes attempt to access and modify the same data concurrently. The primary goal of locking is to prevent race conditions and lost updates, which can occur if operations are interleaved in an undesirable way.

When a transaction needs to access a piece of data, it can request a lock on that data. There are typically two main types of locks: shared locks and exclusive locks. A shared lock, often denoted as 'S', allows multiple transactions to read the data simultaneously but prevents any transaction from modifying it. An exclusive lock, denoted as 'X', grants a single transaction exclusive access to the data, preventing any other transaction from reading or writing to it.

When a transaction acquires an exclusive lock, no other transaction can acquire any type of lock on that data until the first transaction releases its lock. If a transaction requests a lock that is incompatible with an existing lock (e.g., requesting an exclusive lock when a shared lock is already held), the transaction must wait. This waiting period is known as blocking.

The process of acquiring and releasing locks is managed by the database's concurrency control mechanism. Locks are typically released when the transaction completes, either through a commit or a rollback. While locking is essential for data integrity, excessive or improper locking can lead to performance issues, such as increased waiting times for transactions and the potential for deadlocks, where two or more transactions are blocked indefinitely, each waiting for a resource held by the other. Database systems employ various strategies to detect and resolve deadlocks.