How does a database lock work?

A database lock works by preventing simultaneous access to data, ensuring data integrity during transactions.

In a database system, multiple users often access and manipulate data simultaneously. This concurrent access can lead to various data inconsistencies and conflicts. To prevent such issues, database management systems (DBMS) use a mechanism known as 'locking'.

Locking is a method used to serialise access to shared resources, in this case, the data in a database. When a user starts a transaction that reads or modifies data, the DBMS places a lock on the relevant data items. This lock prevents other users from accessing or modifying the data until the initial transaction is complete.

There are two main types of locks: shared locks and exclusive locks. A shared lock allows multiple transactions to read (but not modify) the same data item simultaneously. On the other hand, an exclusive lock allows only one transaction to read and modify a data item at a time.

Locking is crucial for maintaining the ACID (Atomicity, Consistency, Isolation, Durability) properties of a database. It ensures that all transactions are atomic (indivisible and irreducible), maintain database consistency, are isolated from each other, and result in durable changes to the database.

However, locking can also lead to certain issues. For instance, it can cause a deadlock, a situation where two or more transactions indefinitely wait for each other to release resources. To handle such situations, most DBMS have a deadlock detection and resolution mechanism in place.

In summary, database locks are a fundamental aspect of database management, ensuring data integrity and consistency during concurrent transactions. They work by serialising access to data, preventing conflicts and inconsistencies that could arise from simultaneous data access and modification.