Transaction Management and Concurrency Control

1. Transactions

A transaction is a sequence of database operations that are treated as a single unit of work. Transactions ensure that the database remains in a consistent state by adhering to the ACID properties:

• Atomicity: All operations within a transaction are completed successfully, or none are. If any operation fails, the entire transaction is rolled back.
• Consistency: A transaction brings the database from one valid state to another, maintaining all database rules and constraints.
• Isolation: Each transaction is executed in isolation from others, meaning the intermediate states of a transaction are not visible to other transactions.
• Durability: Once a transaction is committed, its changes are permanent and survive any subsequent failures.

Example: In a banking system, transferring money from one account to another involves debiting one account and crediting another. This sequence of operations must be treated as a single transaction to ensure that either both operations succeed or neither does.

2. Concurrency Control

Concurrency control is the management of multiple transactions executing simultaneously to ensure database consistency. It prevents issues such as lost updates, dirty reads, and inconsistent retrievals.

Example: Consider two transactions, T1 and T2, both trying to update the same account balance. Without proper concurrency control, T1 might read the balance, T2 might read the same balance and update it, and then T1 might overwrite T2's update, resulting in a lost update.

3. Locking

Locking is a concurrency control mechanism that ensures transactions have exclusive access to data items. There are two main types of locks:

• Shared Lock (S-Lock): Allows multiple transactions to read a data item simultaneously but prevents any transaction from writing to it.
• Exclusive Lock (X-Lock): Allows a transaction to read and write a data item exclusively, preventing any other transaction from accessing it.

Example: In a library system, if one transaction is reading a book's details (using a shared lock), other transactions can also read the details simultaneously. However, if another transaction wants to update the book's details (using an exclusive lock), it must wait until the shared lock is released.

4. Deadlocks

A deadlock occurs when two or more transactions are waiting indefinitely for each other to release locks. This situation can lead to a system freeze and must be managed to ensure database availability.

Example: Transaction T1 holds a lock on data item A and requests a lock on data item B, while transaction T2 holds a lock on data item B and requests a lock on data item A. Both transactions are waiting for each other, resulting in a deadlock.

Conclusion

Understanding transaction management and concurrency control is crucial for maintaining database integrity and performance. By ensuring transactions adhere to the ACID properties, managing concurrency with locking, and preventing deadlocks, databases can operate efficiently and reliably in multi-user environments.