SQL Transactions and Concurrency Control

What is Concurrency Control?

Concurrency control manages simultaneous operations on a database without conflicting with each other. It ensures that transactions execute in a way that the final state of the database is correct and consistent.

Without concurrency control, issues like lost updates, dirty reads, and inconsistent data can occur.

• Ensures data consistency during concurrent access.
• Prevents conflicts between transactions.
• Maintains isolation between transactions.

Common Concurrency Problems

When multiple transactions run concurrently, several problems can arise if concurrency control is not properly implemented.

• Dirty Read: Reading uncommitted data from another transaction.
• Non-Repeatable Read: Data changes between reads within the same transaction.
• Phantom Read: New rows appear in a subsequent query within the same transaction.
• Lost Update: Two transactions overwrite each other's changes.

Mechanisms for Concurrency Control

SQL databases use several mechanisms to control concurrency and maintain data integrity.

• Locking: Prevents other transactions from accessing data being modified.
• Transaction Isolation Levels: Define how visible changes are between concurrent transactions.
• Timestamp Ordering: Uses timestamps to serialize transactions.
• Optimistic Concurrency Control: Assumes conflicts are rare and checks for conflicts before commit.

Example: Using Transactions with Isolation Levels

Here is an example of how to set the isolation level and use transactions in SQL to control concurrency.