Recap

Concurrency control ensures correct execution of transactions in a multi-user database environment by:

• Preventing race conditions when multiple transactions access the same data.
• Ensuring ACID properties to maintain correctness and consistency.

Motivation: Why Concurrency Control?

Key Problems

1. Lost Updates

• Two transactions modify the same record simultaneously.
• Without concurrency control, changes can be lost.

1. Dirty Reads (Uncommitted Data)

• One transaction reads data modified by another uncommitted transaction.

1. Inconsistent Analysis (Non-Repeatable Reads)

• A transaction sees different data when reading the same value multiple times.

1. Phantom Reads

• A transaction inserts or deletes rows, affecting another transaction's results.

ACID Properties

Example: Transferring $100 from A to B