What is a Transaction?

A transaction is a sequence of operations performed as a single logical unit of work. It either completes fully or rolls back.

ACID Properties

• Atomicity: All operations succeed or none do.
• Consistency: Database moves from one valid state to another.
• Isolation: Concurrent transactions do not interfere with each other.
• Durability: Committed transactions survive system failures.

Concurrency Problems

• Dirty Read: Reading uncommitted data from another transaction.
• Non-repeatable Read: Same query returns different results in one transaction.
• Phantom Read: New rows appear between two reads in one transaction.

Deadlock

Two transactions wait for each other to release locks. Resolved by timeout or deadlock detection and rollback.