ACID Properties

What are ACID Properties?

To ensure data integrity, relational databases follow the ACID model.

1. Atomicity (A)

"All or Nothing"
A transaction is treated as a single unit. It either completes entirely or fails entirely.

• Example: In a money transfer, if the deduction happens but the addition fails, the deduction is rolled back.

2. Consistency (C)

"Rules are Followed"
A transaction must bring the database from one valid state to another valid state, maintaining all constraints (Unique, Foreign Key, etc.).

• Example: You cannot insert a row that violates a PRIMARY KEY constraint, even inside a transaction.

3. Isolation (I)

"Independent Execution"
Transactions executed concurrently should create the same state as if they were executed sequentially.

• One transaction should not see the intermediate (uncommitted) results of another transaction (depending on Isolation Level).

Isolation Levels

• Read Uncommitted: Lowest isolation, can see dirty data.
• Read Committed: Standard for many DBs.
• Repeatable Read: Ensures if you read a row twice, you get the same data.
• Serializable: Highest isolation, strictly sequential.

4. Durability (D)

"Saved Forever"
Once a transaction is committed, it remains committed even in the case of a system failure (power loss, crash). Use of transaction logs ensures this.