What are ACID properties in a database?

A transaction is a collection of instructions. To maintain the integrity of a database, all transactions must obey ACID properties. ACID is an acronym for atomicity, consistency, isolation, and durability. Have a look...

Atomicity

A transaction is an atomic unit; hence, all the instructions within a transaction will successfully execute, or none of them will execute. The following transaction transfers 20 dollars from Alice’s bank account to Bob’s bank account. If any of the instructions fail, the entire transaction should abort and rollback.

A transaction to transfer 20 pounds from Alice's account to Bob's account.

Consistency

A database is initially in a consistent state, and it should remain consistent after every transaction. Suppose that the transaction in the previous example fails after Write(A_b) and the transaction is not rolled back; then, the database will be inconsistent as the sum of Alice and Bob’s money, after the transaction, will not be equal to the amount of money they had before the transaction.

Isolation

If the multiple transactions are running concurrently, they should not be affected by each other; i.e., the result should be the same as the result obtained if the transactions were running sequentially. Suppose B_bal is initially 100. If a context switch occurs after B_bal *= 20, T2 will read the incorrect value of 100 as the updated value will not have been written back to the database. This violates the isolation property as the result is different from the answer that would have been​ obtained if T1 had ​finished before T2.

Durability

Changes that have been committed to the database should remain even in the case of software and hardware failure. For instance, if Bob’s account contains $120, this information should not disappear upon hardware or software failure.