ACID properties (Atomicity, Consistency, Isolation and Durability)

ACID stands for:

• A – Atomicity

• C – Consistency

• I – Isolation

• D – Durability

Together, they ensure that database transactions are processed reliably and securely.

🔹 Atomicity – "All or Nothing"

✅ Definition:

Atomicity ensures that either all operations in a transaction are performed, or none are. If one part fails, the entire transaction is rolled back.

💡 Example:

In a banking system:

• Debit ₹500 from Account A

• Credit ₹500 to Account B

If the debit succeeds but the credit fails, the ₹500 must be restored to Account A. Partial changes are not allowed.

🔁 Handled by:

• COMMIT (to save)

• ROLLBACK (to undo)

🔹 Consistency – "Valid from Start to End"

✅ Definition:

Consistency ensures that the database remains in a valid state before and after the transaction. Any transaction must preserve the rules, constraints, and integrity of the database.

💡 Example:

• If an account can't go below ₹0, a withdrawal transaction violating this will be rejected.

• Foreign key and check constraints are examples of consistency rules.

A transaction cannot leave the DB in an invalid state.

🔹 Isolation – "Transactions Don’t Interfere"

✅ Definition:

Isolation ensures that concurrent transactions do not affect each other's execution. The final result should be as if the transactions ran one after the other (serially).

💡 Example:

Two people buying the last movie ticket at the same time:

• Without isolation: both may book it — data corruption.

• With isolation: one succeeds, the other waits or fails.

🔐 Implemented via:

• Isolation levels: READ UNCOMMITTED, READ COMMITTED, REPEATABLE READ, SERIALIZABLE

🔹 Durability – "Permanent Effect"

✅ Definition:

Once a transaction is committed, the changes are permanent — even in the event of a power failure, crash, or system error.

💡 Example:

If money is transferred and the system crashes, once the transaction is committed, it won’t be reversed — changes are written to disk/logs.

📌 Summary Table