ACID PROPERTIES IN DBMS

🔐 ACID PROPERTIES IN DBMS

Atomicity • Consistency • Isolation • Durability

📘 What are ACID Properties?

ACID properties are four important characteristics that make database transactions reliable and safe.

A = Atomicity   |   C = Consistency   |   I = Isolation   |   D = Durability

A transaction should either complete correctly or leave the database in a safe state.

🔐 Four ACID Properties

Property	Meaning	Simple Idea
Atomicity	All or Nothing	Complete transaction or rollback
Consistency	Valid State	Rules and constraints remain valid
Isolation	Transactions are separated	Concurrent transactions should not interfere incorrectly
Durability	Permanent Result	Committed data survives failures

ACID = Atomicity + Consistency + Isolation + Durability

⚛️ A — Atomicity

Atomicity means a transaction is treated as one complete unit.

Either all operations are completed, or none of them are applied.

Example: Bank Transfer

Account A → ₹1000
Account B → ₹500

Transfer = ₹200

1

₹200 is deducted from Account A.

2

₹200 is added to Account B.

Both operations succeed.

A = ₹800
B = ₹700

Transaction COMMIT.

What if the second operation fails?

The ₹200 deduction from A must also be cancelled.

This is called ROLLBACK.

Therefore:
Either both operations happen, or neither happens.

✅ C — Consistency

Consistency means a transaction must take the database from one valid state to another valid state.

All database rules, constraints and relationships must remain valid.

Example

Total Bank Balance Before = ₹10,000

Total Bank Balance After = ₹10,000

If ₹2,000 is transferred from Account A to Account B, the total amount should remain unchanged.

1

A loses ₹2,000.

2

B gains ₹2,000.

3

Total money remains the same.

Database rules remain valid.

Therefore the transaction preserves consistency.

🔒 I — Isolation

Isolation means that multiple transactions executing at the same time should not incorrectly interfere with one another.

Example

Suppose two transactions are running simultaneously:

Transaction T1 → Withdraw ₹500

Transaction T2 → Deposit ₹1000

1

T1 and T2 execute concurrently.

2

The intermediate changes of one transaction should not incorrectly affect the other transaction.

3

The DBMS controls concurrent execution using mechanisms such as locking or other concurrency-control techniques.

The final result should be equivalent to a correct serial execution.

This is the purpose of Isolation.

💾 D — Durability

Durability means that once a transaction has been successfully committed, its changes should remain saved even if a system failure occurs afterward.

Example

1

A customer deposits ₹5,000.

2

The transaction completes successfully.

3

The DBMS executes COMMIT.

4

Immediately afterward, the computer crashes.

After the database system recovers, the committed ₹5,000 deposit should still be present.

That is Durability.

🏦 Complete Bank Transfer Example

T1: Transfer ₹500 from A to B

Initial State

Account	Balance
A	₹5000
B	₹3000

Transaction

A = A − 500
B = B + 500

Final State

Account	Before	After
A	₹5000	₹4500
B	₹3000	₹3500

Atomicity: Both updates happen or neither.

Consistency: Total balance remains ₹8000.

Isolation: Other transactions do not see an incorrect intermediate state.

Durability: After COMMIT, the result remains saved.

🧮 ACID Transaction Step-by-Step

1

BEGIN TRANSACTION
The transaction starts.

2

READ DATA
The required account information is read.

3

PERFORM OPERATIONS
Money is deducted from one account and added to another.

4

CHECK CONSISTENCY
Database constraints and rules must remain valid.

5

COMMIT
If everything succeeds, the transaction is committed.

6

FAILURE?
If an operation fails, the transaction can be rolled back.

SUCCESS → COMMIT

FAILURE → ROLLBACK

⚠️ What Happens During Failure?

Suppose ₹1000 is transferred from Account A to Account B.

1

₹1000 is deducted from A.

2

System failure occurs before ₹1000 is added to B.

3

Atomicity requires the incomplete transaction to be undone.

4

The DBMS performs recovery/rollback as appropriate.

The database returns to a correct state.

Incomplete transaction → ROLLBACK

📊 ACID Properties Comparison

Letter	Property	Main Question
A	Atomicity	Did everything happen or nothing?
C	Consistency	Are database rules still valid?
I	Isolation	Are concurrent transactions safely separated?
D	Durability	Will committed data survive failure?

A → ALL OR NOTHING

C → VALID STATE

I → SAFE CONCURRENT EXECUTION

D → PERMANENT COMMIT

❓ Short Questions & Answers

Q1. What does ACID stand for?

ACID stands for Atomicity, Consistency, Isolation and Durability. These properties help maintain reliable database transactions.

Q2. What is Atomicity?

Atomicity means that a transaction is treated as one unit. Either all its operations are completed or none are applied.

Q3. What is Consistency?

Consistency ensures that a transaction takes the database from one valid state to another valid state while maintaining database constraints and rules.

Q4. What is Isolation?

Isolation ensures that concurrently executing transactions do not produce incorrect results because of interference between them.

Q5. What is Durability?

Durability means that once a transaction is committed, its changes remain preserved even after a subsequent system failure.

Q6. What is COMMIT?

COMMIT permanently accepts the successful changes made by a transaction.

Q7. What is ROLLBACK?

ROLLBACK cancels the changes of an incomplete or unsuccessful transaction and restores an appropriate previous database state.

B.K. PAUL

Bhairab Ganguly College

DBMS • Transactions • ACID Properties

Interactive Learning Module