ACID is a set of four essential properties that ensure a database transaction is safe, reliable, and accurate.
A transaction means:
A small unit of work that changes data in a database.
Example: Money transfer, bill payment, ATM withdrawal, deposit, login attempt record, etc.
🔥 ACID = Atomicity + Consistency + Isolation + Durability
⭐ 1. Atomicity
Meaning:
- The transaction must happen completely or not at all.
- “All or Nothing Rule.”
Key points:
- No half-completed work is allowed.
- If one step fails → entire transaction is cancelled.
- Database returns to original state.
Example (Banking):
You transfer ₹500 from A to B. A transaction has two steps:
- Deduct ₹500 from A
- Add ₹500 to B
If Step 2 fails due to network issue → Step 1 is also undone.
So A’s balance stays the same.
✔ No money disappears.
✔ No partial update.
⭐ 2. Consistency
Meaning:
- The transaction must move the database from one valid state to another valid state.
- Data should always follow rules/constraints.
Key points:
- Ensures database rules are not violated.
- Guarantees data correctness.
- Ensures integrity constraints (like NOT NULL, UNIQUE, FOREIGN KEY) are not broken.
Example (Banking):
- Account balance cannot be negative.
- Interest rate cannot be > 100%.
- PAN number must be unique.
If a transaction tries to violate rules → it will be rejected.
✔ Maintains accuracy and validity.
✔ Protects business rules.
⭐ 3. Isolation
Meaning:
- Every transaction should work independently.
- One transaction should not affect another while running.
Key points:
- No interference between transactions.
- Ensures parallel transactions give correct results.
- Prevents problems like:
- Dirty reads
- Lost updates
- Uncommitted data access
Example (Banking):
Transaction 1: A withdrawing ₹1000 from ATM.
Transaction 2: A checking account balance on mobile app.
Both happen at the same time.
Isolation ensures the balance shown is correct, without mixing intermediate steps.
✔ Each transaction acts as if it is running alone.
⭐ 4. Durability
Meaning:
- Once a transaction is successfully completed, the data must be permanently saved.
- Even if:
- System crashes
- Power goes off
- Server restarts
Key points:
- Uses logs, backups, and storage techniques.
- Ensures committed data is never lost.
Example (Banking):
You transfer ₹5,000 to someone.
App shows “Transaction Successful”.
Even if:
- ATM loses power
- Mobile app crashes
- Bank server restarts
The transaction will still be recorded in your account statement.
✔ Committed = Permanent.
✔ No loss of confirmed data.
🧠 Quick Memory Table
| Property | Simple Meaning | Key Idea | Banking Example |
|---|---|---|---|
| Atomicity | All or none | No partial update | Money transfer must fully succeed or fail |
| Consistency | Valid state | No rule violation | Cannot have negative balance |
| Isolation | Separate transactions | No interference | ATM withdrawal & balance check |
| Durability | Permanent results | Data persists | Transaction stays after crash |
📝 One-Line Definitions
- Atomicity: A transaction is executed completely or not at all.
- Consistency: Transaction maintains database rules and integrity.
- Isolation: Transactions run independently without affecting each other.
- Durability: Committed data is permanently stored even after failures.
✅ ACID PROPERTIES
🔶 SECTION A: BASIC DEFINITIONS
1. ACID stands for:
A. Atomicity, Consistency, Isolation, Durability
B. Accuracy, Consistency, Integrity, Durability
C. Atomicity, Concurrency, Integrity, Durability
D. Accuracy, Concurrency, Isolation, Data
Answer: A
2. ACID properties are used in:
A. Operating systems
B. DBMS & RDBMS
C. Internet Browsers
D. Compilers
Answer: B
3. ACID ensures:
A. Secure network
B. Reliable transactions
C. Fast response
D. Hardware safety
Answer: B
🔶 SECTION B: ATOMICITY
4. Atomicity ensures:
A. Transaction is partially done
B. Transaction is fully done or not done at all
C. Transaction happens twice
D. Data is encrypted
Answer: B
5. Atomicity is also known as the:
A. Integrity rule
B. Lock rule
C. All-or-nothing rule
D. Consistency rule
Answer: C
6. If a transaction fails in the middle, Atomicity:
A. Deletes the database
B. Undoes all completed steps
C. Saves half the steps
D. Commits partial data
Answer: B
7. In a money transfer, Atomicity ensures that:
A. Only debit happens
B. Only credit happens
C. Both debit and credit happen together
D. None
Answer: C
🔶 SECTION C: CONSISTENCY
8. Consistency ensures database moves from:
A. Valid state to invalid state
B. One valid state to another valid state
C. Invalid to valid only
D. Any state to any state
Answer: B
9. Consistency prevents:
A. Power failure
B. Violation of rules/constraints
C. Network lag
D. File corruption
Answer: B
10. Which of the following may violate Consistency?
A. Primary key duplication
B. Negative account balance
C. Foreign key mismatch
D. All of the above
Answer: D
11. Which ACID property is related to “Integrity Constraints”?
A. Atomicity
B. Consistency
C. Isolation
D. Durability
Answer: B
🔶 SECTION D: ISOLATION
12. Isolation ensures:
A. No data is saved
B. Transactions do not affect each other
C. Data is permanently stored
D. No constraints
Answer: B
13. Isolation is used to avoid:
A. Deadlock
B. Conflict in concurrent transactions
C. Power failure
D. Data deletion
Answer: B
14. Which issue occurs if Isolation is not maintained?
A. Dirty read
B. Lost update
C. Uncommitted read
D. All of the above
Answer: D
15. ATM withdrawal and online balance check require which property?
A. Consistency
B. Isolation
C. Atomicity
D. Durability
Answer: B
🔶 SECTION E: DURABILITY
16. Durability ensures:
A. Data is temporary
B. Data is stored permanently after commit
C. Only half data is stored
D. Data is rolled back automatically
Answer: B
17. Durability protects data from:
A. Power failure
B. System crash
C. Errors
D. All of these
Answer: D
18. Durability depends on:
A. Logs & backups
B. RAM only
C. Internet speed
D. Query language
Answer: A
19. In Durability, committed data will stay even if:
A. App crashes
B. System restarts
C. Power goes off
D. All of the above
Answer: D
🔶 SECTION F: MIXED
20. Which ACID property prevents “partial updates”?
A. Consistency
B. Atomicity
C. Durability
D. Isolation
Answer: B
21. Which ACID property deals with “correctness of data”?
A. Isolation
B. Durability
C. Consistency
D. Atomicity
Answer: C
22. Which ACID property is most related to “concurrency control”?
A. Durability
B. Isolation
C. Atomicity
D. Consistency
Answer: B
23. Commit operation finalizes which property?
A. Durability
B. Isolation
C. Atomicity
D. Consistency
Answer: A
24. Rollback operation is part of:
A. Durability
B. Atomicity
C. Encryption
D. Backup
Answer: B
🔶 SECTION G:
25. Which two properties ensure reliable multi-user transactions?
A. Durability + Consistency
B. Isolation + Atomicity
C. Isolation + Durability
D. Atomicity + Consistency
Answer: C
26. Which ACID property prevents data corruption during system crash?
A. Durability
B. Atomicity
C. Isolation
D. Integrity
Answer: A
27. Violating Consistency may break:
A. Referential integrity
B. Entity integrity
C. Domain integrity
D. All
Answer: D
28. Which property ensures “no dirty read”?
A. Atomicity
B. Isolation
C. Consistency
D. Durability
Answer: B
🔶 SECTION H:
29. Atomicity means partial update is allowed.
True / False
Answer: False
30. Consistency ensures rules are maintained.
True / False
Answer: True
31. Isolation prevents interference between transactions.
True / False
Answer: True
32. Durability ensures committed data is lost on crash.
True / False
Answer: False
🔶 SECTION I: APPLICATION-BASED
33. A bank transfer succeeds but system crashes after commit. Which property ensures that transfer is saved?
A. Atomicity
B. Durability
C. Isolation
D. Consistency
Answer: B
34. Two users withdraw money at the same time. Which property ensures no incorrect balance?
A. Atomicity
B. Isolation
C. Durability
D. Consistency
Answer: B
35. Transaction tries to insert duplicate PAN number. Which property stops it?
A. Consistency
B. Durability
C. Atomicity
D. Isolation
Answer: A
36. If one step of a transaction fails, the whole transaction is cancelled. Which property?
A. Durability
B. Isolation
C. Consistency
D. Atomicity
Answer: D
🔶 SECTION J: ADVANCED/DIFFICULT
37. Which property uses logs and checkpoints internally?
A. Atomicity
B. Durability
C. Isolation
D. Consistency
Answer: B
38. Dirty read occurs when:
A. Transaction reads uncommitted data
B. Database crashes
C. Constraint is violated
D. Two transactions run serially
Answer: A
39. Which property prevents dirty read?
A. Durability
B. Consistency
C. Isolation
D. Atomicity
Answer: C
40. The “serializability” concept is part of:
A. Durability
B. Atomicity
C. Isolation
D. Consistency
Answer: C
🔶 SECTION K:
41. Which ACID property ensures database remains error-free after rollback?
A. Isolation
B. Durability
C. Consistency
D. Atomicity
Answer: C
42. Which property ensures that the effects of completed transactions persist?
A. Atomicity
B. Consistency
C. Isolation
D. Durability
Answer: D
43. Which combination ensures “database correctness”?
A. Atomicity + Isolation
B. Consistency + Atomicity
C. Durability + Isolation
D. None
Answer: B
44. During crash recovery, the DBMS checks logs to maintain:
A. Durability
B. Isolation
C. Atomicity
D. Consistency
Answer: A
🔶 SECTION L: MORE HIGH-VALUE
45. Which ACID property ensures that the database “never reaches an invalid state”?
A. Atomicity
B. Consistency
C. Isolation
D. Durability
Answer: B
46. Which one is directly related to COMMIT and ROLLBACK operations?
A. Durability
B. Atomicity
C. Isolation
D. Consistency
Answer: B
47. Which property is MOST important in a banking environment?
A. Isolation
B. All of them
C. Consistency
D. Durability
Answer: B