- LPU MCA Semester 1
- CAP570
- Unit 2
CAP570 · Unit 2: E-R modeling
ACID properties
9 practice questions · in the MTE and ETE syllabus
Key points
- Atomicity is all or nothing
- Isolation hides partial work from concurrent transactions
- Durability applies only to committed transactions
- Consistency preserves declared rules
A transaction is a logical unit of work. A DBMS guarantees four ACID properties.
| Property | Meaning | Mechanism |
|---|---|---|
| Atomicity | all of the transaction happens or none of it | undo through the log |
| Consistency | the database moves from one valid state to another valid state | constraints and application logic |
| Isolation | concurrent transactions do not see each other's partial work | locking, multiversioning |
| Durability | once committed, changes survive a crash | write-ahead log, redo |
Example: in a transfer of 500 from A to B, atomicity means both the debit and the credit happen or neither does. A crash after the debit is repaired by undoing it. Two clerks both reading a balance of 1000 and each withdrawing 600 is an isolation failure (a lost update). If power fails just after COMMIT, the log lets the system redo the changes, which is durability.
Exam trap: the DBMS enforces only declared rules, so consistency partly depends on the application.
Try two questions
Which property ensures that concurrent transactions do not see each other's partial results?
- Atomicity
- Durability
- Consistency
- Isolation
Show the answer
D. Isolation makes each transaction behave as if it were running alone.
Why not A: Atomicity ensures all or nothing, not invisibility of partial results.
Why not B: Durability concerns persistence.
Why not C: Consistency concerns rules holding before and after.
A transfer debits account A by 500 and the system crashes before crediting B. After recovery, A shows no debit. Which property is demonstrated?
- Atomicity
- Durability
- Isolation
- Consistency alone
Show the answer
A. Undoing the partial debit leaves the transfer as if it never began, which is atomicity.
Why not B: Durability would keep committed data; this transaction never committed.
Why not C: No concurrent transaction is involved.
Why not D: The all-or-nothing rollback is the atomicity guarantee.