Explain ACID Properties with 30 Common Interview Q&As

 

The ACID properties are a set of principles that guarantee reliable processing of database transactions. Explain ACID Properties These properties ensure data integrity, especially in systems where multiple transactions occur simultaneously (like in banking, e-commerce, etc.).

Here’s a breakdown of each ACID property:

🔹 A — Atomicity

• Definition: A transaction is all-or-nothing.
• If any part of the transaction fails, the entire transaction fails and the database is rolled back to its original state.
• Example: If you transfer money from Account A to Account B, both the debit and the credit must occur. If the credit fails, the debit is undone.

 

🔹 C — Consistency

• Definition: A transaction must bring the database from one valid state to another.
• It ensures that all data integrity rules and constraints (like foreign keys, unique constraints) are maintained.
• Example: If a rule requires every employee to belong to a department, you cannot insert an employee record without a valid department ID.

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🔹 I — Isolation

• Definition: Transactions occur independently and concurrently without interference.
• Intermediate states of a transaction are not visible to other transactions.
• Ensures correctness even when transactions run at the same time.
• Example: If two people try to buy the last ticket simultaneously, only one should succeed.

 

🔹 D — Durability

• Definition: Once a transaction is committed, the changes are permanent, even in the event of a crash or power failure.
• The data is safely stored in non-volatile memory (like disk).
• Example: After a successful money transfer, the updated account balances remain saved even if the system crashes right after.

 

Explain ACID Properties with 30 Common Interview Q&As

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Property	Purpose	Example
Atomicity	All or nothing	Debit and credit in a fund transfer must both succeed or both fail.
Consistency	Data remains valid	No violations of rules (e.g., foreign key constraints).
Isolation	No interference between transactions	Concurrent users see consistent data.
Durability	Data is permanent after commit	Survives crashes or reboots.

 

Explain ACID Properties with 30 Common Interview Q&As

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Here are the Top 30 Interview Questions and Answers on ACID Properties in databases, ideal for freshers and experienced professionals preparing for technical interviews:

🔹 1. What does ACID stand for?

Answer:
ACID stands for Atomicity, Consistency, Isolation, and Durability, which are key properties that ensure reliable database transactions.

🔹 2. What is Atomicity in a database transaction?

Answer:
Atomicity ensures that a transaction is all-or-nothing — if any part fails, the entire transaction is rolled back.

🔹 3. Explain Consistency in ACID.

Answer:
Consistency ensures that a transaction brings the database from one valid state to another, maintaining all defined rules like constraints and triggers.

🔹 4. What is Isolation in ACID properties?

Answer:
Isolation ensures that the operations of one transaction are invisible to other transactions until the transaction is complete.

🔹 5. Define Durability in database systems.

Answer:
Durability guarantees that once a transaction is committed, the changes are permanently saved, even in case of a crash.

🔹 6. Why are ACID properties important?

Answer:
They ensure data reliability, consistency, and integrity, especially in systems with concurrent transactions.

🔹 7. What happens if Atomicity is violated?

Answer:
The database may end up in an incomplete or inconsistent state, as partial changes from a failed transaction may persist.

🔹 8. Give a real-world example of Atomicity.

Answer:
Money transfer: Debiting from Account A and crediting to Account B must both occur; otherwise, both are rolled back.

🔹 9. What is a transaction in databases?

Answer:
A transaction is a sequence of one or more SQL operations treated as a single logical unit of work.

🔹 10. Can a transaction be durable but not consistent?

Answer:
Yes. If a logic or constraint is violated before commit, it can persist (durable) but violate rules (inconsistent).

🔹 11. How is Isolation implemented in databases?

Answer:
Using isolation levels like Read Uncommitted, Read Committed, Repeatable Read, and Serializable.

🔹 12. What are Isolation Levels?

Answer:
They define how and when changes made by one transaction become visible to others. Higher levels offer better isolation but can reduce performance.

🔹 13. List common problems due to poor isolation.

Answer:

• Dirty Reads
• Non-Repeatable Reads
• Phantom Reads

🔹 14. What is a Dirty Read?

Answer:
Reading uncommitted data from another transaction.

🔹 15. What is a Non-Repeatable Read?

Answer:
Reading the same row twice during a transaction yields different values because another transaction modified it in between.

🔹 16. What is a Phantom Read?

Answer:
New rows are added or deleted by another transaction, affecting subsequent queries in the current transaction.

🔹 17. How does a database ensure durability?

Answer:
By writing committed changes to non-volatile storage, often using write-ahead logs (WAL).

🔹 18. What is rollback and how is it related to ACID?

Answer:
Rollback undoes all changes in a transaction if an error occurs — it’s part of ensuring Atomicity.

🔹 19. What is commit in a database?

Answer:
A commit finalizes all changes made in a transaction, ensuring Durability.

🔹 20. Is it possible to achieve full ACID in distributed databases?

Answer:
Yes, but it’s complex. Protocols like Two-Phase Commit (2PC) or Three-Phase Commit are used.

🔹 21. What is Two-Phase Commit?

Answer:
A protocol to ensure Atomicity across multiple databases in distributed systems.

🔹 22. Can NoSQL databases be ACID compliant?

Answer:
Some NoSQL databases offer partial or full ACID compliance (e.g., MongoDB, Couchbase, FoundationDB).

🔹 23. Do all RDBMS support ACID?

Answer:
Yes, traditional relational databases like MySQL, PostgreSQL, Oracle, SQL Server are ACID compliant.

🔹 24. How does PostgreSQL handle ACID?

Answer:
PostgreSQL ensures ACID compliance using MVCC (Multi-Version Concurrency Control), WAL, and strict constraints.

🔹 25. What is MVCC?

Answer:
Multi-Version Concurrency Control allows transactions to work with snapshots of data, improving Isolation and consistency.

🔹 26. Difference between Consistency in ACID and in CAP theorem?

Answer:

• ACID consistency = data integrity and constraints.
• CAP consistency = all nodes see the same data at the same time.

🔹 27. Can a transaction violate ACID?

Answer:
Yes, especially under crash scenarios, improper rollback, or with low isolation levels.

🔹 28. What tools or logs help ensure Durability?

Answer:
Write-Ahead Logs (WAL), transaction logs, and database checkpoints.

🔹 29. Is read-only query a transaction?

Answer:
Yes, if executed in a transactional context; it’s a non-modifying transaction.

🔹 30. Why is Isolation often traded off for performance?

Answer:
Higher Isolation levels can lead to locking and reduced concurrency, impacting performance.