ACID properties in HLD - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Which ACID property ensures that a transaction is completed fully or not at all?

In database systems, transactions must be reliable. Which ACID property guarantees that either all steps of a transaction happen or none do, preventing partial updates?

A Durability

B Atomicity

C Consistency

D Isolation

💡 Hint

Think about the property that treats the transaction as a single unit.

Atomicity means the transaction is all or nothing. If any part fails, the whole transaction is rolled back.

❓ Architecture

intermediate

Which ACID property prevents transactions from interfering with each other?

When multiple users access a database at the same time, which ACID property ensures their transactions do not affect each other's results?

A Atomicity

B Durability

C Isolation

D Consistency

💡 Hint

Think about how transactions are kept separate during execution.

Isolation ensures that concurrent transactions do not see each other's intermediate states, preventing conflicts.

❓ scaling

advanced

In a distributed database, which ACID property is hardest to maintain when scaling horizontally?

When a database is spread across many servers to handle more users, which ACID property becomes most challenging to guarantee?

A Durability

B Atomicity

C Isolation

D Consistency

💡 Hint

Consider the difficulty of keeping data the same everywhere at the same time.

Consistency is difficult in distributed systems because all nodes must agree on data state despite network delays or failures.

❓ tradeoff

advanced

Which ACID property can be relaxed to improve database performance but risks reading uncommitted data?

To speed up database operations, sometimes one ACID property is weakened. Which one is it, and what risk does this introduce?

A Isolation; risk of dirty reads

B Atomicity; risk of partial transactions

C Durability; risk of losing committed data

D Consistency; risk of invalid data

💡 Hint

Think about concurrency and seeing changes before they are finalized.

Relaxing Isolation can improve speed but allows dirty reads, where a transaction sees uncommitted changes from others.

❓ estimation

expert

Estimate the minimum number of replicas needed to ensure Durability in a distributed database with potential node failures.

You want to guarantee that committed data is never lost even if some servers fail. What is the minimum number of replicas you need to store the data safely?

A 3 replicas, to tolerate one server failure and maintain quorum

B 2 replicas, to tolerate one server failure

C 1 replica, because one copy is enough if the server is reliable

D 4 replicas, to tolerate two server failures

💡 Hint

Think about how many copies are needed to survive one failure and still agree on data.

Three replicas allow one failure while still having a majority (quorum) to confirm data durability.

Practice

1. Which of the following ACID properties ensures that a transaction is completed fully or not at all, preventing partial updates?

easy

A. Atomicity

B. Consistency

C. Isolation

D. Durability

Solution

1. Step 1: Understand Atomicity in ACID

   Atomicity means a transaction is treated as a single unit. It either happens completely or not at all.

2. Step 2: Compare with other properties

   Consistency ensures data correctness, Isolation manages concurrent transactions, Durability guarantees data persistence after commit.

3. Final Answer:

   Atomicity -> Option A

4. Quick Check:

   Atomicity = All or nothing [OK]

Hint: Atomicity means all steps succeed or none do [OK]

Common Mistakes:

• Confusing Atomicity with Consistency
• Thinking Isolation means no partial updates
• Mixing Durability with Atomicity

2. Which ACID property is correctly described by this statement: Once a transaction commits, its changes are permanent even if the system crashes.

easy

A. Isolation

B. Atomicity

C. Durability

D. Consistency

Solution

1. Step 1: Identify property about permanence after commit

   Durability guarantees that once a transaction commits, changes survive crashes.

2. Step 2: Check other properties

   Isolation controls concurrent access, Atomicity ensures all-or-nothing, Consistency maintains valid data states.

3. Final Answer:

   Durability -> Option C

4. Quick Check:

   Durability = Permanent changes after commit [OK]

Hint: Durability means changes survive crashes [OK]

Common Mistakes:

• Mixing Durability with Atomicity
• Confusing Isolation with Durability
• Thinking Consistency means permanence

3. Consider two transactions T1 and T2 running concurrently. T1 reads a value, T2 updates it and commits. Which ACID property ensures T1 does not see T2's partial changes before commit?

medium

A. Isolation

B. Durability

C. Atomicity

D. Consistency

Solution

1. Step 1: Understand concurrent transaction behavior

   Isolation ensures transactions do not interfere and see only committed data.

2. Step 2: Relate to other properties

   Atomicity is about all-or-nothing, Durability about permanence, Consistency about valid data states.

3. Final Answer:

   Isolation -> Option A

4. Quick Check:

   Isolation = No dirty reads [OK]

Hint: Isolation prevents seeing uncommitted changes [OK]

Common Mistakes:

• Confusing Isolation with Atomicity
• Assuming Consistency controls concurrency
• Mixing Durability with Isolation

4. A developer notices that after a system crash, some committed transactions are lost. Which ACID property is most likely violated?

medium

A. Atomicity

B. Consistency

C. Isolation

D. Durability

Solution

1. Step 1: Identify property related to crash recovery

   Durability ensures committed transactions survive crashes.

2. Step 2: Analyze the problem

   Lost committed transactions after crash means Durability is violated.

3. Final Answer:

   Durability -> Option D

4. Quick Check:

   Durability = No data loss after crash [OK]

Hint: Lost commits after crash means Durability failure [OK]

Common Mistakes:

• Blaming Atomicity for crash data loss
• Confusing Isolation with crash recovery
• Assuming Consistency fixes crash issues

5. You design a banking system where multiple users transfer money concurrently. Which combination of ACID properties is most critical to prevent incorrect balances and lost updates?

hard

A. Consistency and Durability

B. Atomicity and Isolation

C. Isolation and Durability

D. Atomicity and Consistency

Solution

1. Step 1: Identify needs for concurrent money transfers

   Atomicity ensures each transfer completes fully or not at all, preventing partial money moves.

2. Step 2: Ensure no interference between concurrent transfers

   Isolation prevents transactions from seeing intermediate states, avoiding lost updates and incorrect balances.

3. Step 3: Evaluate other properties

   Consistency ensures valid data but depends on Atomicity and Isolation; Durability ensures persistence but not concurrency control.

4. Final Answer:

   Atomicity and Isolation -> Option B

5. Quick Check:

   Atomicity + Isolation = Safe concurrent transfers [OK]

Hint: Atomicity + Isolation prevent partial and conflicting updates [OK]

Common Mistakes:

• Ignoring Isolation leads to lost updates
• Assuming Consistency alone prevents concurrency issues
• Overlooking Atomicity causes partial transfers