The CAP theorem in HLD - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding CAP Theorem Trade-offs

In a distributed system, which two properties can be guaranteed simultaneously according to the CAP theorem?

A Consistency and Availability

B Consistency and Partition Tolerance

C Availability and Partition Tolerance

D Consistency, Availability, and Partition Tolerance

💡 Hint

Remember, the CAP theorem states you can only have two out of three properties at the same time.

The CAP theorem states that in the presence of a network partition, a distributed system must choose between consistency and availability. Therefore, only two of the three properties can be guaranteed simultaneously.

❓ Architecture

intermediate

Designing for Availability in CAP Theorem

You are designing a distributed database that must remain available during network partitions but can tolerate eventual consistency. Which CAP properties does your design prioritize?

A Availability and Partition Tolerance

B Consistency and Partition Tolerance

C Consistency and Availability

D Only Availability

💡 Hint

Think about which property is relaxed to keep the system available during partitions.

Choosing availability and partition tolerance means the system remains responsive during network issues but may not be immediately consistent.

❓ scaling

advanced

Scaling a Distributed System with CAP Constraints

You have a distributed system that must scale globally with strong consistency guarantees. What is the main challenge you will face according to the CAP theorem?

A Scaling without any network partitions

B Ensuring partition tolerance without consistency loss

C Maintaining availability during network partitions

D Avoiding consistency issues by sacrificing partition tolerance

💡 Hint

Think about what happens when network partitions occur and you want strong consistency.

Strong consistency requires the system to reject some requests during partitions, reducing availability.

❓ tradeoff

advanced

Choosing Between Consistency and Availability

In a banking application requiring accurate account balances, which CAP property should be prioritized during network partitions?

A Consistency over Availability

B Availability over Consistency

C Partition Tolerance over both

D Neither Consistency nor Availability

💡 Hint

Consider the importance of accurate data in financial transactions.

Banking systems must ensure data correctness, so consistency is prioritized even if availability is reduced during partitions.

❓ estimation

expert

Estimating Impact of Network Partitions on System Availability

A distributed system has 5 nodes across different regions. Network partitions occur randomly with a 1% chance per hour. If the system prioritizes consistency and partition tolerance, what is the expected availability impact over 24 hours?

A Approximately 50% downtime due to frequent partitions

B Approximately 1% downtime as partitions are rare and availability is prioritized

C No downtime because partition tolerance ensures availability

D Approximately 24% downtime due to unavailability during partitions

💡 Hint

Calculate downtime based on partition probability and availability trade-offs.

With 1% chance per hour over 24 hours, expected downtime is about 24%. Prioritizing consistency means the system becomes unavailable during partitions.

Practice

1. Which two properties can a distributed system guarantee at the same time according to the CAP theorem?

easy

A. Consistency and Availability

B. Availability and Partition tolerance

C. Consistency and Partition tolerance

D. Consistency, Availability, and Partition tolerance all together

Solution

1. Step 1: Understand the CAP theorem basics

   The CAP theorem states that a distributed system can only guarantee two out of three properties: Consistency, Availability, and Partition tolerance.

2. Step 2: Identify which pairs are possible

   Systems can guarantee Availability and Partition tolerance or Consistency and Partition tolerance, but not all three simultaneously. However, only Availability and Partition tolerance can be guaranteed simultaneously in the presence of network partitions.

3. Final Answer:

   Availability and Partition tolerance -> Option B

4. Quick Check:

   CAP theorem = two properties only [OK]

Hint: Remember CAP means pick only two properties [OK]

Common Mistakes:

• Thinking all three properties can be guaranteed simultaneously
• Confusing Availability with Partition tolerance
• Ignoring network partitions in distributed systems

2. Which of the following is the correct definition of Partition tolerance in the CAP theorem?

easy

A. The system can handle unlimited data growth

B. All nodes always return the same data at the same time

C. The system is always available to respond to requests

D. The system continues to operate despite network failures between nodes

Solution

1. Step 1: Define Partition tolerance

   Partition tolerance means the system keeps working even if some network connections between nodes fail or messages are lost.

2. Step 2: Match definitions to options

   The system continues to operate despite network failures between nodes correctly describes Partition tolerance as handling network failures. Other options describe consistency, availability, or scalability.

3. Final Answer:

   The system continues to operate despite network failures between nodes -> Option D

4. Quick Check:

   Partition tolerance = handle network splits [OK]

Hint: Partition tolerance means surviving network splits [OK]

Common Mistakes:

• Confusing Partition tolerance with Availability
• Thinking Partition tolerance means data consistency
• Assuming Partition tolerance means unlimited scaling

3. Consider a distributed database that chooses Availability and Partition tolerance over Consistency. What is the expected behavior during a network partition?

medium

A. The system may return stale or inconsistent data but remains available

B. The system rejects all writes until the partition is resolved

C. The system becomes unavailable to maintain consistency

D. The system automatically merges all data without conflicts

Solution

1. Step 1: Identify chosen CAP properties

   The system chooses Availability and Partition tolerance, sacrificing Consistency.

2. Step 2: Analyze behavior during network partition

   Because it prioritizes Availability, the system stays up and responds to requests, but data may be inconsistent or stale.

3. Final Answer:

   The system may return stale or inconsistent data but remains available -> Option A

4. Quick Check:

   Availability + Partition tolerance = possible inconsistency [OK]

Hint: Availability + Partition tolerance means possible stale data [OK]

Common Mistakes:

• Assuming system rejects writes to keep consistency
• Thinking system becomes unavailable during partition
• Believing automatic conflict-free merges always happen

4. A developer claims their distributed system guarantees Consistency, Availability, and Partition tolerance simultaneously. What is the most likely issue?

medium

A. The system is using eventual consistency, not strong consistency

B. The system is only tested on a single node

C. The system is ignoring network partitions or failures

D. The system uses synchronous replication

Solution

1. Step 1: Recall CAP theorem limitation

   The CAP theorem states all three properties cannot be guaranteed simultaneously in a distributed system.

2. Step 2: Identify the likely mistake

   If the system claims all three, it probably ignores or does not handle network partitions properly, which is unrealistic in real distributed environments.

3. Final Answer:

   The system is ignoring network partitions or failures -> Option C

4. Quick Check:

   CAP theorem forbids all three together [OK]

Hint: All three CAP properties together is impossible [OK]

Common Mistakes:

• Assuming eventual consistency means full consistency
• Thinking synchronous replication solves CAP limits
• Ignoring real network failures in testing

5. You are designing a global online banking system where transactions must be accurate and consistent even during network failures. According to the CAP theorem, which two properties should you prioritize and why?

hard

A. Consistency and Partition tolerance, to maintain accurate data despite network partitions, even if some requests are delayed

B. Availability and Partition tolerance, to keep the system running even if data is temporarily inconsistent

C. All three properties equally, because banking requires perfect reliability

D. Consistency and Availability, to ensure all users see the same data and system is always responsive

Solution

1. Step 1: Understand banking system requirements

   Banking needs accurate, consistent transactions even if network issues occur, so data correctness is critical.

2. Step 2: Apply CAP theorem to design choice

   To keep data consistent during network partitions, the system must prioritize Consistency and Partition tolerance, accepting some availability delays.

3. Final Answer:

   Consistency and Partition tolerance, to maintain accurate data despite network partitions, even if some requests are delayed -> Option A

4. Quick Check:

   Banking = Consistency + Partition tolerance [OK]

Hint: Banking needs consistency over availability during partitions [OK]

Common Mistakes:

• Choosing availability over consistency for critical data
• Assuming all three CAP properties can be met
• Ignoring impact of network partitions on data accuracy