Redundancy and fault tolerance in HLD - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding Redundancy Types

Which type of redundancy involves having multiple identical components running simultaneously to ensure system availability?

A Cold redundancy

B Passive redundancy

C Backup redundancy

D Active redundancy

💡 Hint

Think about components working at the same time to avoid downtime.

Active redundancy means multiple components operate simultaneously, so if one fails, others continue without interruption.

❓ Architecture

intermediate

Designing Fault Tolerant Web Service

You are designing a web service that must remain available even if one server fails. Which architectural pattern best supports this requirement?

A Single server with backup power supply

B Load-balanced cluster of servers with health checks

C Monolithic server with scheduled downtime

D Single server with RAID storage

💡 Hint

Consider how traffic can be distributed and failures detected automatically.

A load-balanced cluster distributes requests across multiple servers and uses health checks to remove failed servers, ensuring availability.

❓ scaling

advanced

Scaling with Fault Tolerance

When scaling a distributed database for fault tolerance, which approach minimizes data loss during node failures?

A Caching data only on client side

B Sharding data without replication

C Replicating data synchronously across nodes

D Using a single master node for writes

💡 Hint

Think about how data consistency and availability are maintained during failures.

Synchronous replication ensures data is copied to multiple nodes before confirming writes, reducing data loss risk.

❓ tradeoff

advanced

Tradeoffs in Fault Tolerant Design

Which tradeoff is most commonly accepted when implementing active-active redundancy in a system?

A Increased latency due to synchronization overhead

B Simpler design with less complexity

C Lower cost because fewer components are needed

D Reduced availability due to single point of failure

💡 Hint

Consider what happens when multiple components must stay in sync.

Active-active redundancy requires components to synchronize state, which can add latency but improves availability.

❓ estimation

expert

Estimating System Availability with Redundancy

A system uses two identical servers in active-passive redundancy. Each server has an availability of 99.5%. What is the approximate overall system availability?

A Approximately 99.995%

B Approximately 99.75%

C Approximately 99.0%

D Approximately 99.25%

💡 Hint

Calculate the probability that both servers fail simultaneously.

System availability = 1 - (probability both fail) = 1 - (0.005 * 0.005) = 0.999975 or 99.9975% approx.

Practice

1. What is the main purpose of redundancy in system design?

easy

A. To simplify the system architecture

B. To increase the speed of data processing

C. To have backup components to avoid single points of failure

D. To reduce the cost of hardware

Solution

1. Step 1: Understand redundancy concept

   Redundancy means having extra components or backups so if one fails, the system still works.

2. Step 2: Identify the main goal of redundancy

   The goal is to avoid single points of failure by having backups ready.

3. Final Answer:

   To have backup components to avoid single points of failure -> Option C

4. Quick Check:

   Redundancy = Backup components [OK]

Hint: Redundancy means backups to prevent failure [OK]

Common Mistakes:

• Confusing redundancy with performance improvement
• Thinking redundancy reduces cost
• Assuming redundancy simplifies architecture

2. Which of the following is a correct example of a fault-tolerant system component?

easy

A. A load balancer distributing traffic to multiple servers

B. A single server with no backup

C. A database without replication

D. A system that shuts down on any error

Solution

1. Step 1: Identify fault tolerance features

   Fault tolerance means the system can keep working even if some parts fail.

2. Step 2: Evaluate options for fault tolerance

   A load balancer sends traffic to multiple servers, so if one fails, others handle requests.

3. Final Answer:

   A load balancer distributing traffic to multiple servers -> Option A

4. Quick Check:

   Fault tolerance = multiple servers with load balancer [OK]

Hint: Fault tolerance needs multiple working parts, not single points [OK]

Common Mistakes:

• Choosing single server as fault tolerant
• Ignoring replication or backups
• Confusing fault tolerance with error shutdown

3. Consider a system with two identical servers running in active-active mode for fault tolerance. If one server fails, what happens to the incoming requests?

medium

A. The system shuts down to prevent errors

B. All requests are lost until the failed server is fixed

C. Requests are queued indefinitely with no processing

D. Requests are automatically handled by the remaining active server

Solution

1. Step 1: Understand active-active mode

   In active-active, both servers handle requests simultaneously to share load and provide backup.

2. Step 2: Analyze failure impact

   If one server fails, the other continues handling all requests without interruption.

3. Final Answer:

   Requests are automatically handled by the remaining active server -> Option D

4. Quick Check:

   Active-active means failover to active server [OK]

Hint: Active-active means all servers handle requests; failure shifts load [OK]

Common Mistakes:

• Thinking requests are lost on failure
• Assuming system queues requests indefinitely
• Believing system shuts down on failure

4. A system uses a single database without replication. It crashes unexpectedly. What is the main fault tolerance issue here?

medium

A. Load balancer misconfiguration

B. No backup database to take over on failure

C. Too many replicas causing conflicts

D. Excessive redundancy causing delays

Solution

1. Step 1: Identify fault tolerance problem

   Fault tolerance requires backups or replicas to continue working if one fails.

2. Step 2: Analyze the system setup

   Single database without replication means no backup exists to handle failure.

3. Final Answer:

   No backup database to take over on failure -> Option B

4. Quick Check:

   No replication = no fault tolerance backup [OK]

Hint: No replication means no fault tolerance backup [OK]

Common Mistakes:

• Confusing replication with too many replicas
• Blaming load balancer for database crash
• Thinking redundancy causes delays here

5. You are designing a fault-tolerant web service that must remain available even if one data center fails. Which design choice best supports this requirement?

hard

A. Deploy the service in two geographically separate data centers with data replication

B. Deploy the service in one data center with multiple servers

C. Use a single powerful server with backup power supply

D. Deploy the service in one data center and rely on daily backups

Solution

1. Step 1: Understand availability requirement

   The service must stay available even if one entire data center fails.

2. Step 2: Evaluate design options for fault tolerance

   Deploying in two separate data centers with data replication ensures if one fails, the other serves requests.

3. Step 3: Compare other options

   Single data center or single server cannot handle full data center failure; daily backups do not provide immediate availability.

4. Final Answer:

   Deploy the service in two geographically separate data centers with data replication -> Option A

5. Quick Check:

   Geo-redundancy + replication = high availability [OK]

Hint: Use multiple data centers with replication for fault tolerance [OK]

Common Mistakes:

• Relying on single data center for fault tolerance
• Confusing backup power with data center failure protection
• Thinking daily backups ensure immediate availability