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.