Redundancy and fault tolerance in HLD - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is redundancy in system design?

Redundancy means having extra components or systems that can take over if the main ones fail. It helps keep the system working without interruption.

beginner

Define fault tolerance in simple terms.

Fault tolerance is the ability of a system to keep working even when some parts fail. It prevents total system failure by handling errors smoothly.

intermediate

Why is redundancy important for fault tolerance?

Redundancy provides backup parts or systems. Fault tolerance uses these backups to continue working when failures happen, making the system reliable.

intermediate

Name two common types of redundancy used in system design.

1. Hardware redundancy: extra physical devices like servers or disks.
2. Data redundancy: copies of data stored in multiple places.

advanced

What is the difference between active and passive redundancy?

Active redundancy means backup components run alongside the main ones all the time.
Passive redundancy means backups only start working when the main components fail.

What does fault tolerance help a system do?

A Reduce data storage

B Run faster under load

C Use less power

D Keep working despite failures

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

Which of these is an example of hardware redundancy?

A Using a faster CPU

B Compressing data to save space

C Multiple servers running the same service

D Encrypting data for security

Hardware redundancy involves extra physical devices like servers to provide backup.

Passive redundancy means backups:

A Run all the time alongside main components

B Only activate when main components fail

C Are never used

D Are faster than main components

Passive redundancy backups start working only when the main parts fail.

Which term means having extra copies of data stored in different places?

A Data redundancy

B Load balancing

C Caching

D Sharding

Data redundancy means storing copies of data in multiple locations for safety.

What is the main goal of redundancy in system design?

A To improve system reliability

B To reduce system cost

C To increase system complexity

D To slow down system response

Redundancy aims to improve reliability by providing backups.

Explain how redundancy and fault tolerance work together to keep a system reliable.

Think about backups and how the system keeps running when parts fail.

Describe the difference between active and passive redundancy with examples.

Focus on when backups are running or activated.

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