Redundancy and fault tolerance in HLD - System Design Guide

Problem Statement

When a critical component in a system fails, the entire service can become unavailable, causing downtime and loss of user trust. Without backup components or mechanisms, a single failure can cascade and bring down the whole system.

Solution

Redundancy duplicates critical components so that if one fails, others can take over immediately. Fault tolerance ensures the system continues to operate correctly despite failures by detecting faults and switching to backups without interrupting service.

Architecture

Client App

↓

Load Balancer (LB)

↓

Server A

(Primary)

This diagram shows a client sending requests to a load balancer that distributes traffic between two servers. Server B acts as a backup to Server A, providing redundancy and fault tolerance.

Trade-offs

✓ Pros

→

Improves system availability by eliminating single points of failure.

→

Enables seamless failover to backup components without user impact.

→

Supports maintenance and upgrades without downtime by switching traffic.

→

Increases system reliability and user trust.

✗ Cons

→

Adds hardware and operational costs due to duplicate components.

→

Increases system complexity requiring monitoring and failover logic.

→

May introduce slight latency due to health checks and failover mechanisms.

Use when system uptime is critical and expected traffic exceeds 1,000 requests per second or when downtime costs are high.

Avoid when system is small-scale with low traffic (under 100 req/sec) or when cost constraints outweigh availability needs.

Real World Examples

Netflix

Uses redundant edge servers and fault-tolerant load balancers to ensure uninterrupted streaming even if some servers fail.

Amazon

Employs multiple availability zones with redundant databases and services to maintain fault tolerance during data center failures.

Google

Implements redundancy in its global network and data centers to provide fault tolerance and high availability for search and cloud services.

Alternatives

Graceful degradation

Instead of full redundancy, the system reduces functionality under failure to maintain partial service.

Use when: Use when full redundancy is too costly but some service continuity is required.

Circuit breaker

Prevents cascading failures by stopping calls to failing components rather than duplicating them.

Use when: Use when failures are transient and you want to isolate faults quickly.

Summary

Redundancy duplicates critical components to avoid single points of failure.

Fault tolerance enables systems to continue operating despite component failures.

Together, they improve availability but add cost and complexity.

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