Single point of failure identification in HLD - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a single point of failure (SPOF) in system design?

A single point of failure is a part of a system that, if it fails, causes the entire system to stop working.

beginner

Why is identifying SPOFs important in system design?

Identifying SPOFs helps prevent total system outages by allowing designers to add redundancy or failover mechanisms.

beginner

Name a common example of a single point of failure in a web application.

A single database server without replication is a common SPOF because if it fails, the app cannot access data.

intermediate

What is a simple method to identify SPOFs in a system?

Trace the request flow and find components that have no backup or alternative path if they fail.

intermediate

How can redundancy help in eliminating SPOFs?

Redundancy adds duplicate components so if one fails, another can take over, keeping the system running.

What happens if a single point of failure fails in a system?

A The entire system may stop working

B Only a small part of the system is affected

C The system speeds up

D Nothing changes

A single point of failure causes the whole system to fail if it breaks.

Which of these is a way to reduce single points of failure?

A Ignore failures

B Use only one server

C Add redundancy

D Remove backups

Adding redundancy means having backups to avoid SPOFs.

A database without replication is an example of:

A Load balancing

B A single point of failure

C Caching

D Failover

Without replication, the database is a SPOF.

What is the first step to identify SPOFs in a system?

A Trace the request flow

B Add more servers

C Ignore logs

D Restart the system

Tracing request flow helps find components with no backup.

Which component is least likely to be a SPOF?

A A single power supply

B A single network switch

C A single database server

D A load balancer with multiple servers

Load balancers distributing traffic to multiple servers reduce SPOFs.

Explain what a single point of failure is and why it matters in system design.

Think about what happens if one part breaks and the whole system stops.

Describe how you would find single points of failure in a simple web application architecture.

Follow the path a user request takes and look for weak spots.

Practice

1. Which component in a system is called a single point of failure?

easy

A. A part that never fails

B. A component that has multiple backups

C. A part that, if it fails, stops the whole system from working

D. A component that only handles logging

Solution

1. Step 1: Understand the definition of single point of failure

   A single point of failure is a part of the system that can cause the entire system to stop working if it breaks.

2. Step 2: Match the definition to the options

   A part that, if it fails, stops the whole system from working describes a component that stops the whole system if it fails, which matches the definition.

3. Final Answer:

   A part that, if it fails, stops the whole system from working -> Option C

4. Quick Check:

   Single point of failure = component causing total failure [OK]

Hint: Look for the component whose failure stops everything [OK]

Common Mistakes:

• Confusing components with backups as single points of failure
• Thinking a component that never fails is a single point of failure
• Assuming logging components cause total failure

2. Which of the following is the correct way to reduce a single point of failure in a database server?

easy

A. Disable backups to improve speed

B. Use a single database server with no backups

C. Store all data on a local hard drive only

D. Add a replica database server for failover

Solution

1. Step 1: Identify how to reduce single points of failure

   Reducing single points of failure means adding backups or alternatives to critical components.

2. Step 2: Evaluate options for database server

   Add a replica database server for failover adds a replica server, providing failover if the main server fails, reducing single point of failure.

3. Final Answer:

   Add a replica database server for failover -> Option D

4. Quick Check:

   Adding backups = reducing single point of failure [OK]

Hint: Add replicas or backups to critical components [OK]

Common Mistakes:

• Choosing single server without backups
• Ignoring failover options
• Thinking disabling backups improves reliability

3. Consider a system with a load balancer, two web servers, and replicated database servers. Which component is the single point of failure?

medium

A. Load balancer

B. Web servers

C. Database server

D. All components have backups

Solution

1. Step 1: Analyze redundancy of components

   There are two web servers and replicated database servers, so they are redundant. The load balancer is only one.

2. Step 2: Identify which component lacks backup

   The load balancer is a single device with no backup, so it is a single point of failure.

3. Final Answer:

   Load balancer -> Option A

4. Quick Check:

   Single device without backup = single point of failure [OK]

Hint: Find the component with no backup or alternative [OK]

Common Mistakes:

• Assuming web servers are single points due to multiple instances
• Ignoring load balancer as critical single device
• Thinking database servers are a single point of failure

4. A system has a single DNS server and multiple application servers. Users report downtime when DNS fails. What is the main issue?

medium

A. DNS server is a single point of failure

B. Application servers lack backups

C. Users have slow internet

D. Application servers are overloaded

Solution

1. Step 1: Understand the role of DNS server

   DNS translates domain names to IPs; if it fails, users cannot reach the system.

2. Step 2: Identify the cause of downtime

   Since downtime happens when DNS fails and there is only one DNS server, it is a single point of failure.

3. Final Answer:

   DNS server is a single point of failure -> Option A

4. Quick Check:

   Single DNS failure causes downtime = single point of failure [OK]

Hint: Check if critical service has only one instance [OK]

Common Mistakes:

• Blaming application servers without evidence
• Ignoring DNS role in connectivity
• Assuming user internet issues cause downtime

5. You design a payment system with one payment gateway, one database, and multiple app servers. Which change best removes single points of failure?

hard

A. Add more app servers only

B. Add a second payment gateway and database replica

C. Use a single faster database

D. Remove payment gateway to simplify

Solution

1. Step 1: Identify single points of failure

   Payment gateway and database each have only one instance, so they are single points of failure.

2. Step 2: Choose the best fix

   Adding a second payment gateway and a database replica adds redundancy, removing single points of failure.

3. Final Answer:

   Add a second payment gateway and database replica -> Option B

4. Quick Check:

   Adding redundancy removes single points of failure [OK]

Hint: Add backups to all critical components [OK]

Common Mistakes:

• Adding only app servers ignores gateway and database
• Using a single faster database does not add redundancy
• Removing payment gateway breaks system