Single point of failure identification in HLD - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Identify the Single Point of Failure in a Web Application Architecture

Consider a simple web application architecture where a single database server handles all data storage, a single application server processes all requests, and a load balancer distributes traffic to multiple web servers. Which component is the single point of failure?

A The load balancer, because if it fails, no traffic reaches the web servers.

B The database server, because if it fails, the application cannot access data.

C The web servers, because they handle user requests directly.

D The application server, because it processes all requests.

💡 Hint

Think about which component has no backup or redundancy and would stop the entire system if it fails.

The load balancer is the single point of failure because it is a single instance without redundancy. If it fails, no traffic can reach the web servers, causing the entire system to fail. The database server and application server may also be single instances, but the load balancer is the critical entry point. The web servers have redundancy.

❓ Architecture

intermediate

Spot the Single Point of Failure in a Distributed System Diagram

Given a distributed system with multiple microservices communicating via a message queue, a single message broker instance is used. Which component is the single point of failure?

A The microservices, because they depend on the message queue.

B The message broker instance, because it handles all message routing.

C The database, because it stores persistent data.

D The client applications, because they initiate requests.

💡 Hint

Consider which component, if it stops working, would block communication between microservices.

The single message broker instance is the single point of failure because all microservices rely on it for communication. If it fails, messages cannot be routed, and the system stops functioning properly. Other components have redundancy or are distributed.

❓ scaling

advanced

Evaluate Single Point of Failure in a Cloud-Based Storage System

A cloud storage system uses multiple storage nodes behind a single metadata server that manages file locations. Which component is the single point of failure?

A The client devices, because they access the storage.

B The network switches, because they connect storage nodes.

C The storage nodes, because they hold the actual data.

D The metadata server, because it manages file locations and access.

💡 Hint

Think about which component, if it fails, would prevent access to data even if storage nodes are healthy.

The metadata server is the single point of failure because it manages file locations. If it fails, clients cannot find or access files, even if storage nodes are operational. Storage nodes are multiple and redundant, but the metadata server is a single instance.

❓ tradeoff

advanced

Choosing Between Redundancy and Cost in System Design

A company wants to eliminate single points of failure in their payment processing system but has a limited budget. Which tradeoff is most likely to occur?

A Removing redundant components reduces cost and improves availability.

B Adding redundant components reduces cost but decreases availability.

C Adding redundant components increases cost but improves availability.

D Removing redundant components increases cost and decreases availability.

💡 Hint

Think about how redundancy affects cost and system reliability.

Adding redundant components like backup servers or databases increases the system's cost but improves availability by removing single points of failure. Removing redundancy lowers cost but risks downtime.

❓ estimation

expert

Estimate Impact of Single Point of Failure on System Uptime

A system has a single database server with an uptime of 99.9%. The rest of the system is fully redundant with 99.99% uptime. What is the approximate overall system uptime considering the single point of failure?

A Approximately 99.9%, limited by the database server uptime.

B Approximately 99.999%, combining both uptimes.

C Approximately 99.99%, because the rest of the system is redundant.

D Approximately 99.0%, due to compounded failures.

💡 Hint

Consider that the system is only as reliable as its weakest component.

The overall system uptime is limited by the single point of failure, the database server, which has 99.9% uptime. Even if other parts are more reliable, the system cannot be more available than its weakest link.

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