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.