Availability checking in LLD - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding Availability in Distributed Systems

Which of the following best describes the concept of availability in a distributed system?

A The system's ability to process requests within a fixed time limit under all conditions.

B The system's ability to remain operational and accessible for use when needed.

C The system's capacity to store large amounts of data without failure.

D The system's ability to prevent unauthorized access to data.

💡 Hint

Think about what it means for a system to be ready and usable at any time.

Availability means the system is up and ready to serve requests whenever users need it. It focuses on uptime and accessibility, not speed or security.

❓ Architecture

intermediate

Designing for High Availability

You are designing a web service that must be highly available. Which architectural choice most directly improves availability?

A Deploying multiple servers behind a load balancer with health checks.

B Implementing a caching layer that refreshes data every 24 hours.

C Encrypting all data stored in the database to prevent breaches.

D Using a single powerful server with a large database to handle all requests.

💡 Hint

Consider how to avoid a single point of failure.

Multiple servers with a load balancer and health checks ensure if one server fails, others can serve requests, improving availability.

❓ scaling

advanced

Scaling Availability for Peak Traffic

Your service experiences sudden spikes in traffic. Which scaling strategy best maintains availability during these spikes?

A Vertical scaling by upgrading the CPU and RAM of the existing server.

B Reducing the number of servers to save costs during low traffic periods.

C Horizontal scaling by adding more servers dynamically based on traffic load.

D Caching all responses indefinitely to avoid hitting the backend.

💡 Hint

Think about adding capacity quickly and flexibly.

Horizontal scaling adds more servers as needed, distributing load and maintaining availability during traffic spikes.

❓ tradeoff

advanced

Tradeoffs Between Consistency and Availability

In the context of the CAP theorem, which statement correctly describes the tradeoff between consistency and availability?

A During network partitions, a system must choose to sacrifice either consistency or availability.

B A system can guarantee both consistency and availability at all times during network partitions.

C Consistency and availability are unrelated and can be optimized independently.

D Availability is always more important than consistency in distributed systems.

💡 Hint

Recall the CAP theorem's core principle about network partitions.

The CAP theorem states that during network partitions, a distributed system can only guarantee either consistency or availability, not both.

❓ estimation

expert

Estimating Required Server Capacity for 99.99% Availability

You operate a service with a target of 99.99% availability. Your current single server has an average downtime of 1 hour per month. How many identical servers with automatic failover do you need to deploy to meet the availability target?

Assume failures are independent and failover is instant.

A 4 servers

B 1 server

C 3 servers

D 2 servers

💡 Hint

Calculate availability per server and use the formula for combined availability with failover.

One server availability = (total time - downtime) / total time = (720 - 1) / 720 ≈ 0.9986 (99.86%). Two servers with failover availability = 1 - (1 - 0.9986)^2 ≈ 0.99999796 (99.9998%), which exceeds 99.99% target.