How to Approach System Design

System design interviews can be daunting. This guide provides a structured framework to help you navigate through the process effectively.

1. List Functional and Non-functional Requirements

The first step is to clarify the scope of the problem.

Functional Requirements

• Focus on 2-3 main features at the beginning.
• Add more detailed requirements as the discussion progresses.
• Example: For a URL shortener, functional requirements would be shortening a URL and redirecting.

Non-functional Requirements

• Clearly state details like Read/Write ratio.
• Discuss Consistency vs Availability (CAP Theorem).
• Define Latency and Throughput targets.

2. Back-of-the-envelope Calculation

Estimating the scale helps in making architectural decisions.

Common Formulas

• QPS (Queries Per Second): DAU * Avg Requests/User / 86400 seconds
• (Remember to mention the peak time and the average time QPS if necessary)
• Storage: DAU * Avg Item Size * Retention Period
• Unit Conversion:
  • 1 PB = 1,000 TB
  • 1 TB = 1,000 GB
  • 1 GB = 1,000 MB
  • 1 MB = 1,000 KB
  • 1 KB = 1,000 B
  • 1 B = 8 bits

Key Principles

1. Order of magnitude first: Give a rough estimate first, then refine it if necessary.
2. Focus on scale: The goal is to understand the system’s scale, not to be mathematically perfect.

3. High-level Design

Once the requirements and scale are clear, move to the architecture.

1. Draw a high-level architecture diagram: Identify the main components (Load Balancer, Web Servers, Database, Cache, etc.).
2. Basic solution first: Ensure the “happy path” is working before addressing edge cases.

4. Design Deep Dive

The interviewer will usually pick one or two areas for a more detailed discussion.

• Identify interest: Find what the interviewer is interested in (e.g., database sharding, caching strategy, or message queues).
• Concept focus: Focus on high-level concepts and let the interviewer drive the depth.
• Clarify assumptions: Always ask clarifying questions and state your assumptions explicitly.
• Trade-offs: Always talk about trade-offs (e.g., why choose SQL over NoSQL).

5. Tips for Success

• Don’t memorize solutions: Understand the principles instead.
• Collaboration is key: Chat with your interviewer like discussing a project with a partner.
• Avoid over-engineering: Start simple and scale as needed.
• Stay positive: Don’t argue with your interviewer; listen to their hints.