Approaching System Design Questions

The Uncomfortable Truth About System Design Interviews

Let’s be honest: system design interviews terrify most engineers. And unlike coding interviews where there’s a clear right answer (your solution either works or it doesn’t), system design is fundamentally different. There’s no single “correct” architecture. You could design Twitter with a monolith, microservices, or a hybrid approach, and all could be defensible depending on constraints.

Here’s what you need to know: interviewers aren’t looking for the perfect design. They’re evaluating your thought process, how you communicate, and whether you can navigate ambiguity with confidence. That’s the good news. You don’t need to know everything. You need to have a process.

The 4-Step Framework

The secret to approaching any system design question is having a repeatable framework. This one has worked for thousands of candidates because it mirrors how real architects actually think:

Step 1: Understand the Problem and Gather Requirements (5 minutes)

Before you draw anything, you need to understand what you’re actually building.

• Ask clarifying questions about functional requirements (what features should the system have?)
• Identify the users and their journeys
• Establish non-functional requirements (scale, latency, consistency)
• Clarify edge cases and constraints

Pro tip: Write the requirements down on the whiteboard as the interviewer answers. This shows discipline and gives you something to reference when the design gets complex.

Step 2: Estimate Scale and Define Constraints (5 minutes)

Put numbers on the problem. How many users? How many requests per second? How much storage?

• Calculate daily active users and derive requests/second
• Estimate storage and bandwidth needs
• Understand the read-to-write ratio
• Account for peak traffic (typically 2-5x average)

This step is crucial because it determines your entire architecture. A system for 1,000 requests/second is fundamentally different from one handling 100,000 requests/second.

Step 3: Design High-Level Architecture (15-20 minutes)

Now you can sketch the system at 10,000 feet.

• Draw boxes for major components (load balancers, web servers, databases, caches, queues)
• Show data flow between components
• Identify single points of failure
• Sketch how requests move through the system

Don’t obsess over details yet. This is about getting the big picture right. Think of it like sketching a building’s floor plan before worrying about electrical outlets.

Step 4: Deep Dive into Critical Components (15-20 minutes)

This is where you show your depth. Pick 2-3 components that are critical to the system and go deep.

• Database schema design and indexing strategy
• Caching strategy (what to cache, where, TTL)
• How to handle specific challenges (consistency, replication, sharding)
• Trade-offs you made and why

The interviewer will often guide you here. “Tell me more about your database design” or “How would you handle consistency?” Follow their lead.

What Interviewers Actually Evaluate

Understanding the evaluation criteria helps you understand what the interviewer is listening for:

Criterion	What They’re Looking For	Red Flag
Structured Thinking	Do you have a process? Do you ask questions before jumping to solutions?	Jumping straight to technology choices without understanding requirements
Communication	Can you explain your reasoning clearly? Can you discuss trade-offs?	Designing silently, then struggling to explain decisions
Trade-off Analysis	Do you consider alternatives and justify your choices?	Defending your design without acknowledging drawbacks
Technical Depth	Can you go deep on critical components? Do you know how databases, caches, and queues work?	Shallow understanding of the technologies you’re proposing
Pragmatism	Are your solutions realistic? Do you consider operational complexity?	Over-engineering, designing for Google scale when the problem says “startup”

Pro tip: Interviewers would much rather see you admit “I’m not sure about that — let me think through it” than give a confident but incorrect answer.

Common Mistakes (and How to Avoid Them)

We’ve seen these patterns repeatedly in interviews. Avoid them:

Mistake 1: Jumping to the solution without asking questions

You hear “Design Twitter” and immediately start drawing a feed microservice, tweet cache, and notification system. But hold on — are we designing for millions of users or a startup with 10,000? Can we tolerate stale data? How important is strong consistency?

Fix: Ask questions first. Always.

Mistake 2: Over-engineering for scale you don’t need

Designing a chat application for a startup with 1,000 users using a globally distributed, strongly consistent system? That’s premature optimization. Keep it simple until the requirements demand complexity.

Fix: Design for the scale stated in the problem, not Google scale.

Mistake 3: Under-communicating while you design

You’re drawing boxes and the interviewer doesn’t understand what you’re thinking. Is that box a load balancer or a cache? Are you replicating synchronously or asynchronously?

Fix: Explain out loud as you draw. Narrate your thinking.

Mistake 4: Ignoring non-functional requirements

You design a system that works but doesn’t address availability, consistency, or latency targets. Non-functional requirements are often more important than the feature list.

Fix: Explicitly confirm non-functional requirements and refer back to them as you design.

Mistake 5: Getting lost in one component

You spend 20 minutes debating database indexing strategy while ignoring how caching fits in or whether you need a message queue. This loses the big picture.

Fix: Keep reminding yourself: “How does this component fit into the overall system?”

Mistake 6: Not drawing diagrams

Boxes, arrows, and labels. Visuals help both you and the interviewer think through the design. A good diagram is worth 100 words of explanation.

Fix: Get up and draw early. Update the diagram as you evolve the design.

How to Practice Effectively

System design is a skill you build through deliberate practice, not passive reading.

Mock Interviews

Find someone to interview you. Use platforms like Exponent, Interview Kickstart, or find peers in your network. The real-time feedback is invaluable — you’ll discover communication gaps you didn’t know existed.

Practice With a Timer

Set a 45-minute timer. Gather requirements (5 min), estimate scale (5 min), high-level design (20 min), deep dive (15 min). You’ll quickly learn how to pace yourself.

Explain Out Loud

Even practicing alone, explain your design decisions verbally. This catches fuzzy thinking fast. If you can’t explain it clearly, you don’t understand it well enough.

Study Real-World Architectures

Read how companies actually design systems. Twitter engineering blog, Amazon’s architecture whitepaper, Uber’s blog. Not to memorize their design, but to see how senior engineers think through problems.

Key Takeaways

• System design interviews evaluate your process and communication, not your knowledge of obscure technologies
• The 4-step framework (gather requirements → estimate scale → high-level design → deep dive) works for any system design question
• Non-functional requirements are just as important as features
• Draw diagrams and communicate out loud — never design in silence
• Interviewers care more about your reasoning than your final answer
• Practice with time pressure and get real-time feedback

Practice Exercise

Pick one of these systems and apply the framework. Spend 10 minutes gathering requirements and estimating scale:

1. Design a URL shortener
2. Design a real-time multiplayer game backend
3. Design a notification system for a social media platform

For each, write down:

• The key functional requirements
• The key non-functional requirements (scale, latency, consistency)
• Estimated QPS and storage needs
• 3-5 major components for the high-level architecture

Next: Gathering Requirements Effectively

Now that you have the framework, let’s dive deep into the first step. Gathering requirements seems simple until you realize how many wrong assumptions engineers make. We’ll learn the difference between functional and non-functional requirements, and how to ask the right questions that show senior-level thinking.