Design Interview Methodology

This skill provides a structured framework for approaching system design interviews and architectural discussions.

When to Use This Skill

Keywords: system design interview, whiteboard design, architecture discussion, technical interview, design framework

Use this skill when:

• Preparing for a system design interview
• Structuring a whiteboard architectural session
• Teaching others how to approach design problems
• Practicing with design exercises
• Leading architectural discussions with stakeholders

The 4-Step Framework

System design interviews typically last 45-60 minutes. This framework ensures you cover all bases while demonstrating structured thinking.

Step	Time	Purpose
1. Requirements	5-10 min	Clarify scope, constraints, scale
2. High-Level Design	10-15 min	Draw major components and data flow
3. Deep Dive	15-20 min	Detail 1-2 critical components
4. Wrap-Up	5-10 min	Trade-offs, bottlenecks, improvements

Step 1: Requirements Gathering (5-10 minutes)

Goal: Understand what to build and constraints before designing.

Functional Requirements

Ask about core functionality:

• "What are the primary use cases?"
• "What should users be able to do?"
• "What are the must-have vs. nice-to-have features?"

Non-Functional Requirements (NFRs)

Clarify quality attributes:

Category	Questions to Ask
Scale	How many users? DAU/MAU? Read/write ratio?
Performance	Latency requirements? Throughput targets?
Availability	Uptime requirements? (99.9% = 8.76 hours downtime/year)
Consistency	Strong or eventual consistency?
Durability	Data loss tolerance? Backup requirements?

Back-of-Envelope Estimation

Quick calculations to inform design:

Example: Design a URL shortener

Users: 100M monthly active
Writes: 100M URLs/month = ~40 writes/second
Reads: 10:1 ratio = 400 reads/second
Storage: 100M * 500 bytes = 50GB/month = 600GB/year

Detailed estimation techniques: See estimation-techniques skill.

Requirements Checklist

Before proceeding to design:

• Core use cases identified
• Scale understood (users, data, requests)
• Read/write ratio known
• Latency/throughput targets set
• Consistency requirements clear
• Availability targets established

Step 2: High-Level Design (10-15 minutes)

Goal: Draw the major components and show data flow.

Start with the Obvious

Begin with the simplest architecture that could work:

Client --> API Gateway --> Service --> Database

Then add complexity as needed based on requirements.

Core Components to Consider

Component	When to Include
Load Balancer	Multiple servers, horizontal scaling
API Gateway	Authentication, rate limiting, routing
CDN	Static content, global users
Cache	Read-heavy, latency-sensitive
Message Queue	Async processing, decoupling
Database	Persistent storage (SQL vs NoSQL decision)
Search	Full-text search, complex queries
Object Storage	Large files, media content

Data Flow Narration

Walk through the system explaining each step:

"When a user creates a short URL:
1. Request hits the load balancer
2. API Gateway validates the request and checks rate limits
3. Service generates a unique short code
4. Short code and URL are stored in the database
5. Cache is updated for fast reads
6. Response returns the shortened URL"

API Design

Sketch key API endpoints:

POST /urls          - Create short URL
GET  /{shortCode}   - Redirect to long URL
GET  /urls/{id}/stats - Get click analytics

Database Schema Sketch

Show key tables/collections:

URLs table:
- id: primary key
- short_code: indexed, unique
- long_url: original URL
- user_id: foreign key
- created_at: timestamp
- expires_at: nullable timestamp

Step 3: Deep Dive (15-20 minutes)

Goal: Demonstrate depth on 1-2 critical components.

How to Choose What to Deep Dive

Let the interviewer guide, or choose based on:

1. Most complex component - Shows technical depth
2. Most critical for requirements - Shows understanding
3. Your strength area - Play to your expertise

Common Deep Dive Topics

Topic	What to Cover
Database scaling	Sharding strategy, replication, indexes
Caching	Cache strategy, invalidation, hit ratio
Data consistency	Conflict resolution, distributed transactions
Search	Indexing, ranking, query optimization
Messaging	Delivery guarantees, ordering, dead letters
Rate limiting	Algorithm choice, distributed implementation

Example Deep Dive: URL Shortener ID Generation

Option 1: Auto-increment
  Pros: Simple, guaranteed unique
  Cons: Predictable, single point of failure, hard to scale

Option 2: UUID
  Pros: No coordination needed
  Cons: Too long (36 chars), not URL-friendly

Option 3: Base62 encoding of counter
  Pros: Short, URL-friendly
  Cons: Requires coordination for distributed systems

Option 4: Pre-generated IDs
  Pros: Fast, no runtime coordination
  Cons: ID exhaustion, more complex

Recommendation: Pre-generated ID ranges + Base62 encoding
- Each server gets a range of IDs
- IDs are Base62 encoded for short URLs
- Handles distributed scale without coordination per request

Quantify Trade-offs

Always explain trade-offs with specifics:

• "This adds latency of ~5ms but improves reliability from 99% to 99.9%"
• "This uses 3x more storage but reduces query time from 100ms to 10ms"
• "This increases complexity but allows horizontal scaling to 10x traffic"

Step 4: Wrap-Up (5-10 minutes)

Goal: Summarize, identify issues, and propose improvements.

Identify Bottlenecks

Where will the system break first as scale increases?

"The main bottleneck is the database. At 10x current scale:
- Write throughput becomes limiting
- Solution: Implement sharding by URL prefix
- Fallback: Read replicas for analytics queries"

Discuss Trade-offs Made

Summarize key decisions and alternatives:

"We chose eventual consistency for the redirect cache:
- Benefit: Lower latency, simpler architecture
- Cost: Up to 5 seconds of stale data possible
- Alternative: Strong consistency with higher latency"

Propose Improvements

If you had more time, what would you add?

Improvement	Benefit
Analytics pipeline	Usage insights, business value
Abuse detection	Malware/spam URL protection
Geographic distribution	Lower latency globally
A/B testing capability	Feature experimentation

Handle Edge Cases

Mention edge cases you'd address:

• What if the database is down?
• What if a URL expires mid-redirect?
• What if a malicious user spams URL creation?
• What about duplicate long URLs?

Common Pitfalls to Avoid

1. Jumping to Solution

Problem: Starting to design without understanding requirements. Fix: Spend 5-10 minutes on requirements first.

2. Over-Engineering

Problem: Adding every component you know. Fix: Start simple, add complexity only when justified by requirements.

3. Not Quantifying

Problem: Vague statements like "it's fast" or "it scales." Fix: Use numbers: "handles 10K requests/second with p99 latency of 50ms."

4. Ignoring Trade-offs

Problem: Presenting only benefits of your design. Fix: Actively discuss what you're sacrificing for each decision.

5. Silent Designing

Problem: Drawing without explaining. Fix: Narrate your thought process constantly.

Interview Signals: What Interviewers Look For

Strong Signals

• Asks clarifying questions before designing
• Starts with requirements, not solutions
• Quantifies scale with back-of-envelope math
• Explains trade-offs for each decision
• Identifies bottlenecks proactively
• Acknowledges what they don't know

Red Flags

• Jumps to favorite technology without justification
• Ignores scale/performance requirements
• Can't explain why a component is needed
• No consideration of failure scenarios
• Unable to identify trade-offs
• Defensive when challenged

Time Management Tips

Situation	Response
Running short on time	Skip to wrap-up, summarize trade-offs
Interviewer redirects	Follow their lead, they're guiding you
Stuck on a component	Acknowledge, move on, come back if time
Asked unknown technology	Explain your reasoning, ask about constraints

Practice Strategy

Before the Interview

1. Study common design problems (see design-problem-catalog skill - Phase 4)
2. Practice back-of-envelope calculations (see estimation-techniques skill)
3. Know your quality attributes (see quality-attributes-taxonomy skill)
4. Time yourself on practice problems

During Practice

• Set a 45-minute timer
• Practice on a whiteboard or paper (not IDE)
• Narrate your thoughts out loud
• Record yourself and review

Related Skills

• estimation-techniques - Back-of-envelope calculations for scale
• quality-attributes-taxonomy - NFRs and the "-ilities"
• cap-theorem - Consistency/availability trade-offs (Phase 2)
• design-problem-catalog - Common interview problems (Phase 4)

Related Commands

• /sd:design <problem> - Interactive design session (Phase 4)
• /sd:estimate <scenario> - Capacity calculations
• /sd:explain <concept> - Explain any concept

Related Agents

• system-design-interviewer - Mock interview practice (Phase 4)
• capacity-planner - Back-of-envelope calculations
• architecture-critic - Challenge your designs (Phase 4)

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Version History

• v1.0.0 (2025-12-26): Initial release

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Last Updated

Date: 2025-12-26 Model: claude-opus-4-5-20251101