20 — System Design Interviews

System design interviews assess your ability to design large-scale systems. Unlike coding interviews, there’s no single correct answer — interviewers evaluate your thought process, trade-offs, and communication.

Analogy: A system design interview is like being an architect asked to design a stadium. The interviewer doesn’t expect you to know every beam and bolt. They want to see how you gather requirements, think about capacity, make trade-offs, and communicate your design.

flowchart TB
    Step1["1️⃣ Requirements<br/>5 min"] --> Step2{"Functional &<br/>Non-Functional"}
    Step2 --> Step3["2️⃣ Capacity Estimation<br/>5 min"]
    Step3 --> Step4["3️⃣ Data Model<br/>5 min"]
    Step4 --> Step5["4️⃣ High-Level Design<br/>10 min"]
    Step5 --> Step6["5️⃣ Deep Dive<br/>15 min"]
    Step6 --> Step7["6️⃣ Trade-offs & Scale<br/>5 min"]

    style Step1 fill:#f59e0b,color:#fff
    style Step3 fill:#3b82f6,color:#fff
    style Step5 fill:#7c3aed,color:#fff
    style Step6 fill:#059669,color:#fff
    style Step7 fill:#ef4444,color:#fff

1. Requirements (5 minutes) — Understand the problem before designing

2. Capacity Estimation (5 minutes) — Rough calculations

• DAU → Requests per second → Storage → Bandwidth → Servers
• Use round numbers (1M DAU, 10K RPS, 10TB storage)

3. Data Model (5 minutes) — Define the core entities

• Tables/collections, key fields, relationships
• Choose database type (SQL vs NoSQL) based on access patterns

4. High-Level Design (10 minutes) — Draw the architecture

• Start with a simple diagram: Client → LB → Server → DB → Cache
• Add components as needed (CDN, queue, search, analytics)

5. Deep Dive (15 minutes) — Focus on one area

• The interviewer will ask you to go deeper on a specific component
• Discuss algorithms, data structures, trade-offs

6. Wrap-up (5 minutes) — Trade-offs and scaling

• What would you do differently with more time?
• How would you scale to 10x users?

flowchart TB
    Clients["📱 Clients<br/>App / Browser"] --> LB["⚖️ Load Balancer"]
    LB --> CDN["🌍 CDN<br/>Static assets"]
    LB --> GW["🚪 API Gateway"]

    GW --> Service["🧩 Microservices"]
    Service --> Service1["User Service"]
    Service --> Service2["Content Service"]
    Service --> Service3["Notification Service"]

    Service1 & Service2 & Service3 --> Cache["⚡ Cache Layer<br/>Redis"]
    Service1 & Service2 & Service3 --> DB["🗄️ Database<br/>SQL / NoSQL"]
    Service1 & Service2 & Service3 --> Queue["📨 Message Queue<br/>For async tasks"]

    Cache --> DB
    Queue --> Workers["🔧 Background Workers"]

    style Clients fill:#f59e0b,color:#fff
    style LB fill:#3b82f6,color:#fff
    style GW fill:#7c3aed,color:#fff
    style Service fill:#059669,color:#fff
    style Cache fill:#ef4444,color:#fff
    style DB fill:#6366f1,color:#fff

1. Walk through your approach to designing a system from scratch.
2. How do you handle the trade-off between consistency and availability?
3. What numbers do you memorize for capacity estimation?
4. How do you decide when to use a cache vs a database?
5. What’s the most important non-functional requirement in system design?

• Requirements first — ask what the system should do before designing
• Draw the architecture — start simple (LB → Server → DB → Cache)
• Do rough math — DAU → RPS → servers → storage
• Discuss trade-offs — every design choice has pros and cons
• Deep dive where the interviewer asks — they’ll guide you to interesting areas
• Communicate — think out loud, explain your reasoning, ask clarifying questions
• There’s no perfect design — interviewers want to see your thought process, not a single answer
• Practice with mock interviews — system design is a skill that improves with practice