How do you run a design interview?

Running a design interview involves creating an open, collaborative environment where the candidate feels encouraged to think critically, communicate clearly, and showcase their problem-solving skills. Here’s a step-by-step guide to running a productive design interview:

1. Set the Stage with a Brief Introduction

Begin by explaining the format and objectives of the interview. Let the candidate know what you’re looking for, such as their approach to problem-solving, communication, and understanding of scalable design principles.

Example Introduction: “Today, we’ll work through a system design problem. I’m interested in hearing your thought process, design choices, and how you address scalability and reliability. Feel free to ask questions and clarify assumptions as we go.”

2. Present the Problem Clearly

Choose a well-defined, relevant design problem that gives the candidate enough room to showcase their skills without overwhelming them. Common examples include designing a URL shortener, social media feed, or messaging system.

Example Prompt: “Let’s design a URL shortener similar to TinyURL. Users should be able to input a long URL and receive a shortened version that redirects to the original URL. What are your thoughts on how to approach this?”

3. Encourage Requirement Clarification

A strong design interview begins with clarifying requirements. Allow the candidate to ask questions, as this shows they understand the importance of defining functional and non-functional requirements.

Encouragement: “Great! Let’s focus on the must-have requirements first, but feel free to ask for any details you think are essential for a solid design.”

4. Observe How They Define the System Requirements

Look for a structured approach where the candidate identifies functional and non-functional requirements, such as scalability, latency, and consistency needs. Taking notes helps you track which areas they cover and where they might need a prompt.

Functional Requirements: Key features the system must have.
Non-Functional Requirements: Performance, scalability, availability, and reliability.
Guiding Question (if needed): “Have you considered the traffic or scalability requirements for this system?”

5. Encourage a High-Level Architecture Design

Once requirements are clarified, prompt the candidate to draw a high-level architecture diagram. Encourage them to focus on the main components and data flow. This shows how they approach structuring a complex system.

Key Components to Look For: Client interface, load balancer, application servers, database, caching, etc.
Follow-Up Prompt: “Could you describe the main components and how they would interact in your design?”

6. Allow Deep Dives into Key Components

After they’ve presented a high-level design, ask them to dive deeper into specific components. This gives insight into their technical knowledge and ability to address complex aspects of system design.

Common Deep Dive Areas:
- Database Design: Choice of SQL vs. NoSQL, schema, partitioning.
- Caching Strategy: What to cache and where, and cache invalidation.
- Load Balancing and Scaling: How they’d handle increased traffic.
- Data Consistency and Replication: Trade-offs between consistency and availability.
Prompt if Needed: “Could you explain how you would ensure quick data retrieval in your design? What caching strategy might you use?”

7. Assess Their Approach to Scalability and Reliability

Prompt the candidate to address scalability and reliability to understand if they know how to handle large-scale systems.

Topics to Encourage:
- Horizontal Scaling: Adding more servers to balance the load.
- Data Partitioning and Replication: To distribute data and increase availability.
- Failover Mechanisms: Ensuring the system can handle failures without downtime.
Prompt if Needed: “How would your design handle a sudden increase in traffic or ensure the system remains available if a server fails?”

8. Discuss Trade-Offs and Decision-Making

Ask the candidate to explain their decision-making process and address any trade-offs involved. This gives insight into their ability to make informed choices based on system requirements.

Common Trade-Offs to Explore:
- Consistency vs. Availability: Understanding the CAP theorem.
- Performance vs. Cost: When to prioritize performance improvements.
- Complexity vs. Simplicity: When a simple solution may be preferable.
Sample Prompt: “I see you chose eventual consistency here. Could you explain why and any potential trade-offs?”

9. Consider Security and Data Privacy

If applicable, ask the candidate to consider security measures, especially if the system handles sensitive information.

Security Aspects to Cover:
- Authentication and Authorization
- Data Encryption
- Rate Limiting and Abuse Prevention
Follow-Up Prompt: “How would you secure user data in your design, particularly for authentication and preventing abuse?”

10. Allow Time for Scaling and Optimization Discussions

Encourage the candidate to think about possible optimizations or cost-efficiency improvements in their design.

Areas to Focus On:
- Caching Optimization: Reducing database load.
- Auto-Scaling: Scaling resources up and down based on traffic.
- Cost-Saving Measures: Using cloud services or serverless functions.
Prompt: “If we wanted to optimize for cost, how would you modify your design?”

11. Encourage Clear Communication and Use of Diagrams

Encourage the candidate to draw diagrams and explain their thought process at each step. This helps clarify their ideas and ensures the interviewer can follow along.

Communication Tips for the Candidate: Use clear language, avoid jargon, and walk through each design component step-by-step.
Diagram Prompt: “Could you use a diagram to illustrate how data flows through your design?”

12. Ask Follow-Up and What-If Questions

Introduce follow-up or “what-if” scenarios to test how the candidate would handle changes to requirements or unforeseen issues. This assesses adaptability and resilience in problem-solving.

Example Scenarios:
- “What if traffic suddenly doubled—how would you adapt the design?”
- “How would you add real-time data updates if that became a requirement?”

13. End with Feedback and a Wrap-Up

Close the interview by giving the candidate a chance to ask any questions or clarify parts of their design. Offer constructive feedback and highlight any strengths or areas for improvement.

Feedback Tips: Be specific, noting what the candidate did well and any parts of the design that could be strengthened.
Wrap-Up Statement: “Thank you for walking me through your design. I appreciated how you handled [specific part of design], and I encourage you to keep focusing on [area for improvement].”

By following this structured approach, you can effectively assess a candidate’s technical skills, problem-solving abilities, and communication style in a system design interview. This approach not only helps evaluate the candidate thoroughly but also creates an interactive and supportive environment where they feel encouraged to think critically and do their best.