The purpose of a system design interview is to assess a candidate's ability to design and understand complex and large-scale software systems. It's a crucial part of the hiring process for roles that involve software systems and architectures, such as software engineers, system architects, and DevOps engineers.

Unlike coding interviews that have specific solutions, system design questions are open-ended – there isn’t a single “correct” answer, and the emphasis is on the approach and reasoning behind the design​. The goal is to evaluate how you think through large-scale problems and make engineering decisions, rather than to produce a snippet of code.

Why do companies conduct system design interviews?

In essence, it’s to assess whether you can apply engineering knowledge to real-world system architecture challenges. Key purposes include:

• Assessing problem-solving skills: Can the candidate navigate complex, ambiguous problems and figure out a viable system design? Interviewers look at how you approach an open-ended challenge from scratch. It’s not just about the final answer, but how you break the problem down and work towards a solution.
• Evaluating technical knowledge: System design interviews gauge your understanding of various system architecture components – for example, databases, APIs, caching, load balancing, and network protocols. The interviewer wants to see that you know how different pieces of a large system fit together.
• Considering scalability and reliability: A good design accounts for important qualities like scalability, reliability, and maintainability of the system. You’re expected to discuss how your design can handle growth (more users, more data) and remain stable and performant over time.
• Trade-off analysis and decision-making: There are many ways to design a system, so interviewers pay attention to why you choose a certain approach. You should explain the trade-offs of your design decisions (e.g. why choose one database or architecture over another) and demonstrate sound decision-making in balancing factors like performance, complexity, and cost.
• Communication and collaboration: System design interviews are typically interactive. You’ll be explaining your ideas on the fly, answering questions, and maybe whiteboarding diagrams. Clear communication is essential – the interviewer wants to see that you can articulate your design and collaborate on ideas (much like you would with teammates).

What System Design Interview is Not About

• It’s not about getting the perfect answer. In fact, there often isn’t one “correct” solution in system design.
• It’s not a test of memorization. While familiarity with certain tools and technologies is helpful, understanding the concepts is more important.

Conclusion

The system design interview is a critical tool for employers to assess not just your technical abilities, but also your problem-solving approach, communication skills, and overall fit for roles that require designing and working with large-scale systems. It's a comprehensive evaluation of how you would handle real-life challenges in system architecture and engineering.