System design interviews often feel vague at a high level. You may be asked to design a large-scale system, one that took years to build, in under an hour. Clearly, it is impossible to design such a system in full detail within that time. So what, then, is the real purpose of a system design interview?
A system design interview is not about building a production-ready system. It is an exercise designed to evaluate your ability to reason about complex problems, structure a solution, and explain design decisions along with their tradeoffs. Interviewers are far more interested in how you think than in the final architecture you propose.
Every system design problem is different, and there is no one-size-fits-all solution. That said, there are common steps you can follow to approach most system design interviews effectively.
Below is a simple and practical template for navigating system design interviews.
A 4-step process for effective system design interviews
Step-1 Understand the problem and establish design scope**
Do not rush into designing the system before fully understanding the problem. Start by clarifying the requirements and constraints.
Key questions to ask include:
- What does the system do?
- How many users will the system support?
- What guarantees must the system provide?
- Availability?
- Consistency?
- Scalability?
- What is the expected growth pattern?
- Are there existing services or components we can leverage?
These questions help define the scope of the system and guide your design decisions and tradeoffs.
Note that these are only sample questions. The exact questions you ask will depend on the system you are asked to design. Sometimes the interviewer will provide direct answers, and other times you will be expected to make reasonable assumptions.
As you go, clearly document all requirements and assumptions. This ensures you do not miss important details and gives you a solid reference point to design against.
Example:
Say you are asked to design a news feed system.
The conversation between you and your interview may look something like this.
Interviewer:
Design a news feed system.
Candidate:
Before I start designing, I would like to clarify the core requirements. What are the main features we want to support?
Interviewer:
Users should be able to see a personalized feed of posts from accounts they follow.
Candidate:
Understood. To size the system properly, how many users do we expect to support?
Interviewer:
Around 100 million monthly active users, with about 20 percent daily active users.
Candidate:
So roughly 20 million daily active users. How frequently do users create posts?
Interviewer:
Each active user creates about one post per day.
Candidate:
How often do users read or refresh their feed compared to posting?
Interviewer:
Users read their feed much more frequently, about 50 feed reads per post.
Candidate:
What are the latency and freshness requirements?
Interviewer:
Feed loads should be under 200 milliseconds, and new posts should appear in followers’ feeds within a few seconds.
Step 2: Propose a high-level design and get buy-in
Once the problem and scope are clear, propose a high-level design. At this stage, treat the interviewer as a teammate and align on the overall approach before going deeper.
Ask yourself what the main components of the system are and how they interact to solve the problem.
For the news feed example, the system can be broken down into two main flows:
- Feed publishing: A user creates a post, which is validated and persisted.
- Feed building: A user’s feed is generated by aggregating posts from followed accounts, typically in reverse chronological order.
This level of abstraction ensures you and the interviewer agree on the system’s structure before diving into details.
Step 3: Deep dive into key components
After agreeing on the high-level design, prioritize the most critical components and dive deeper into them. Focus on areas that are central to the system’s functional requirements.
This is where you discuss:
- Data models
- Read versus write tradeoffs
- Scalability strategies
- Caching
- Bottlenecks and failure modes
Explain why you choose certain approaches and what tradeoffs they introduce. This step is often the core of the interview.
Step 4: Wrap up and discuss improvements
In the final stage, the interviewer may ask follow-up questions such as:
- How would you improve the system?
- What happens if the user base grows by 100 times?
- What are the main bottlenecks?
- Where does the system fall short?
No system is perfect. Being open about limitations and discussing how you would address them leaves a strong impression and demonstrates maturity in system design thinking.
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