How I Structured My System Design Interviews (L5/L6)

Note: This is mostly directed towards senior/staff engineers, whereas junior engineers would likely spend less time on optimizations.

Whether a senior/staff system design interview (check out the differences between L5 and L6 interviewing expectations here) ends up meriting an offer often depends on how well-prepared a candidate is and how much they know about the interview’s structure. The system design interview is a crucial phase in technical evaluations, especially for engineering roles involving the design of complex systems. Conducting a well-structured interview ensures that candidates’ problem-solving skills, architectural understanding, and optimization abilities are thoroughly evaluated. In this blog post, we will outline a structured approach for a one-hour system design interview, highlighting key steps and focus areas to assess candidates effectively.

  • Gather Requirements and Understand Scalability (5 minutes):

The first five minutes of the interview are dedicated to gathering functional and non-functional requirements. Functional requirements define what the system should do, while non-functional requirements focus on performance, scalability, availability, and other quality attributes. Additionally, understanding the system’s scalability characteristics helps in designing a system that can handle increasing loads.

During this phase, the interviewer will pose relevant questions to assess the candidate’s ability to identify requirements and potential bottlenecks (and gauge depth of knowledge). For example, if the candidate is designing a social media platform, questions could revolve around user authentication, data storage, and the expected number of active users.

  • Propose API Endpoints (4-5 minutes):

The next phase involves proposing API endpoints, which serve as a foundation for the system’s architecture. Candidates should outline the core functionalities and interactions between different system components. The focus is on the data flow among different modules – often rushing through this part results in a system where something is missing (i.e. a video platform that serves content, but the upload process is left undiscussed).

In this part of the interview, the interviewer can evaluate the candidate’s ability to break down a problem into smaller components and identify how different modules communicate with each other. For example, for a ride-hailing application, candidates should discuss endpoints like request ride, cancel ride, driver location update, etc.

  • Develop the High-Level Design (5-10 minutes):

With the API endpoints proposed, candidates can now elaborate on the High-Level Design (HLD). The HLD should encompass various design-specific components, their interactions, and supplementary components such as load balancers, caches, and queues. Candidates should consider the pros and cons of different design choices and select the most suitable ones based on the system’s requirements.

During this phase, the interviewer can assess the candidate’s ability to create an effective system architecture. For example, for a messaging application, candidates should discuss the database schema for storing messages, user profiles, and any other relevant data.

  • Focus on Optimizations (40-45 minutes):

The majority of the remaining interview time should be dedicated to optimizing crucial parts of the system. Candidates should identify potential bottlenecks and propose optimizations to enhance system performance, scalability, and efficiency. Here are ways one might deep-dive for two different problems:

a. Search Engine Optimization:

  • Discuss optimizing the inverted index to reduce search latencies.
  • Explore techniques like caching popular search results.
  • Introduce sharding to distribute search load efficiently.

b. Twitter Clone Optimization:

  • Address latencies in pushing tweets to millions of followers.
  • Propose fan-out-on-write or fan-out-on-read strategies for follower updates.
  • Explore techniques like compression to reduce storage requirements.

Conducting a well-structured system design interview requires a systematic approach that covers gathering requirements, proposing API endpoints, creating a High-Level Design, and focusing on optimizations. By following this approach, interviewers can effectively assess candidates’ architectural and optimization skills, ensuring the selection of the best-suited individuals for complex engineering roles. Remember to encourage open discussions, explore different scenarios, and appreciate innovative solutions during the interview process. Happy interviewing!

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