Rubric for System Design Interviews

In system design interviews, you’re assessed on your ability to build performant, scalable, efficient, and fault-tolerant systems. You’re also assessed on how you communicate your design decisions.

The expectations for an advanced candidate are higher, especially around identifying bottlenecks, discussing tradeoffs, integrating business objectives, and mitigating risk.

While system design interview rubrics vary based on companies and roles, this lesson provides a general overview of the key rubric signals:

• Problem understanding and requirement collection: assesses your ability to ask clarifying questions and understand the problem, system requirements, and relevant constraints.
• Technical design and tradeoffs: assesses your ability to make and justify tradeoffs between design choices, given the associated risks with each key design decision.
• Scalability and performance: assesses how you approach scaling techniques to handle increased load or growth.
• Fault tolerance and reliability: assesses how you incorporate fault tolerance and reliability in your design decisions.
• Communication and collaboration: assesses your ability to communicate your design choices, discuss technical concepts, and collaborate with the interviewer.

An interviewer will assess these signals using a rating scale of “very weak” to “very strong.” The overall rating of the five rubric signals translates to:

• Very Weak or Missing: no hire
• Weak: no hire, but interviewer can be convinced if candidate did exceptionally well in other interview rounds
• Strong: hire, but interviewer may be convinced to no hire if candidate did poorly in other rounds
• Very Strong: strong hire, interviewer will advocate for the candidate, even if other rounds went poorly. Can lead to “upleveling.”

Problem understanding and requirement collection

• Very Weak or Missing: Fails to ask clarifying questions and jumps into the design without scoping the problem according to the system requirements. Overlooks the big picture and/or goes down a rabbit hole solution that doesn’t work and wastes time.
• Weak: Asks questions, which demonstrates baseline interview prep on how to approach the question, but doesn’t utilize the interviewer’s responses to guide the answer. Starts solutioning without fully exploring product and/or scale requirements.
• Strong: Asks thoughtful questions to identify important system requirements and constraints. Clarifies most of the ambiguity around product requirements and scale requirements.
• Very Strong: Proactively explores the design space by asking clarifying questions about what is needed vs. what has already been implemented. Clearly identifies the problem statement, system requirements, and any relevant constraints. Scopes the problem to fit the interview's time constraints. May anticipate future project and business requirements that exceed the interviewer’s expectations.

An example that aces this rubric category:

• For an interview question about an unreliable network, you might ask: “Do we prefer to ensure the successful delivery of a message (at the cost of certain redundancy), or can we tolerate a lossy message transfer?” This clarifying question considers potentially conflicting design requirements/constraints. It identifies the most common operating conditions that satisfy the requirements for the given constraints.

Technical design and tradeoffs

• Very Weak or Missing: Fails to explain why design choices were made and/or does not explain how the chosen components present tradeoffs.
• Weak: Implements some parts of a working solution and identifies some tradeoffs, but doesn’t explain how they apply to the problem. Doesn’t justify design decisions nor anticipate potential points of failure in the system.
• Strong: Implements a working solution, considers key tradeoffs, and justifies design choices with a detailed understanding of how the components interact with each other.
• Very Strong: Shows technical depth by deep-diving into specific parts of the system. Assesses tradeoffs by analyzing risks and justifying design decisions with real-world evidence.

Examples that ace this rubric category:

• Performance vs. cost: Performance-enhancing resources, such as computational power, memory, or network bandwidth, come at a cost. Strike a balance between optimal performance and budgetary constraints.
• Consistency vs. availability: Strong consistency across a distributed system can ensure data integrity and prevent conflicts, but it often requires coordination and synchronization, which can hinder system availability.
• Latency vs. consistency: In distributed systems, ensuring strong consistency can increase latency, which compromises the user experience. Balance the need for low latency with the level of consistency required for the application.
• Security vs. usability: Implementing stringent security measures, such as complex authentication or encryption protocols, to enhance system security can introduce usability challenges and hinder the user experience. Balance the need for strong security and usability to ensure both are prioritized appropriately.
• Storage efficiency vs. read/write performance: Optimizing storage efficiency by using compression or compact data formats to reduce storage costs can compromise read and write performance due to the additional processing required. Determine the optimal tradeoff based on the specific requirements of the system.

Scalability and performance

• Very Weak or Missing: Overlooks potential bottlenecks and implements ill-fitted scaling techniques. Uses arbitrary numbers to calculate solutions for bottlenecks and system performance. May also misidentify the bottleneck, due to a lack of experience and/or large-scale knowledge.
• Weak: Identifies some bottlenecks, but provides incomplete or overcomplicated mitigation strategies. May blindly use certain technologies without sufficient reasoning, such as using a NoSQL database because it “scales better.” Demonstrates poor intuition when estimating data by making implausible calculations.
• Strong: Demonstrates a strong understanding of how to scale the system by leveraging industry best practices to address bottlenecks.
• Very Strong: In addition to leveraging industry best practices, provides creative and insightful scaling strategies to handle increased load or growth. Chooses the appropriate algorithms, frameworks, and technologies to address potential bottlenecks. Uses concrete numbers, such as back-of-the-envelope calculations, to satisfy the specific scaling and performance requirements.

Examples that ace this rubric category:

• Horizontal scaling vs. vertical scaling: Horizontal scaling adds more instances or nodes to the system to handle increased load. Explore approaches like load balancing, sharding, or partitioning to distribute the workload across multiple servers or resources. Introduce vertical scaling, which increases the resources of individual nodes, such as CPU or memory, to handle increased load. Consider opting for horizontal scaling for a large distributed system, given that it's a more common and cost-effective pattern.
• Caching and content delivery: Caching helps reduce the load on backend systems and improve response times. Explore caching strategies like in-memory caches, distributed caching, or content delivery networks (CDNs) to serve static content closer to users.
• Asynchronous processing: For offline/non-real-time tasks, asynchronous processing could remove a large portion of a system from a latency-critical or resource-sensitive path. Consider message queues, event-driven architectures, or asynchronous workflows to decouple components and improve system responsiveness.
• Database scaling: Discuss approaches like database partitioning, sharding, replication, or utilizing distributed databases to handle increasing data volumes and achieve high performance.

Fault tolerance and reliability

• Very Weak or Missing: Fails to identify the system’s points of failure. Does not consider fault-tolerant strategies that help the system maintain high availability.
• Weak: Implements some strategies to support fault tolerance and reliability, but overlooks potential points of failure. Provides an incomplete solution if asked to solve a point of failure by the interviewer.
• Strong: Considers appropriate strategies to support fault tolerance and reliability, with potential points of failure in mind.
• Very Strong: Designs a resilient system that tolerates failures and ensures high availability. Identifies potential points of failure, such as hardware failures, software bugs, network issues, and/or external dependencies. Acknowledges that there may not be perfect solutions to specific failure scenarios and presents tradeoffs based on key parameters, such as business/product requirements and/or customer behavior.

Examples that ace this rubric category:

• Redundancy and replication: Explore using multiple instances, replicating data across multiple servers or regions, and/or implementing backup systems. Explain how these measures mitigate the impact of failures. Explain how you would design backup and restore mechanisms, implement data replication across multiple geographic regions, or leverage cloud services to achieve geo-redundancy.
• Automated failure recovery: Discuss strategies for automated failure recovery, such as self-healing, automatic scaling, or fault recovery algorithms. Explain how you would handle failover scenarios, automatically reroute traffic, or restore failed components without manual intervention.
• Failure detection and monitoring: Discuss techniques such as health checks, heartbeat mechanisms, or distributed monitoring systems to identify and respond to failures promptly.
• Error handling and retry mechanisms: Discuss approaches such as exponential backoff, circuit breakers, or retry queues to handle transient failures and prevent cascading failures.
• Data durability and consistency: Explain concepts like write-ahead logging, distributed transactions, or eventual consistency models to handle failures without compromising data integrity.
• Testing and simulations: Discuss techniques such as chaos engineering, fault injection, or stress testing to proactively identify weaknesses and validate the system's fault tolerance mechanisms.

Communication and collaboration

• Very Weak or Missing: Fails to check in with the interviewer to ensure a thorough understanding of the requirements and constraints. Presents the solution in a confusing and disorganized manner. Ignores hints from the interviewer, demonstrating a lack of receptiveness to feedback.
• Weak: Struggles to collaborate with the interviewer, toggling between communicating design decisions and working silently. Interviewer frequently intervenes and drives the discussion. Fails to clarify points of confusion and integrate the interviewer’s hints into the design.
• Strong: Demonstrates active listening and clear communication skills throughout the interview. Any confusion or clarification requests from the interviewer are minor. Checks in with the interviewer to see whether the solution is on the right track and/or stops to see if the interviewer wants to deep dive on a specific topic.
• Very Strong: Effectively communicates design choices, discusses technical concepts, and collaborates with the interviewer. Shows genuine interest in the interviewer's feedback and is receptive to constructive criticism.

Examples that ace this rubric category:

• Active listening: Pay attention to the interviewer's questions and comments. Clarify any ambiguities and ask follow-up questions to ensure a thorough understanding of the requirements and constraints.
• Structured approach: It’s often desirable to present the solution in a hierarchical approach, providing a high-level overview of the system first before diving into details. Break down complex problems into smaller components, explain the reasoning behind your choices, and clearly justify your design tradeoffs.
• Visual aids: Utilize visual aids, such as diagrams or whiteboard sketches, to illustrate your system design. Visual representations can help convey complex ideas more effectively and facilitate better understanding with the interviewer.
• Collaboration: Seek the interviewer’s input and feedback, and encourage a dialogue to foster a collaborative atmosphere. If faced with a challenging problem during the interview, actively collaborate with the interviewer to explore potential solutions. Engage in a discussion, brainstorm alternative ideas, and consider different perspectives.
• Clear and concise communication: Use clear and concise language to convey your ideas. Avoid technical jargon or unnecessary complexity that may confuse the interviewer.
• Contextualize your design: Clearly explain the context and assumptions underlying your design decisions. Discuss the specific problem domain, user requirements, and any constraints that influenced your design. This demonstrates your ability to understand the broader context of the system.
• Clarity in documentation: If asked to document your design, ensure your documentation is clear, well-structured, and easily understandable. Use proper headings, diagrams, and concise descriptions to effectively communicate your design to others.
• Positive attitude: Maintain a positive and enthusiastic demeanor throughout the interview. Show genuine interest in the interviewer's feedback and be receptive to constructive criticism. Demonstrating a positive attitude indicates your willingness to collaborate and learn from others.

Depending on your background and specific roles, the system design interview might involve other grading rubrics, including:

• Security and privacy: For security-related positions, you’re also assessed on your ability to implement security components such as authentication, authorization, encryption, secure communication, security auditing/logging, and be mindful of data privacy. Techniques such as password hashing, token-based authentication, OAuth (Open Authorization), and protocols like SSL (Secure Sockets Layer), TLS (Transport Layer Security), and RBAC (Role-Based Access Control) help ensure the security and privacy of user data.
• API design: You might be guided to show system APIs that fulfill the functional requirements. Well-designed APIs simplify usages for end users and the interactions between different system components.

As you can imagine, arriving at great system designs in an interview scenario takes a lot of practice. It also takes a structured approach. In our How to Answer System Design Interview Questions lesson, we teach you a framework for working through these open-ended questions. You have a lot of freedom to adjust the framework to suit you, but we strongly recommend applying a framework since it is easy to get off track.