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Study Guide

Repetition is the mother of all skills

Tony Robbins

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  %% Kickoff container at top
  subgraph Kickoff["**Kickoff**"]
    direction TB
    style Kickoff stroke-dasharray:5,5,fill:none,stroke-width:2px
    A[<a href="#understand-what-a-system-design-interview-is-really-about">What is a system design interview <em>really</em> about?</a>]
    B[<a href="#alex-xus-four-phase-framework">Alex Xu's 4 steps </a>]    
    C[<a href="#abc-cheat-code">ABC Cheat Code</a>]   
    D[<a href="#cracking-the-system">Cracking the system</a>]
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  subgraph GapAssessment["**Gap Assessment**"]
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    GA2[<a href="#gap-assessment">Try sample solved interviews and compare your results</a>]
    GA1[<a href="#refresh-terms-and-building-blocks">Learn/refresh terms and building blocks</a>]
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  %% Personal Plan container
  subgraph PersonalPlan["**Personal Learning Plan**"]
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    style PersonalPlan stroke-dasharray:5,5,fill:none,stroke-width:2px
    %% use href to point both to Personal Plan header  
    PP1[<a href="#personal-learning-plan">Harness your personal strengths in the learning process</a>]
    PP2[<a href="#personal-learning-plan">Shortcuts?</a>]
  end

  %% Learning Loop container
  subgraph LearningLoop["**Learning Loop**"]
    direction LR
    style LearningLoop stroke-dasharray:5,5,fill:none,stroke-width:2px
    %% use href to point both to Learning Loop header  
    GapAnalysis["Goal"]
    LearnTry["Learn / Solve / See Solutions"]
    Test["Check"]
    Assess["Assess Gaps"]
  end


  %% Container flow
  Kickoff --> GapAssessment
  GapAssessment --> PersonalPlan
  PersonalPlan -->  LearningLoop


  %% Internal flows
  A --> B
  B --> C  
  C --> D
  D --> B

  GA1 --> GA2
  GA2 --> GA1
  PP1 --> PP2
  PP2 --> PP1


  GapAnalysis --> LearnTry --> Test --> Assess --> GapAnalysis

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Understand What a System Design Interview Is Really About

Many interviewees fail the system design interview—not for lack of technical knowledge, but for misunderstanding what it's really about. Many candidates focus on memorizing architectures instead of engaging in a collaborative design discussion. In reality, interviewers want to see how you lead a targeted conversation to solve a scoped problem, not how fast you can sketch an entire production system.

Key Principles for the System Design Interview

Drive the Dialogue by Asking and Suggesting. Unlike coding interviews where you follow prompts, here you're expected to take the lead. Propose directions (“Shall we dive in?”), offer ideas, solicit feedback (“Does this design match your expectations?”), and iterate in real time. You are not just allowed—but expected—to guide the conversation.
Scoping Is Your Responsibility. You won't build an entire production system in 45 minutes; instead, you should lead the conversation to define a manageable subset that simulates real-life constraints without overwhelming detail. Define a narrow, realistic scope—“Which core features should we prioritize?”—so you don't drown in complexity. Think of this as a negotiation with the interviewer. You bring your own expertise, which might have some overlaps with the interviewer a priori vision of the right solution. Scoping is your chance to steer the discussion (aka interview) towards areas where your knowledge is strongest, while still aligning with what the interviewer considers a valid solution.
Strive to Get Buy-In. Remember, this is a dialogue, not a monologue. Don't explain the solution—seek agreement at each key stage: after defining the scope, after presenting your high-level design, and during any deep dives. Think of it as selling your approach. A successful “sale” solves a problem the “buyer” (interviewer) cares about. If you're not getting buy-in, try to identify the gap and adjust your proposal accordingly.

✅ ABC Cheat Code

Alex Xu's have a list of do's and don'ts in his book "System Design Interview - An Insider's Guide". Let's create a cheat code for remembering them, with the above key principles. Just think on it as triple ABC:

We divided the key principles into 3 parts:

Preparation - Things to do before the interview
Dialogue Loop - How to communicate with the interviewer
Thinking Process - How to think about the problem

🎯 ABC Preparation

Adopt a Winning Mindset

Stay positive, never give up before or during the interview—resilience is often more important than getting everything right.
Come in with confidence and determination to ask questions and listen to the interviewer.

Build Step by Step, your knowledge and solutions

Tackle problems incrementally.
Validate each assumption before moving to the next step.

Come Prepared

Study typical interview questions and scenarios.
Know the key concepts, patterns, and tools before you enter the room.

🗣️ ABC Dialogue Loop

Use this simple loop to build clear, structured conversations, especially useful in interviews, meetings, or collaborative design sessions. This ABC loop is a good way to memorize the above 3 key principles we started with.

Ask - Start by Asking.
Brief - Rephrase what you heard to ensure mutual understanding and get Buy-in.
Clarify - Ask for any missing information or corrections, and Clarify the next step / sell.

Loop until the end of the interview.

🔁 Interview ABC Loop

flowchart LR

   C[Clarify] --> A[Ask] -->  B[Brief] --> C[Clarify]

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  style B fill:#FFD580,stroke:#AA6600,stroke-width:4px,rx:6,ry:6
  style C fill:#CCFFCC,stroke:#009900,stroke-width:4px,rx:6,ry:6

🧠 ABC - Thinking Process

Communicate Your Thoughts Effectively

Ask for guidance

Don't hesitate to request hints if you're stuck.
A good interviewer is a collaborator, not just an evaluator.

Bounce ideas off the interviewer

Think out loud and share your directions, doubts, reasoning and suggestions.
Use the interviewer as a sounding board to refine your direction.

Communicate continuously

Narrate your thoughts.
Avoid silent problem-solving.
Verbalize your thought process clearly, even if it's still evolving.

A well-structured system design interview isn't about blurting out the “right” design—it's about owning the conversation. Rather than simply answering, you must lead: ask clarifying questions, listen to sub text, expose hidden constraints, and keep the interviewer engaged. Alex Xu's four-phase framework helps you do just that.

Alex Xu's four-phase framework

Phase 1: Understand the Problem & Establish Design Scope

Drive the dialogue with questions. Who are our users? What scale do we expect? Which features matter most? Every question you ask both narrows scope and demonstrates leadership.
Confirm alignment. Restate constraints and assumptions back (“So we need to handle 10 M DAUs with ≤100 ms P99 latency, correct?”) to lock in buy-in before diving deeper.

Phase 2: Propose a High-Level Design & Get Buy-In

Sketch components at a glance. Draw clients, gateways, services, data stores—just enough to show your initial blueprint architecture.
Engage actively. Rather than declaring “Here's my design,” invite feedback: “Does this match what you had in mind?” If you sense any hesitation, ask “What part feels off?” to expose hidden expectations.
Think out loud. Communicate your thoughts and make sure if you need to do back of the envelope calculations to validate your initial design.

Phase 3: Deep Dive into Key Components

Select your hotspots. Based on the feedback you got till here, usually you managed to get a good scope and direction for the deep dive, If this is not the case you need to try to ask more questions to get a better understanding of the problem. If after asking you still don't have a good direction, you can ask the interviewer for suggestions. Such as shall I talk about the main tradeoffs or focus on the areas with the most risk or complexity (e.g., caching, partitioning, queueing)?
Try not to get into unnecessary details Focus on proving your ability to design scalable systems.
Validate continuously. After each trade-off discussion, check back: “Does that satisfy our latency goals given the consistency needs?”

Phase 4: Wrap Up

Close the loop. Ask, “Are there any areas you'd like me to revisit?” If there's lingering doubt, probe its root and address it on the spot.
Recap of the design, and the feedback you got. “We defined scope, sketched the system, deep-dived on X and Y... however we understood that we need to revisit..., ”. This is important mainly if you had some design options to choose from.
Ask the interviewer's opinion about the next steps.
Propose other refinements if you have time. e.g. Potential improvements, error cases, Operational issues (metrics). Note where you might shard more, add rate limiting, back-pressure, or introduce caching layers as the workload grows.

By taking ownership of the conversation—asking clarifying questions, seeking continuous buy-in, and guiding the interviewer through each phase—you transform a static Q&A into a collaborative design session. This is the approach that empowers you to steer a system design interview successfully.

Cracking the system

Make the 4 Phases Work for You: From Insight to Interview Mastery

Let's reverse engineer the process of the system design interview from the interviewer point of view.

Interviewer's Goals in the 4 Phases of a System Design Interview

Evaluate Communication and Clarification Skills During the problem clarification phase, the interviewer wants to see if the candidate can ask the right questions, uncover hidden constraints, and frame the problem with precision — just like they would in real-world ambiguity.
Assess High-Level Thinking and Tradeoff Awareness In the high-level design phase, the goal is to understand how the candidate breaks a big system into components, balances trade-offs (e.g., latency vs. throughput), and considers scalability, availability, and reliability.
Test Technical Depth and Practical Decision-Making During the deep dive, the interviewer looks for depth — how well the candidate understands core components (e.g., databases, queues, caching) and whether they can explain and defend technical choices under pressure.
Gauge Thought Process, Flexibility, and Maturity In the wrap-up, the interviewer wants to know how the candidate handles open questions: can they reflect, summarize trade-offs, suggest improvements, or identify edge cases and limitations?

Phase 1: Understand the Problem & Establish Design Scope

🎯 Interviewer's Goal

Evaluate Communication and Clarification Skills The interviewer wants to see if you can ask the right questions, uncover hidden constraints, and precisely frame the problem — just like in real-world ambiguous scenarios.

✅ Candidate's Strategy

Ask clear, somewhat pre-defined open-ended questions that surface assumptions, constraints, and the essence of the system. Aim for high signal questions that map the system's shape without going deep into implementation yet. Ask about requirements regarding: Fault Tolerance, Reliability, Security, Privacy, Scalability, Maintenance and Operational Aspects.

🧭 Phase one ABC - Ask wide open questions on the below subjects:

Ask - "What are the main users, use cases and main features of the system?" (Actors, Business Scenarios, Core Features)
Brief So those are our main Assumptions, Boundaries (scope), Constraints ? Did I explained MVP (Minimum Viable Product) correctly?
Clarify Any important topics we need to talk about the problem scope for this phase Before we Continue

🔍 Example Dialogue (Pastebin)

Interviewer: Design Pastebin.com.

Candidate: Thanks. To clarify and understand the scope, may I start with a few quick questions?

Candidate: What are the main features, users, and use cases of the system?

Interviewer: Pastebin is a simple site to share plain text. Users paste content and get a short link to share. Pastebin includes anonymous users as well as logged in users.

Candidate: Great. So can we scope the problem to 2 main flows:

User creates a paste and gets a link to share.
User accesses a paste using the link.

Interviewer: Yes, that's a good start.

Candidate: What are the other important topics we need to consider?

Interviewer: Pastebin supports two types of users:

Anonymous users - who can create and share content without an account
Authenticated users - who can create content and customize their sharing links (e.g., custom URLs, expiration dates, access controls).

Candidate: Understood. For this phase, can we focus on Anonymous users?

Interviewer: You mean that we should ignore the requirements for authenticated users?

Candidate: No, I want to clarify. I am suggesting and asking your confirmation - to be effective I think we can start dealing with the main flows of the system. We will bear in mind the authenticated requirement and deal with them later on.

Interviewer: Later on in the interview, or later on in the product life cycle?

Candidate: Both. Let me explain. Let's for now assume that we will use REST API to write and read the content. We can have 2 different api endpoints for anonymous and authenticated users, and we can have shared functions that can be used by both. This way we can focus on the main flows of the system. On the other hand we can talk about the authenticated requirements now, such as OAuth 2.0.

Interviewer: Ok.

Candidate: So for now, is it ok to focus on the anonymous users?

Interviewer: Yes.

Candidate: What are the other important topics we need to consider? What about traffic assumptions / load?

Interviewer: 10M writes per month, 100M reads. High read-to-write ratio.

Candidate: Got it. Any other specific requirements, assumptions or constraints, data flows?

Interviewer: We do track monthly stats. Links can have optional expiration. Expired pastes are auto-deleted.

Candidate: Is there anything more we should discuss in terms of latency, availability, or other non-functional constraints?

Interviewer: Reads should be low-latency. High availability is expected.

Candidate: Cost efficiency, scaling and security matter, but I suggest to digest those in the next phases.

Interviewer: Ok.

Candidate: Thanks — that's clear and helps scope my design. Let's me summarize the scope as i understood it and the assumptions to make sure we are on the same page.

Interviewer: Ok.

Candidate: ... (see the full example in the solutions section)

🧭 For more examples for this phase

see the Solutions phase 1 examples

Phase 2: Propose a High-Level Design & Get Buy-In

🎯 Interviewer's Goal

Assess High-Level Thinking and Tradeoff Awareness In this phase, the interviewer wants to see how you break a system into logical components, understand their relationships, and think in terms of scalability, availability, performance, and trade-offs.

🛠️ Candidate's Strategy

Translate the clarified requirements and use cases into a high-level architecture:

Identify the core read and write operations
Establish data processing/preparation workflows
Understand timing (sync/async, triggered vs. scheduled)
Propose a high-level architecture (building blocks + interactions)
Evaluate trade-offs and constraints and Alternative Approaches
Optimize for Latency and availability
Ask about Fault Tolerance, Reliability, Security, Privacy, and Scalability that might be relevant for the system

🧭 Phase two ABC - Propose a high-level design

Artifacts - "What are the core data models, read and write operations?"
Bursting - "What is processed in synchronic mode and what is processed in the background? How do we handle the bursty nature of the traffic?"
Capacity - "How do we scale the system? What are the bottlenecks? What are the trade-offs?"

Examples for Phase 2 in the solutions section

Phase 3 & 4

we will use the same ABC loop for the deep dive and the wrap up. However, those phases are more specific and detailed therefore it is harder to build a rigid template for those steps. Therefore, the best way to prepare for those phases is to practice many examples.

Phase 3: Deep Dive into Key Components

🎯 Interviewer's Goal

Test Technical Depth and Practical Decision-Making Interviewer looks for how well you understand core components (e.g., databases, queues, caching) and whether you can explain and defend choices under pressure.

ABC Process for Phase 3

Align your strengths with interview exceptions
Broadcast your thoughts augmented with your knowledge, and questions
Confirm you cover the expectations and get buy-in

Phase 4: Wrap Up

🎯 Interviewer's Goal

Gauge Thought Process, Flexibility, and Maturity Interviewer wants to see how you reflect, summarize trade-offs, suggest improvements, and identify edge cases.

🛠️ Candidate's Strategy

Close the Loop: Ask if any area needs revisiting or deeper detail.
Summarize Design: Recap scope, high-level architecture, deep-dive decisions, and trade-offs made.
Next-Step Suggestions: Propose enhancements (e.g., metrics/monitoring, rate limiting, canary deployments).
Solicit Feedback: Invite the interviewer's perspective on your summary and ideas.

ABC Process for Phase 4

Ask for Gaps: “Are there any parts you'd like to revisit?”
Brief Recap: “Here's what we covered: scope, design, deep dive, and trade-offs.”
Co-create Next Steps: “Given more time, I'd add X, Y, Z or improve metrics and failover strategies. What do you think?”

This final loop ensures alignment, demonstrates reflection, and leaves a strong closing impression.

Gap Assessment

The best way to assess your gap is to jump over to solved examples and read the assignment, then write the 4 phases and walk your way through a solved example and map gaps.

There are 2 different goals for you to master. The best way to master them is to practice, practice and practice. The first goal is system design knowledge. The second goal is system design interview practice. You might be good in one of them and weak in the other, but you need to master both to ace the system design interview.

Try to test your memory about the system design guides content and to practice the 4 phases. After you test see the solutions and map the gaps.

Refresh Your knowledge

After mapping your gaps prioritize your learning and learn and practice. Learn / Refresh your knowledge by reading the system design topics and practice by solving the interview questions.

Comment: We are working on a tools to help you with this.

Personal Learning Plan

Timeline matters

Short timeline - Aim for breadth with system design topics. Practice by solving some interview questions.
Medium timeline - Aim for breadth and some depth with system design topics. Practice by solving many interview questions.
Long timeline - Aim for breadth and more depth with system design topics. Practice by solving most interview questions.

	Short	Medium	Long
Start with this document, Study Guide	👍	👍	👍
Do a first pass on System design topics to get a broad understanding of how systems work	👍	👍	👍
Learn about the company you addressing, search for tips and resources about their interview questions and process	👍	👍	👍
Work through System design interview questions with solutions and repeat	Few times	More times	Know by heart
Review other books and guides and resources in the web	Some	More	Most
Read through a few articles in the Company engineering blogs for the companies you are interviewing with			👍
Read through a few Real world architectures			👍

Learning Loop

Time to practice.

Repetition is the mother of all skills

FAQ

Q: For interviews, do I need to know everything here?

A: No, you don't need to know everything here to prepare for the interview.

What you are asked in an interview depends on variables such as:

How much experience you have
What your technical background is
What positions you are interviewing for
Which companies you are interviewing with
Luck

More experienced candidates are generally expected to know more about system design. Architects or team leads might be expected to know more than individual contributors. Top tech companies are likely to have one or more design interview rounds.

Start broad and go deeper in a few areas. It helps to know a little about various key system design topics. Adjust the following guide based on your timeline, experience, what positions you are interviewing for, and which companies you are interviewing with.

Source(s) and further reading

Check out the following links to get a better idea of what to expect:

References

Alex Xu, book, System Design Interview - An Insider's Guide, Second Edition, 2020.
GeeksforGeeks, "Common Mistakes to Avoid in a System Design Interview".
Exponent, "Five common system design interview mistakes 😅".

More (original repo) resources:

23 KiB Raw Blame History

Study Guide

Understand What a System Design Interview Is Really About

Key Principles for the System Design Interview

✅ ABC Cheat Code

🎯 ABC Preparation

🗣️ ABC Dialogue Loop

🔁 Interview ABC Loop

🧠 ABC - Thinking Process

Alex Xu's four-phase framework

Cracking the system

Make the 4 Phases Work for You: From Insight to Interview Mastery

Interviewer's Goals in the 4 Phases of a System Design Interview

Phase 1: Understand the Problem & Establish Design Scope

🎯 Interviewer's Goal

✅ Candidate's Strategy

🧭 Phase one ABC - Ask wide open questions on the below subjects:

🔍 Example Dialogue (Pastebin)

🧭 For more examples for this phase

see the Solutions phase 1 examples

Phase 2: Propose a High-Level Design & Get Buy-In

🎯 Interviewer's Goal

🛠️ Candidate's Strategy

🧭 Phase two ABC - Propose a high-level design

Examples for Phase 2 in the solutions section

Phase 3 & 4

Phase 3: Deep Dive into Key Components

🎯 Interviewer's Goal

ABC Process for Phase 3

Phase 4: Wrap Up

🎯 Interviewer's Goal

🛠️ Candidate's Strategy

ABC Process for Phase 4

Gap Assessment

Refresh Your knowledge

Personal Learning Plan

Timeline matters

Learning Loop

Repetition is the mother of all skills

FAQ

Source(s) and further reading

References

23 KiB

Raw Blame History