System Design Interview Prep

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Crack the system design interview

How to actually answer "design X", out loud, on a whiteboard, under time.

You freeze when the interviewer says design Twitter. Learn the structure, clarifying questions, and out-loud reasoning that turns the system design loop from a guessing game into a script.

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Overview

Most candidates lose system-design rounds in the first five minutes, not because they don't know the patterns, but because they freeze on scoping, miss the back-of-envelope step, and never get to the interesting trade-offs. This course is the interview-shaped version of system design: structure, scoping, narration, and the small repertoire of designs interviewers actually ask.

What you'll learn

By the end, you'll be able to do these, not just have read about them.

  • Structure a 45-minute design interview with confidence

  • Clarify scope, drive requirements, and propose a defensible design

  • Evolve a design under interviewer pressure without losing the thread

  • Spot the deep-dive question before the interviewer asks it

Who this is for

  • You're interviewing for senior, staff, or principal engineering roles in the next 6–12 weeks.

  • You can build the system you'd be asked to design, but you haven't practiced talking through one.

  • You've already done a fundamentals pass and want pure interview reps.

Prerequisites

  • Working knowledge of the system design fundamentals, caches, queues, sharding, replication.

  • You have at least one upcoming interview loop (or are about to start one).

Suggested chapters

This is the typical chapter list. Your version is generated against your background and adapts as you go. It may compress, expand, or reorder these.

  1. 01

    The 5-minute structure

    Clarify, scope, capacity-estimate, high-level design, deep dives, trade-offs, the script you can run on autopilot.

  2. 02

    Asking the right questions

    Functional, non-functional, scale, edge cases, the order matters and signals seniority.

  3. 03

    Back-of-envelope, fast

    QPS, storage, bandwidth, getting to a number in 60 seconds without a calculator.

  4. 04

    Read-heavy systems

    News feed, search, video, the canonical read-path designs interviewers love.

  5. 05

    Write-heavy systems

    Chat, payments, ride-hailing, sequencing, idempotency, exactly-once-ish.

  6. 06

    Real-time & geo

    Live updates, presence, geospatial queries, where it gets weird and how to stay clear.

  7. 07

    Trade-offs as a skill

    How to argue for a choice, name what you give up, and pivot when challenged.

  8. 08

    Mock interviews & feedback

    Three end-to-end mocks against the course you just built, with structured feedback.

Real-world projects

  • 01Talk through 6 canonical designs (Twitter, Uber, WhatsApp, YouTube, Dropbox, payments).
  • 02Run 3 end-to-end mock interviews against a graded rubric.
  • 03Build your own one-page cheat-sheet for capacity estimation.

Tools & concepts

Real tools and ideas covered. Octo brings them in when they fit your stack.

  • Interview structure
  • Capacity estimation
  • CAP
  • Sharding
  • Replication
  • Caching
  • Queues
  • Consistent hashing
  • Geohashing
  • Pub/sub
  • WebSockets

Where this leads

  • 01

    Senior, staff, and principal engineering interviews at FAANG-tier companies.

  • 02

    Engineering-management interviews where the design round is on the loop.

  • 03

    A repeatable script you can use again next time you switch jobs.

Common questions

  • I haven't done fundamentals, can I skip there?

    If you're brand new to system design, start with System Design Fundamentals. This course assumes you know the building blocks and just need interview reps.

  • How many mock designs do I get?

    You walk through 6 canonical designs and run 3 graded end-to-end mocks against the same rubric companies use.

  • Is this a fixed course, or is it built for me?

    Built for you. The chapter list below is a typical outline. Your actual course is generated against your role, experience, and what you already know, then adapts as you go.

  • How long does it take?

    Most learners finish in 2–6 weeks at a normal pace, depending on the topic. Octo compresses where you're strong and slows down where you're weak.

  • Is there a fixed schedule or cohort?

    No. You start when you start. There's no live session, no calendar, no deadline.

  • Can I ask questions while I'm learning?

    Yes, every module has an AI Sidekick in the margin. Ask for a different example, push back, or get a clarifying analogy without leaving the page.

  • What do I get at the end?

    A verifiable, HMAC-signed certificate with a public verify page. It records the modules passed, scores, and capstone, not just attendance.

  • How much does it cost?

    Octo is in research preview, courses are open. We'll be transparent before pricing changes.

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