Everything from modules 1-19, distilled into a working playbook. A 6-phase framework, a question bank, the most common mistakes — and a simulator that walks you through any system design problem the way a senior engineer would. The last module. The one you'll come back to.
If you walk into a system design interview expecting to be tested on whether you know "the right way" to design Twitter, you've already lost. There is no single right way. The interviewer isn't comparing your answer to a key in their head. They're watching how you think when handed an ambiguous, large, real-world problem — exactly the kind you'll handle in the actual job. The interview is theater, but it's theater for a very specific skill: structured thinking out loud, under uncertainty.
That last one matters a lot. Strong candidates frequently say "I'm not sure" — about a specific technology, a specific number, a specific tradeoff. Then they describe how they'd resolve the uncertainty (read the docs, run a benchmark, ask the team). Weak candidates fake certainty because they think it's expected, and get caught when the interviewer probes a layer deeper. Calibrated confidence — knowing what you know and what you don't — is what's actually being measured.
So what does a structured thinker do at the whiteboard? They follow a framework — a sequence of phases that ensures they hit the right concerns in the right order. That framework is exactly what we'll spend the rest of this module on. Once you've internalized it, every "design X" question becomes the same exercise: walk the phases, fill in the details.
Every well-conducted system design interview follows roughly the same arc. The phases below are the canonical decomposition — taught at FAANG companies, used in mock-interview books, and (most importantly) the actual structure senior engineers use in real production design reviews. The order matters. Do them out of order and the rest of the interview drifts.
POST /tweet, GET /feed. Forces you to commit to what the system actually exposes.A few things to internalize about this framework. First, the time estimates are loose — they shift based on the question and the interviewer's interests. But the relative weight is right: Phase 5 (architecture) is the biggest chunk, and Phase 6 (deep dive) is where senior signals land. Spend 20+ minutes on Phase 1, and you'll be rushed through the parts that actually matter.
Second, Phase 1 is non-negotiable. Even when you "know" the question is about Twitter, you still clarify: are we doing the feed, or the post step, or both? Public or private accounts? Just text or also media? The clarification questions are the answer to "do you scope problems before solving them" — which is what the interviewer is measuring.
Third, narrate the framework out loud. Tell the interviewer: "Let me start with requirements, then I'll do some back-of-envelope math, then sketch the API, data model, and architecture, and at the end I'll dive into whichever part you want to explore." This single sentence at the start of the interview signals you have a framework. It is shockingly effective.
Below: pick one of five canonical system design questions. Step through the 6 phases. Each phase shows the prompts you should be hitting plus a model answer specific to that question. Switch questions to see how the same framework adapts. This is the actual playbook — bookmark it before your next interview.
The framework above gets you 80% of the way. The last 20% is avoiding the failure modes that even experienced engineers fall into when they're nervous. Each of these has a simple counter-move — and just knowing the trap exists halves your chance of falling into it.
"Just to confirm what we're building..." is a phrase that gets you points every time."I'm choosing X over Y because [reason], though Y would be better for [other case]." This signals you considered alternatives. Senior engineers don't pick best; they pick best for these requirements."Simplest version: one app server, one database." Then identify what breaks at scale and add components to fix it. Each addition justified by the previous bottleneck. This is exactly what M.17 taught."I'm thinking about whether to shard by user_id or by tweet_id... If we shard by user_id, then the user's tweets stay together which is good for the feed query, but..." Even false starts are signal. Silent thinking shows nothing to evaluate.One more anti-pattern worth flagging: defending your first answer at all costs. If the interviewer pushes back — "what about hot keys?", "what if traffic 10×s?", "how do you handle the celebrity case?" — they're inviting you to refine the design, not catching you in a lie. Welcome the pushback. Update the design out loud. "Good catch, you're right — in that case I'd add..." is one of the best things you can say. Stubbornness reads as lack of judgment; flexibility reads as senior.
Before the final quiz, take a moment to see how all 20 modules link to the framework you just learned. Every concept from this track shows up somewhere in a real interview. The "deep dive" phase, especially, is where the deeper concepts pay off — that's the moment the interviewer is deciding whether to push for the next level up.
M.01 Systems M.02 Client-Server M.03 HTTP/HTTPS M.04 REST APIs
M.05 SQL vs NoSQL M.06 Indexes M.07 Caching M.08 Latency
M.09 Scaling M.10 LB M.11 DNS M.12 CDN M.13 Stateless M.14 Auth M.15 Obs M.16 Reliability
M.17 URL Shortener M.18 Image App M.19 C4 Diagrams M.20 Interview
Every "deep dive" topic in an interview maps to one of these modules. "How would you cache the feed?" — Module 7. "What if the DB falls over?" — Modules 9, 16. "How do you keep sessions across servers?" — Module 13. "Tell me about CDN caching policies" — Module 12. The list goes on. You don't have to recall all 20 modules during the interview; you just have to recognize which one applies to the question you're being asked.
These show up in every system design conversation — interviews and real design reviews alike.
The last test of the track. Click an answer; explanation drops in instantly.
The toolkit is yours. Twenty modules, one playbook, ready when the next whiteboard question comes.
If you take only three things from the whole track, take these.
Six phases, same every time. Knowing them well beats memorizing the "right answer" for any specific question — because the right answer comes out of the framework.
There is no best system. There's only best-for-these-requirements. Senior engineers narrate tradeoffs out loud; junior engineers state choices as facts.
"I don't know but here's how I'd find out" beats confident-but-wrong every time. The interview measures judgment, not omniscience.
Twenty modules. Four phases. Dozens of interactive labs. From "what's a system?" to "design Twitter at scale" — the full arc. You now carry the vocabulary, the patterns, and the framework that opens every senior-level conversation in software engineering.
Now go write some better software.