Open Swiggy at 8 PM on a weeknight. Somewhere between your tap and your food appearing on the map, a request travels through load balancers, caches, databases and queues spread across machines you will never see — and stays up even while millions of other people are doing the exact same thing at the exact same moment. The interview round that tests whether you can reason about that kind of system is called High-Level Design (HLD), also called system design, and it is one of the highest-weight rounds in almost every SDE-II and above loop.
This is a free, complete HLD course built the way you would actually reason through it in an interview: start with the most boring possible system — one server, one database — watch it break under real numbers, and fix exactly the part that broke. No memorised diagrams. By the end, a brand-new "design X" prompt should feel like a familiar process, not a cold start.
What is HLD?
HLD is the "zoomed out" design round: which services exist, which databases hold what, how a request flows between machines, and what happens when one of those machines dies. It is the sibling of Low-Level Design (LLD), which zooms into the classes inside one of those services — this course stays zoomed out, at the level of boxes, arrows, and the machines those boxes run on.
Every HLD answer, no matter the system, is really built from the same five questions: what does it need to do (functional requirements), how fast and how often (non-functional requirements and scale), what is the simplest thing that could possibly work, where does that simplest thing break first, and what is the specific, named trade-off of the fix. This course spends fourteen lessons teaching you to ask those five questions of any system.
A first example, with real numbers
Say your college puts its results portal on one server with one database. On a normal day, 200 students check their result — no problem. On result day, 40,000 students refresh the same page in the same ten minutes. That is roughly 40,000 / 600 seconds ≈ 67 requests/second sustained, with sharp bursts far above that average — and a single server that was fine at 200 requests spread over a day falls over completely.
Notice what just happened: the SAME system, described with SAME words ("a results portal"), needs a completely different design once you attach real numbers to it. That single habit — never designing anything before estimating its scale — is the single most valuable thing this course will teach you, and it is lesson 2.
Why learn HLD
Beyond the interview round itself (a mandatory bar at Amazon, Google, Microsoft, Meta, Uber and virtually every product company's SDE-II+ loop), HLD is the skill that separates "writes correct code" from "can be trusted to own a service that real users depend on." If you want to see how this shows up in a specific company's rounds, check Cognizant's interview questions or the TCS NQT guide for how technical rounds are structured end to end, and this course's own company-wise pages (linked in the sidebar) for exactly what Amazon, Google, Microsoft and others have asked in their HLD rounds specifically.
How this course works
- Simplest system first. Every lesson starts from the most boring version of the problem, then adds exactly one piece of pressure at a time.
- Real numbers, every time. No "this scales well" without a QPS or storage estimate backing it up.
- Trade-offs as the main character. Almost nothing in HLD is a free upgrade — every technique costs something, and naming that cost out loud is what a strong answer sounds like.
- Real tools, not made-up boxes. When a lesson says "a cache," you will see Redis's actual logo; when it says "a queue," you will see Kafka's or RabbitMQ's.
- Two full systems built end to end. A URL shortener and a distributed rate limiter, each taken from a one-line prompt all the way to a complete architecture.
- Practice at the end of every lesson, tagged with the companies that have reported asking that exact idea, plus a full company-wise PYQ section afterward.
The full course roadmap
Fourteen lessons, in the order you should read them — foundations first, then the core building blocks, then two complete systems, then a repeatable framework for any new prompt.
| # | Lesson | What you'll be able to do |
|---|---|---|
| 1 | What is HLD? | Know what the interview actually tests, and the simplest-system-first rule. |
| 2 | Requirements & Capacity Estimation | Turn a one-line prompt into real QPS, storage and bandwidth numbers. |
| 3 | Latency, Throughput, Availability & Reliability | Use these four words precisely instead of interchangeably. |
| 4 | Scaling & Load Balancers | Fix "one server can't take the load" two different ways. |
| 5 | Caching Explained | Add the single highest-leverage box in system design, correctly. |
| 6 | SQL vs NoSQL, Replication & Sharding | Scale a database three different ways, and know when each applies. |
| 7 | Message Queues & Async Processing | Stop making users wait for work that doesn't need to be synchronous. |
| 8 | CDN & Object Storage | Get large files out of your database and close to your users. |
| 9 | Consistency Models & CAP Theorem | Answer "strong or eventual consistency?" per-feature, with a reason. |
| 10 | Rate Limiting & API Design | Protect a service from abuse, and design retry-safe APIs. |
| 11 | Reliability & Failure Handling | Answer "what happens when this box dies?" for every box you draw. |
| 12 | Design a URL Shortener | Put nearly every earlier lesson together on a classic warm-up question. |
| 13 | Design a Rate Limiter | Take an algorithm from the whiteboard to a real distributed system. |
| 14 | The HLD Interview Framework | Answer ANY new "design X" prompt with a repeatable 15-step method. |
And once the lessons are done, the sidebar's Company-wise HLD PYQs section is your mock-round layer — real reported questions from Amazon, Google, Microsoft, Meta, Uber, Flipkart and more, three per company.
How to study this course
Before reading how a lesson solves a problem, try to estimate the numbers yourself first — even roughly. Capacity estimation is a habit, not a formula, and the only way to build the habit is to guess first and check your guess against the lesson. Read lessons 1–11 in order (each one assumes the last), then work through the two system-design lessons by sketching your OWN architecture before reading this course's version — the gap between your sketch and the lesson's answer is exactly what you still need to learn.
FAQ
Is HLD the same as system design?
Yes — "system design interview" and "HLD interview" refer to the same round in almost every company. Some companies also run a separate LLD (low-level design / machine coding) round that zooms into class-level design instead.
Do I need to memorize specific architectures like 'how Netflix works'?
No — and doing so often backfires, because interviewers ask follow-up questions a memorized diagram can't answer. This course teaches the underlying building blocks (caching, sharding, queues, CAP) so you can derive an architecture for ANY prompt, familiar or not.
How much math is actually involved?
Just arithmetic — multiplication, division, and orders of magnitude (thousands, millions, billions). Capacity estimation is about reasoning with round numbers quickly, not precise computation.
How long will this course take?
Roughly two weeks at one lesson a day. Short on time before an interview? Lessons 1–7 (requirements through queues) plus the URL shortener lesson cover what most SDE-II HLD rounds actually test.
Ready? Lesson 1: What is HLD? →