A system design interview is a round, common at product companies, where you are asked to design a large-scale system — for example, a URL shortener, a chat app, or a news feed — and defend your decisions. The interviewer evaluates how you gather requirements, estimate scale, choose databases and caches, handle failures, and reason about trade-offs between scalability, consistency, and cost. Common system design interview questions include designing a rate limiter, a WhatsApp-style chat backend, an Instagram-style feed, or a food delivery platform.

Start with the fundamentals — load balancing, caching, database sharding and replication, message queues, and consistency trade-offs — then study 15–20 classic designs such as URL shorteners, notification systems, and news feeds. Delivery matters as much as knowledge, so practise explaining designs out loud with diagrams. Most importantly, learn how to approach a system design interview step by step: clarify requirements, estimate scale, sketch the high-level design, deep-dive into components, and finish with bottlenecks and trade-offs. Round off your prep with a few mock interviews under real time pressure.

Practice by designing, not just reading solutions. Pick one common prompt at a time — a chat system, a ride-hailing backend, a rate limiter — set a 40–45 minute timer, and talk through your design aloud while drawing it, exactly as you would in the actual room. Afterwards, compare your design against reference solutions to find missed requirements or bottlenecks. Once every week or so, replace a solo session with a mock interview, because real-time follow-up questions from an experienced engineer are what most candidates are least prepared for.

Grokking the System Design Interview is a popular prep course that teaches a repeatable framework through classic problems such as the URL shortener, chat system, and news feed, which is why it appears in almost every prep recommendation list. It is worth it mainly if you are facing your first design rounds, as it quickly makes the format familiar. So, is Grokking the System Design Interview worth it for you? For beginners, yes — as a foundation. But pair it with live design practice and feedback, because courses alone will not prepare you for unscripted follow-up questions.

The System Design Interview by Alex Xu is one of the best starting resources — its volumes break down standard problems like rate limiters, URL shorteners, and distributed message queues and teach a solid step-by-step framework. On its own, though, it is not enough. Interviewers now expect you to justify trade-offs, handle curveballs, and communicate clearly, which only comes from practising designs out loud. Use the book to build your base, then pressure-test it through live design practice and mock sessions with feedback.

High level design (HLD) describes the system's overall architecture — services, databases, caches, queues, and how components interact at scale. Low level design (LLD) zooms into one component and defines its internals: classes, objects, relationships, design patterns, and detailed flows. A quick way to remember the difference: HLD answers "what are the building blocks and how do they connect?", while LLD answers "how is each block actually built?". Product companies typically test both — system design rounds for HLD and machine coding or LLD rounds for detailed design.

The classics repeat across product companies: parking lot, elevator system, Splitwise-style expense splitter, BookMyShow-style ticket booking, vending machine, chess or tic-tac-toe, LRU cache, and food delivery order management. What is really being tested is your approach, not the problem — clarifying requirements first, designing extensible classes, applying SOLID principles, and handling follow-ups like "add a new vehicle type" or "change the pricing strategy" without breaking existing code. Practising 8–10 of these end to end covers most of the patterns interviewers use.

Follow a sequence instead of jumping into random problems: first strengthen OOP fundamentals (encapsulation, inheritance, polymorphism, abstraction), then learn SOLID principles, and then design patterns — this is the standard low level design roadmap most candidates follow. After the theory, learn by doing: design real systems like a parking lot, expense splitter, or booking system with class diagrams and working code. Books and structured tutorials speed up the basics, but LLD truly clicks only when you build solutions and get them reviewed by someone who has evaluated these rounds.

Treat practice like the real machine coding round: pick a problem statement (LRU cache, parking lot, food delivery order flow), give yourself 60–90 minutes, and produce clean, working, extensible code — not just a class diagram. After every attempt, review your design for tight coupling, missing abstractions, and places where a new requirement would force a rewrite. The fastest improvement comes from feedback, so every few sessions do a mock LLD interview with an experienced engineer who can show you exactly where your design breaks and how interviewers would score it.

Focus on the ones that repeatedly appear in LLD problems: Factory and Builder for object creation, Singleton for shared resources, Strategy for swappable behaviours like payment methods, Observer for event-driven flows like notifications, Decorator for adding features dynamically, and State for machines like elevators and vending machines. Do not memorise definitions — practise recognising which pattern a problem is hinting at, because interviewers judge how naturally you apply patterns to keep the design extensible, not how many names you can recite.

A low level design document specifies how a system will actually be implemented — class diagrams, entity relationships, database schema, API contracts, and component-level interactions — whereas an HLD stays at the architecture level. In interviews you will not write a full document, but you are essentially producing its core contents on the whiteboard: classes, their responsibilities, relationships, and key flows. Knowing what goes into one helps you structure your LLD answers in a logical, interviewer-friendly order.

Prepare topic-wise instead of solving randomly: arrays, strings, hashing, two pointers, sliding window, binary search, stacks, linked lists, trees, heaps, graphs, and dynamic programming, roughly in order of frequency. For each topic, solve a curated set of standard problems, understand the underlying pattern rather than memorising solutions, and maintain a revision list of problems you initially struggled with. In the final two to three weeks, move to timed practice and mock interviews, because explaining your approach while coding under pressure is a separate skill from solving problems quietly.

Follow a structured flow: restate the problem and clarify constraints, propose a brute-force solution with its complexity, and then optimise it while explaining your reasoning — interviewers evaluate your path to the solution, not just the final code. Once you have chosen an approach, dry-run it on an example, write clean code, and test it against edge cases out loud. Candidates who think aloud at every step consistently score better than those who silently jump to a solution.

The highest-frequency themes are arrays and hashing (Two Sum, group anagrams), two pointers and sliding window, binary search, stacks (valid parentheses, next greater element), linked lists, trees and BSTs, graph traversal with BFS and DFS, heaps, and dynamic programming classics like knapsack and longest common subsequence. Freshers are usually tested on arrays, strings, and linked lists, while experienced rounds push towards graphs, DP variations, and tricky optimisation follow-ups. Learn the approach and complexity analysis behind each classic question, since interviewers often wrap the same patterns in new stories.

Yes — for most candidates they are the highest-return step of preparation. A mock exposes what silent practice hides: freezing under pressure, skipping clarifying questions, losing track of time, and explaining trade-offs poorly. The impact is biggest in system design and LLD rounds, which are conversational and scored on how you think, not just what you output. Do a few sessions before your interviews, ideally with someone who has actually sat on the interviewer's side of the table, so the feedback mirrors what real panels evaluate.