Low level design and high level design test two different skills: LLD focuses on the internals of a single component — classes, objects, their relationships, design patterns, and clean extensible code — while HLD deals with the big-picture architecture of servers, databases, caches, load balancers, and how the system handles scale. Indian product companies typically test LLD in machine coding rounds and HLD in later system design rounds, so you eventually need both.

Start with strong OOP fundamentals (encapsulation, inheritance, polymorphism, abstraction), then learn SOLID principles and class diagrams, and then move to core design patterns like Factory, Strategy, and Observer. Once the theory is in place, the real learning happens by designing complete systems — parking lot, Splitwise, BookMyShow, elevator — on your own. Working engineers usually pick up LLD much faster in a structured, practice-heavy format than from scattered videos.

The most effective way is to simulate real interview conditions: pick one classic problem (parking lot, cab booking, chess, BookMyShow), write down the requirements, sketch the class diagram, and then write working code in Java, Python, or C++ within 60–90 minutes. After every attempt, compare your design with a good reference solution and note what you missed in extensibility. Practising out loud or with a peer/mock partner is what converts knowledge into actual interview performance.

The most frequently asked low level design interview questions in Indian product companies include Parking Lot, BookMyShow, Splitwise, Elevator System, Cab Booking (Uber/Ola style), Tic-Tac-Toe or Chess, Vending Machine, and Logger. Interviewers evaluate your class diagram, your use of SOLID principles and design patterns, and whether your code actually runs and is extensible — not just rough pseudo-code.

Before enrolling in any low level design course, check four things: live problem-solving instead of only recorded theory, coverage of classic interview problems, personal feedback on your code, and mock interview practice. Live cohorts such as the Live LLD Cohort run by Pradeep Kumar (IIIT Delhi, ex-NetApp, ex-Qualcomm) are structured exactly this way for working engineers. Avoid programs that only share PDFs and pre-recorded videos.

The low level design patterns that show up repeatedly are Singleton, Factory/Abstract Factory, Strategy, Observer, Builder, Decorator, and Adapter. What interviewers actually test is whether you can pick the right pattern for a requirement and justify the trade-off, so learn them through problems rather than by memorising definitions.

The most useful low level design books are "Head First Design Patterns" for patterns, followed by "Clean Code" and "Clean Architecture" for writing maintainable, well-structured code. Books build your foundation well, but LLD interviews are a live coding exercise, so pair your reading with hands-on design of complete systems and feedback on your solutions.

A practical way to learn high level design is to first master the building blocks — scalability, load balancing, caching, SQL vs NoSQL, sharding, replication, and message queues — and then study how real systems like WhatsApp, Uber, and Netflix are architected. Follow this up by designing 10–15 standard systems (URL shortener, chat app, notification system) yourself, ideally with feedback from someone senior, since HLD is judged on your reasoning and trade-offs.

A simple plan for how to prepare for a system design interview is 6–8 weeks: first learn the core concepts (caching, load balancing, databases, queues), then go deep on 15–20 classic problems like URL shortener, rate limiter, Twitter feed, and BookMyShow, and finish with mock interviews where you present on a whiteboard. Senior engineers are expected to drive trade-offs like consistency vs availability and SQL vs NoSQL, so practise articulating the "why", not just the "what".

Here's how to approach a system design interview step by step: clarify functional and non-functional requirements, estimate scale (users, storage, QPS), define the API, draw the high-level diagram, then deep-dive into the most critical components and discuss bottlenecks. Product companies score communication as heavily as the final design, so keep thinking aloud and involve the interviewer in your trade-off decisions.

The classics among system design interview questions are URL shortener, WhatsApp/chat system, Twitter/X feed, Instagram, Uber dispatch, BookMyShow ticket booking, distributed rate limiter, and notification system. If you can confidently design these 8–10 systems with scale numbers, caching, and database choices, you can handle most variations interviewers throw at you.

System Design Interview by Alex Xu (Volumes 1 and 2) is the most popular system design interview book and a solid starting point because it explains concepts like consistent hashing, caching, and proxies through real problems. Treat it as your base reading, but remember the interview is a live discussion — supplement the book with practice designing systems out loud and mock rounds.

Grokking the System Design Interview is a widely used text-based course that teaches system design through repeated classic problems (URL shortener, Uber, WhatsApp) using a fixed template. It is useful for getting familiar with the standard flow, but since it is read-only, pair it with actual whiteboard practice so you can handle follow-up questions and trade-off discussions in real interviews.

The most used coding interview preparation websites are LeetCode for DSA practice, HackerRank for basics and company assessments, and platforms hosting interview experiences for company-specific patterns. The website matters less than consistency — a structured 3–4 month plan with revision and mock interviews beats randomly solving hundreds of problems.

A coding interview preparation course is worth it if you keep postponing your prep, fail to convert interviews despite knowing the concepts, or need guidance from someone who has already cleared these rounds — which is common for working engineers with limited time. If you are highly self-disciplined with a clear roadmap, free resources and consistent self-practice can also get you there.