Low-Level Design in a Hurry

Low-level design interviews test your ability to structure code for a self-contained problem. You'll be given a system - a connect-four game, an elevator controller, a parking lot system, etc. - and asked to design the classes, interfaces, and relationships that make it work.
The focus of the interview is on code organization: What are the right objects? How do they interact? What methods do they expose? Can the design be extended without rewriting everything?
What is this guide?
This is a practical guide to low-level design interviews, written by interviewers and engineers with decades of experience in big tech. It's focused on modern, practical strategies for performing well in real interviews, rather than how software design appears in a classroom.
Most LLD content online reads like a textbook. It teaches abstract principles, exhaustive pattern catalogs, and heavily academic versions of object-oriented design. Much of it is outdated, and almost none of it is optimized for making fast, correct decisions in an interview.
We deliberately avoid outdated overfitting to classical OOP. Modern production systems favor composition over inheritance, simple state over deep hierarchies, and pragmatism over pattern worship. You'll still learn the patterns that matter, but only the ones that actually show up in real interviews and real codebases.
LLD interviews vary widely across companies, regions, and levels. Rather than trying to cover every edge case, we teach to the mode. You'll learn the approach that works across the widest range of interviews, with clear callouts where expectations meaningfully differ.
We start by defining a Delivery Framework that we suggest you use to structure your interview.
We then provide a refresher on the basics to help you get back up to speed on the fundamentals of object-oriented design quickly. This includes
Design Principles
Object-Oriented Concepts
Design Patterns
Finally, we provide a (ever-growing) collection of Problem Breakdowns where we walk through how we'd tackle common LLD problems from the perspective of an interviewer.
We firmly believe you need to practice to ensure you're comfortable the day of your actual interview. A common failure mode for candidates is to have consumed a lot of material but stumble when it comes time to actually apply it. For this our guide includes worked solutions to common problems as well as our Guided Practice which walks you through the steps of an LLD interview with personalized feedback.
What is the difference between Low-Level Design and System Design?
Despite the similar names, these interviews have almost nothing in common. System design is about architecture at scale. You talk through traffic, storage, consistency, caching, sharding, and the tradeoffs behind each choice. The goal is to show how you think about large systems that serve real users. It is usually done on a shared whiteboard where you sketch boxes and arrows to represent a distributed system. There is no coding in system design and almost no discussion of classes, methods, or interfaces because the focus is on architecture, not implementation.
For low-level design, on the other hand, you are defining objects, modeling data, and shaping the interactions that make a single feature or service work. Instead of boxes and arrows, you work through classes, methods, relationships, and state transitions.
A simple example makes the difference obvious. If the prompt is to design a ride sharing backend, a system design interview has you sketch the major services on a whiteboard. You draw a matching service, a pricing service, a location service, a datastore, a queue, and show how data flows between them. You talk through scaling each piece, handling load spikes, keeping data consistent, and making the system reliable.

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