What to Expect in a Low-Level Design Interview Round

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Low-level design (LLD) interviews focus on the detailed design of software components and classes.

In these interviews, candidates translate high-level requirements into a workable object-oriented design.

LLD is essentially the “detailed blueprint” of the system – defining how individual modules, classes, and functions will work together.

Companies consider LLD interviews important because they help identify developers who can write clean, modular, and reusable code that is easy to maintain and scale.

A strong LLD shows that you can plan a robust implementation (with proper class structure, interactions, and error handling) before writing actual code. This preparation leads to software that is less buggy and easier to extend.

In short, LLD rounds evaluate your ability to design like an engineer, not just code like a programmer.

What is Evaluated in an LLD Interview?

In a low-level design interview, you will be evaluated on several key skills and design aspects:

Object-Oriented Programming (OOP) Concepts: Interviewers expect a solid understanding of OOP fundamentals like classes, objects, and the four pillars of OOP (encapsulation, abstraction, inheritance, polymorphism). You should know how to apply these concepts to model real-world entities in code. For example, using encapsulation to hide internal data of a class and inheritance to reuse common logic in subclasses demonstrates good grasp of OOP. (We’ll recap OOP principles in the next section.)
Designing Scalable & Maintainable Components: You need to show the ability to design software components that are modular (each class or module has a clear purpose), extensible (easy to add new features), and maintainable (easy to fix or update). This often involves applying SOLID principles – a set of guidelines for good object-oriented design – so your codebase doesn’t turn into spaghetti code. Mastering the S.O.L.I.D principles helps ensure your design is modular, scalable, and maintainable. The interviewer might ask how your design would handle future changes or increased load, to gauge its flexibility and scalability.
Use of Design Principles & Patterns: Beyond basic OOP, LLD rounds assess your familiarity with common design patterns (reusable design solutions) and design principles. For instance, do you know when to use a Factory pattern versus a Singleton? Are you applying the DRY (Don’t Repeat Yourself) principle and not duplicating code? Good designs often leverage well-known patterns for typical problems – demonstrating this knowledge shows you can build on proven solutions rather than reinventing the wheel.
Clean, Modular, Reusable Code: Low-level design is ultimately about how code is organized. Interviewers look for designs that would lead to clean implementation. This means classes with single responsibilities, methods that are not too large, and logical separation of concerns. A good LLD implies that if we wrote code from it, the code would be easy to test, debug, and extend. You might even be asked to sketch some pseudocode or class skeletons – focus on clarity and proper naming if you do.
Class Structure & Relationships: You should be comfortable designing an efficient class hierarchy and relationships between classes. This includes choosing the right relationships (association, composition, inheritance) to model the problem. Interviewers may evaluate if you know when to use an abstract class or interface, how to enforce encapsulation, and how to prevent tight coupling between classes. In essence, they’re checking if your design’s class diagram makes sense for the given problem.
Communication of Design: Finally, an often underrated aspect is communication. In an LLD interview, you should explain your approach clearly – using a whiteboard or notepad to sketch class diagrams and flows. Interviewers evaluate how well you can articulate design decisions (e.g. why you chose a pattern or how classes interact). Clear communication and reasoning show that you not only can come up with a good design, but you can also work through it collaboratively and justify your choices.

Common Topics in Low-Level Design Interviews

LLD interview questions can span a wide range of design topics.

As a beginner, you should familiarize yourself with the following core areas, since these are commonly brought up in interviews:

Object-Oriented Design Principles

The foundation of low-level design is solid understanding of OOP principles. Make sure you know these four principles and can illustrate them in your design:

Principle	Description
Encapsulation	Bundling data and methods inside a class, and restricting direct access to some of the object’s components. This “data hiding” ensures internal representation is hidden from outside. For example, a class keeps its fields `private` and provides public getters/setters.
Abstraction	Exposing only the necessary features of an object while hiding complex details. It’s about modeling a real-world concept in a simplified way. For example, a `Database` class might provide high-level `save()` and `query()` methods without exposing the nitty-gritty of file handlers or network calls.
Inheritance	Allowing a new class (subclass) to acquire the properties and behaviors of an existing class (superclass). Inheritance supports code reuse – common attributes/methods are defined once in the base class and reused by derived classes. e.g. `Car` and `Motorcycle` classes inheriting from a common `Vehicle` class.
Polymorphism	The ability of objects to take on many forms or be used through a common interface. In practice, polymorphism lets you treat subclass instances as their parent type, and the correct overridden methods will be called. This allows one interface to represent different underlying forms (e.g. a `Shape` interface with multiple implementations).

Understanding these principles is crucial – many LLD questions explicitly prompt you to apply encapsulation (e.g. keep data private), use inheritance appropriately for extending classes, or illustrate polymorphism (perhaps via method overriding or interface implementation). If you’re clear on these, you can lay a strong groundwork for any design.

Learn about essential software design principles.

Design Patterns

Design patterns are proven solutions to common design problems.

Interviewers often expect you to know at least a few classic patterns and use them where appropriate in your design.

Some patterns frequently mentioned in LLD interviews include:

Factory Pattern: Provides an interface for creating objects without specifying the exact class. A Factory method helps to encapsulate object creation logic. For instance, an EnemyFactory class can generate different enemy objects (Goblin, Troll, Dragon) based on input, instead of littering the code with new calls. This promotes loose coupling by delegating creation to the factory.
Singleton Pattern: Ensures a class has only one instance globally and provides a global access point to it. Useful for cases like a database connection pool or logger where a single instance is desirable. (Be cautious: singletons can introduce global state, so use them judiciously.)
Observer Pattern: Defines a one-to-many dependency so that when one object (subject) changes state, all its dependents (observers) are notified automatically. This is like a publisher-subscriber model; e.g. UI components observing data changes, or a news feed where followers get updates when someone posts.
Strategy Pattern: Encapsulates interchangeable behaviors and algorithms, allowing the choice of algorithm at runtime. For example, a sorting class might use a SortStrategy interface so that it can swap between QuickSort and MergeSort strategies based on data size, without changing its own code.

There are many patterns (Builder, Adapter, Decorator, etc.), but as a beginner focus on the common ones above, since they solve problems often encountered in interview scenarios.

The goal is not to memorize definitions, but to recognize scenarios where a pattern can clean up your design.

If a question involves creating families of objects, think Factory.

If it requires a single point of configuration, consider Singleton. For event-based notifications, Observer is a good fit, and for replaceable algorithms, Strategy helps. Being able to name and apply these patterns can significantly impress your interviewer.

Learn about the top design patterns.

Class Diagrams & UML

In LLD interviews, you will often sketch a class diagram to illustrate your solution.

A class diagram is a UML (Unified Modeling Language) diagram showing classes, their attributes and methods, and the relationships between classes.

Using such diagrams can greatly aid communication in an interview, making it easier for the interviewer to follow your design.

You don’t have to be a UML expert, but you should know basic notations for: classes (boxes with three compartments for name/attributes/operations), associations (lines connecting classes), inheritance (a line with a triangular arrow), etc.

When representing class relationships, pay attention to multiplicities (one-to-one, one-to-many) and types of relationships:

Association: a generic link between classes (e.g. a User has an association with Order if users place orders).
Aggregation: a special association where one class is a container for others, but those others can exist independently (open diamond arrow in UML). E.g. a Team aggregates Player objects – players can exist without a team.
Composition: a stronger form of aggregation where the part cannot exist without the whole (filled diamond arrow). E.g. a Order composes OrderItems – if the order is deleted, its items should not exist.
Inheritance: depicted with an open arrow from subclass to base class (also called generalization).
Interface Realization: a dashed arrow indicating a class implements an interface.

Designing a clear class structure is key in LLD.

Identify the main entities in the problem and figure out their relationships. Drawing a quick diagram helps ensure you cover all interactions.

Database Design

Some LLD interview questions involve designing the data model or database schema as part of the system. If the problem has a data storage aspect (for example, designing a library system or a shopping cart), be prepared to discuss how you would structure the database tables and relationships.

Key points include:

Schema Design: Identify the entities and relationships in your system and design tables accordingly (e.g. a Users table, an Orders table, etc.). Define primary keys for each table and foreign keys for relationships (one-to-many, many-to-many join tables, etc.). Good schema design captures all necessary data while minimizing redundancy.
Normalization: Ensure your design follows normalization principles (especially 3NF) to avoid data anomalies. For instance, don’t store the same data in multiple tables unnecessarily – separate into lookup tables or related tables to keep data consistent.
Indexes for Performance: Consider which fields to index for efficient queries. In an interview, you might mention indexing columns that are frequently searched or used in lookups (like user ID, email, or foreign keys) to speed up data retrieval. Also consider the volume of data – if a system will have millions of records, your design should support quick searches (indexes, maybe sharding or partitioning if advanced).
Example: If asked to design a movie ticket booking system, you’d likely propose tables like Movie, Theater, Show, Seat, Booking, and define relationships (each Show is in one Theater, each Booking links a User, Show, and specific Seat, etc.). Mentioning how you’d enforce constraints (like a unique constraint on a seat per show per booking to prevent double booking) shows attention to detail.

In summary, show that you can create an efficient and consistent data model as part of your design. Even if the role isn’t database-heavy, demonstrating database design insight can set you apart.

API Design & Interfaces

Another common aspect is designing clear interfaces or APIs for your system’s components.

Many modern systems expose functionality via RESTful APIs, so knowing API design best practices is useful:

Resource-Oriented Thinking: Design your API around resources (nouns), not actions. For instance, in an e-commerce system design, think of resources like /products, /carts, /orders rather than having endpoints named as verbs. Each endpoint should represent an entity or collection.
Use HTTP Methods Correctly: Follow REST conventions for methods: use GET for retrieving data, POST for creating, PUT/PATCH for updating, and DELETE for deletion. Using methods as intended makes your API intuitive (e.g., GET /items/123 returns item 123, DELETE /items/123 deletes it).
Consistent URL Structure: Keep your URLs simple and predictable. Use plural nouns for collections (e.g., /users/{id}/orders for orders of a user). Avoid mixing lower/uppercase or including unnecessary verbs in the path. Consistency helps developers guess or remember endpoints easily.
Data Format & Representation: Decide on JSON (most common) or XML for responses, and keep the format consistent. Clearly define what the request and response bodies look like for each operation. For example, if designing a REST API for a social media feed, specify that GET /feed returns a JSON list of posts, each with fields like id, content, timestamp, authorId, etc..
Versioning: Mention how you’d version your API (like starting URLs with /v1/), especially if designing a public API or one that may evolve. This allows adding new features without breaking old clients.
Error Handling: Define a clear strategy for errors (HTTP status codes and error response structure). E.g., use 404 for not found, 400 for bad requests, etc., and include a JSON error message. Consistent error responses (with maybe an error code and message) make it easier to debug issues.
Security & Rate Limiting: If relevant, touch on securing the API (using API keys or tokens for auth, and HTTPS) and possibly rate limiting to prevent abuse. This shows you think beyond just functionality.
Interfaces in Code: If the interview is not about web APIs but more about internal interfaces, you should describe the interfaces or abstract classes in your design. For example, if designing an ATM, you might have an interface CardValidator that different classes (DebitCardValidator, CreditCardValidator) implement – explaining this interface segregation is good design practice.

Remember, you don’t need to go overboard on API details unless asked.

However, demonstrating that you know how to design clean interfaces – whether web APIs or internal abstractions – will convey that you write systems that are easy to interact with and modify. It’s all part of designing software that plays well with others.

Example LLD Interview Questions

LLD interviews often involve designing a specific system or component. Here are some common examples of questions and what interviewers expect:

Design a Parking Lot System: This is a classic LLD question. You’d need to model entities like ParkingLot, ParkingFloor, ParkingSpot, Vehicle, Ticket, Payment, etc. Consider various components such as different spot types (handicapped, compact, large), vehicle types (car, bike, truck), entry/exit gates, and payment calculation. Focus on an efficient parking spot allocation strategy and how to handle capacity. For instance, when a vehicle enters, how do you assign a spot? A good answer would involve classes for each entity and possibly a strategy pattern for choosing spots. (Don’t forget edge cases like a full lot, or payments for lost tickets.) Learn how to design a parking lot.
Design a Movie Ticket Booking System: Think of an online movie booking like BookMyShow. Identify classes like Movie, Theater, Showtime, Seat, Booking, UserPayment. You must handle seat selection and locking (to prevent two people booking the same seat at once), payment flow, and possibly notifications (confirmation SMS/Email). A clear class diagram might show that a Theater has multiple Shows, each Show has many Seats, and a Booking ties a User to a specific Show’s seat(s). Also consider how to model different seat categories (Regular, VIP) and pricing. This question tests if you can design for concurrency (seat holds) and integrate components like payment and user accounts smoothly.
Design an Online Shopping Cart: Model an e-commerce shopping cart where users add products and place orders. Key components include Product catalog, Cart (with items), Order, Payment, and maybe Inventory. Explain the flow: users add/remove items from a Cart, then checkout to create an Order. Discuss how to calculate totals, apply discounts, and update inventory stock. The interviewer may expect you to mention design patterns like Singleton (for a Cart instance per user session) or a Factory (to create different types of products or promotions). Don’t forget aspects like cart persistence (if the user logs out, the cart is saved) and handling multiple concurrent users.
Design an ATM Machine: Break this down into components like ATM (the machine), Account/Card, CashDispenser, Keypad, Screen, Transaction (with subclasses for Withdrawal, Deposit, Transfer, etc.), and BankService (to verify PIN and account balance). The design should cover how a user inserts a card, enters PIN (authentication), and performs transactions. Consider class relationships: an ATM has a composition of Screen, Keypad, Dispenser; a Transaction uses an Account; the ATM connects to a central bank system (could be an interface in design). Focus on workflow and state: e.g., ATM states (Idle, Authenticating, Serving user, Out of service) could be an extra detail. Mentioning how to handle errors (wrong PIN, insufficient funds, machine out of cash) will show completeness. This question checks understanding of interfacing with external systems and managing complex state in design. Learn how to design ATM
Design a Social Media News Feed: Here you design the backend for a news feed (like Facebook/Twitter feed). Important classes might be User, Post, FeedManager, and possibly FollowerGraph or FeedAlgorithm. The challenge is how to generate a feed for each user – you might discuss pulling posts from users they follow, ordering them (by time or rank), and storing feeds. While the full system involves heavy backend engineering, at LLD focus on class design: e.g., a FeedManager class that fetches relevant posts (maybe using an Observer pattern if you push updates to followers when someone posts). Consider how to design for performance: maybe use caching for feeds, or maintain a priority queue of top posts. Also, mention how to represent relationships (a User has a list of followers/followees). Interviewers are looking to see if you can handle a more algorithmic design and think about scale (since feeds involve lots of data). Learn how to design social media application.

Pro Tip: No matter the question, a great way to start is by clarifying requirements and constraints. For example, ask whether the parking lot is multi-level, or if the movie booking needs to handle payments, etc. This not only buys you time to think but also shows the interviewer you approach design systematically.

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Best Practices to Succeed in LLD Interviews

Even with solid knowledge of principles and patterns, it’s important to approach the interview methodically. Here are some best practices:

Break Down the Problem: Don’t jump straight into drawing classes. First, understand and clarify requirements (functional and non-functional). Then break the problem into logical components or modules. For instance, if designing a ride-sharing app, identify sub-problems: user management, ride matching, payment, etc. Tackle each part step by step rather than trying to design everything at once. A structured approach might be: clarify requirements → identify core classes → define relationships → refine with design patterns → consider edge cases. This systematic breakdown ensures you cover all aspects and makes the problem less overwhelming.
Communicate Design Decisions: Treat the interview as a discussion. Clearly narrate your thought process as you design. Explain why you’re introducing a class or a pattern (“I’ll use a Strategy pattern here to allow different payment methods, so adding new payment types in future is easy.”). Also, mention alternatives you considered and why you chose this approach. If the interviewer asks questions or offers hints, incorporate that feedback. Being able to justify your design decisions and discuss trade-offs (e.g. why choose a composition over inheritance in a certain relationship) is key. It shows maturity in design thinking. Use diagrams and simple examples to illustrate if needed.
Apply SOLID and Other Principles: As you design, keep the SOLID principles in mind (even if you don’t name them explicitly). Ensure your classes have a single clear purpose (Single Responsibility), try to design via interfaces/abstract classes so implementation can vary (Open-Closed, Liskov Substitution), separate out unrelated interfaces (Interface Segregation), and depend on abstractions rather than concretes (Dependency Inversion). Avoid big “God classes” that do everything. Write methods that are cohesive. Also, remember DRY – don’t repeat logic in multiple places, structure your design to reuse code via functions or base classes. If you notice your design violating a principle, mention how you’d improve it. Interviewers appreciate a self-critical and improving approach.
Keep It Simple and Elegant: A common mistake is over-engineering the solution. You don’t get extra points for adding needless classes or patterns. In fact, it may work against you if the design becomes convoluted. Strive for simplicity: use only the patterns that make sense, and keep class count manageable. For example, if a simple list will do, don’t insist on implementing a complex data structure. Remember, simplicity is key to maintainability. A simple, clearly reasoned design trumps a fancy but confused one. If the interviewer asks to extend the design with a new feature, it should be relatively straightforward in your simple design – that’s a sign you did well.
Consider Edge Cases and Constraints: Great candidates differentiate themselves by thinking of edge cases and error handling in their design. Before concluding, ask yourself: does my design handle invalid inputs or error conditions? What if the parking lot is full? What if two users try to book the last seat at the same time? Mentioning these cases and proposing solutions (even briefly) shows thoroughness. Additionally, consider constraints like performance (will my approach scale to thousands of users? do I need caching?), and concurrency (do I need locks or synchronized access for shared resources?). In an LLD interview, you might not need to design the entire concurrency control, but acknowledging it is important. This tells the interviewer you’re not just designing for the sunny day scenario but for the real world.
Practice with Mock Interviews & Examples: Finally, one of the best ways to get better is to practice common LLD problems beforehand. Design the parking lot, library system, elevator system, etc., on your own or with peers. This helps you recognize patterns in problems and reuse successful approaches. Practicing also makes you faster at coming up with class structures. Try mock interviews or write down your designs and have someone review them. By regularly practicing a variety of scenarios, you’ll enter the interview with a toolkit of ideas and the confidence to tackle whatever is asked.

Final Thoughts

Low-level design interviews may seem intimidating at first, but with the right preparation, they become an opportunity to shine.

To recap, focus on strong fundamentals – understand OOP principles deeply and how to apply them, and become familiar with a handful of common design patterns.

Always approach a design problem by thinking it through out loud, structuring your solution, and justifying your choices. Remember that interviewers are not only evaluating what you design but also how you arrive at it.

Keep your designs simple, handle edge cases, and show an enthusiasm for building robust software.

With consistent practice and a clear approach, you’ll gain the confidence to tackle any LLD problem.

Every system design you create, whether it’s a tiny module or a full application, is a chance to demonstrate creativity and engineering thought.