How to Prepare for
Low Level Design Interviews
This is a 7-Day plan to help you prepare for Low Level Design Interviews using common design patterns.
It will guide you to clear LLD rounds of Amazon, Uber, Flipkart, Microsoft and other top product companies for SDE-1, SDE-2 and Senior Software Engineer positions.
It will guide you to clear LLD rounds of Amazon, Uber, Flipkart, Microsoft and other top product companies for SDE-1, SDE-2 and Senior Software Engineer positions.
Languages: As of now only Java and Python are supported.
We won't be covering every detail of large systems like movie ticket booking system or food ordering app which can have hundreds of components and classes.
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Instead we will deep dive into the core features of these questions which interviewers expect to discuss with you in a LLD interview.
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YouTube videos will also teach you how to explain your solution to the interviewer step by step.
LLD Interview Essentials
What your interviewer is looking for?
In an object-oriented design interview, the interviewer is typically evaluating these four things.
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List the core requirements first
Identify only the most important features before discussing classes. Adding unimportant requirements wastes interview time and leaves less time for the core design. Here are a few core feature examples.
Design a Parking Lot:
park()unpark()
Design Food ordering System:
orderFood()rateOrder()displayRestaurants()
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Break the problem into multiple classes
Start by listing the main entities and, wherever required, their corresponding manager classes. Give every class a clear responsibility. Lets take the example of a food ordering system like Uber Eats, Zomato etc.
Restaurant system entities: Restaurant, Order and FoodItem.
Possible managers: RestaurantsManager and OrdersManager.
Possible managers: RestaurantsManager and OrdersManager.
3
Use design patterns for the core features
Be familiar with the implementation and common use cases of the design patterns most frequently discussed in LLD interviews.
Strategy
Factory
Observer
Singleton
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Discuss multi-threading when relevant
After the core design is clear, interviewers may ask how it behaves in a multi-threaded environment. Be ready to discuss locks, synchronization and thread-safe data structures. This blog is a helpful read for Java multi-threading discussions during interviews.
https://codezym.com/blog/13-top-3-questions-multithreading-java-interviews
Q. Will I have to write code or will UML diagrams be enough?
ANS: Yes, you have to write code or discuss the logic for a few functionalities. Only drawing UML diagrams or writing the names of classes will not be enough.
Q. I don't have much time. Which are the most important design patterns I should study first?
ANS: Factory, Strategy, Observer and Singleton.
Q. But how can I explain such large systems in a 45-minute interview? I always run out of time.
ANS: A vast majority of candidates fail because they are not able to present their solution properly within a limited time. Watch this YouTube video, where I explain how to handle this problem.
Q. Are questions like LRU Cache and Search Autocomplete System also asked in LLD rounds?
ANS: Yes. Depending on the interviewer, you can either get a pure LLD question such as designing a parking lot or food-ordering system, or a DSA-based design question like the ones above. Especially Microsoft loves asking DSA based design questions in their LLD rounds. I know you hate this extra preparation, but companies ask both types, so you need to prepare for both. The silver lining is that you have already prepared for DSA-based design questions while preparing for DS & Algo rounds.
Start each day with a few short videos to get into preparation mode,
and then solve one coding question covering one or more design patterns.
YouTube videos are free to watch. But to practise the coding questions, you will need to Buy Premium.
There are two types of low level design interview formats:
1. 75 to 90 minutes of machine Coding
2. 45-60 minutes of face to face discussion
Watch the adjacent Youtube Video for details.
1. 75 to 90 minutes of machine Coding
2. 45-60 minutes of face to face discussion
Watch the adjacent Youtube Video for details.
During LLD Interviews, it is not enough to just come up with a solution. Interviewers at top product companies tweak the questions and add custom scenarios. You should be able to change your design to handle those.
You are expected to defend your design decisions like why did you use a certain design pattern or kept certain data inside a given class. You can learn all this only through practicing questions.
Most candidates who clear FAANG/MAANG companies, practice machine coding of questions rather than just watching videos.
You are expected to defend your design decisions like why did you use a certain design pattern or kept certain data inside a given class. You can learn all this only through practicing questions.
Most candidates who clear FAANG/MAANG companies, practice machine coding of questions rather than just watching videos.
These are the four most common design patterns asked in LLD interviews.
Strategy, Observer, Factory and Singleton
We will practice them first and then move on to other design patterns.
Strategy, Observer, Factory and Singleton
We will practice them first and then move on to other design patterns.
Practice Problem
Design a Parking Lot
https://codezym.com/question/1
Parking Lot design is sort of the Hello World of low level design questions. Most people start their LLD interview preparation with this question.
Python solution video on the right takes you through the core features like park, remove vehicle, search vehicle and also teaches you how to clearly explain your design to the interviewer step by step in a 45-60 minute LLD interview.
For Java , we have to implement our solution for a multi-threaded environment using locking and thread safe data structures.
Video on the right show a simple solution using synchronized keyword which is the popular locking mechanism in Java.
However, using synchronized or any other lock is simple but not efficient as it locks out other threads from doing write operations concurrently.
Design a Parking Lot
https://codezym.com/question/1
Parking Lot design is sort of the Hello World of low level design questions. Most people start their LLD interview preparation with this question.
Python solution video on the right takes you through the core features like park, remove vehicle, search vehicle and also teaches you how to clearly explain your design to the interviewer step by step in a 45-60 minute LLD interview.
For Java , we have to implement our solution for a multi-threaded environment using locking and thread safe data structures.
Video on the right show a simple solution using synchronized keyword which is the popular locking mechanism in Java.
However, using synchronized or any other lock is simple but not efficient as it locks out other threads from doing write operations concurrently.
Video on the left shows a more efficient solution using a thread safe list ConcurrentLinkedDeque to store free spots on each floor.
It ensures that on each floor rather than using brute force to find a free spot, we can add/remove parking spots in O(1)
It ensures that on each floor rather than using brute force to find a free spot, we can add/remove parking spots in O(1)
Strategy Design Pattern
Strategy Pattern is used when we need different algorithms for the same functionality. Some examples are :
1. Different moves in chess: straight, diagonal, 2+1 move
2. different ways to order list of items e.g. by price, rating, most ordered etc
Strategy Pattern is used when we need different algorithms for the same functionality. Some examples are :
1. Different moves in chess: straight, diagonal, 2+1 move
2. different ways to order list of items e.g. by price, rating, most ordered etc
Practice Problem
Design a Parking Lot using Strategy Pattern
https://codezym.com/question/7
We revisit the Parking Lot design again. However, in this question we have to implement two different parking strategies. So you should use strategy pattern here.
Python solution video takes you through the core features like park, remove vehicle, search vehicle and it also teaches you how to clearly explain your design to interviewer step by step in a 45-60 minute LLD interview.
Python video has slightly better explanation than the Java video below it.
https://codezym.com/question/7
We revisit the Parking Lot design again. However, in this question we have to implement two different parking strategies. So you should use strategy pattern here.
Python solution video takes you through the core features like park, remove vehicle, search vehicle and it also teaches you how to clearly explain your design to interviewer step by step in a 45-60 minute LLD interview.
Python video has slightly better explanation than the Java video below it.
Observer Design Pattern
Observer Pattern is a behavioural design pattern and it is used when you want to have loose coupling between data which is getting changed and multiple views or observers which need to receive the changes.
example: Giving 1 to 5 star rating to a product on ecommerce website needs to be observed by the class which shows list of top rated products.
Observer pattern is generally used along with strategy pattern to inform different strategy classes about changes occurring in the common data structure.
example: a customer support system may have different strategy classes to assign an agent to a customer support call. Also each strategy need to be informed whenever an agent is assigned a case so that they can decide whether to assign new cases to this agent or not.
Observer pattern will be used in this case.
Practice Problem
Design a food ordering and rating system like Zomato, Swiggy, DoorDash
https://codezym.com/question/5
This system will have functionality to order food, rate orders and list top restaurants based on their rating
Now whenever an order is rated then it has to observed by classes which return top restaurants lists based on their rating. So this is an ideal use case for observer design pattern
Watch Java and Python video solutions for explanation.
Design a food ordering and rating system like Zomato, Swiggy, DoorDash
https://codezym.com/question/5
This system will have functionality to order food, rate orders and list top restaurants based on their rating
Now whenever an order is rated then it has to observed by classes which return top restaurants lists based on their rating. So this is an ideal use case for observer design pattern
Watch Java and Python video solutions for explanation.
Factory Design Pattern
Factory Pattern is a creational design pattern and it is used to decouple object's creation logic from their usage. This is required when we may have to create similar objects which follow the same interface or are subclasses of same superclass.
Some example are :
- Creating different pieces in chess e.g. pawn, king, queen etc.
- Creating different type of cars in a car race video game.
Singleton Design Pattern
Singleton Pattern is used when we need a single instance of an object. e.g. A connection pool manager for managing database connection will be a singleton.
Generally factories are implemented as a singleton
Singleton Pattern is used when we need a single instance of an object. e.g. A connection pool manager for managing database connection will be a singleton.
Generally factories are implemented as a singleton
Practice Problem
Design Chess Game
https://codezym.com/question/8
In Low Level Design of chess we use following design patterns
1. Chess Piece Factory to create different chess piece objects like king, queen, pawn etc
2. Strategy pattern to implement different moves e.g. straight move, diagonal move etc.
3. Singleton pattern to ensure there is a single instance of chess piece factory object.
Actually this question also serves as a counter example as to where we should not use singleton pattern.
Even though we need only once instance of Chess Piece Factory but it is being used from only one place as you will see in solution videos. So making ChessPieceFactory a singleton is unnecessary in this case.
Design Chess Game
https://codezym.com/question/8
In Low Level Design of chess we use following design patterns
1. Chess Piece Factory to create different chess piece objects like king, queen, pawn etc
2. Strategy pattern to implement different moves e.g. straight move, diagonal move etc.
3. Singleton pattern to ensure there is a single instance of chess piece factory object.
Actually this question also serves as a counter example as to where we should not use singleton pattern.
Even though we need only once instance of Chess Piece Factory but it is being used from only one place as you will see in solution videos. So making ChessPieceFactory a singleton is unnecessary in this case.
Flyweight Design Pattern
Flyweight design pattern is used when we need to create a large number of similar objects It helps us save memory space by sharing constant state among objects.
Some examples are
- Sharing 3-d images of thousands of soldiers in a computer war game
- Sharing styles of characters in a text editor like Microsoft word.
Practice Problem
Design a Text Editor/Word Processor like Microsoft Word
https://codezym.com/question/9
In a text editor, each character can have a different style in terms of parameter like colour, font size etc
Also there can be millions of characters in a text editor file,but total number of different styles used will be just from a few 100's to a few 1000's
So it makes sense to share character style as constant intrinsic state and reduce memory footprint of program
Design a Text Editor/Word Processor like Microsoft Word
https://codezym.com/question/9
In a text editor, each character can have a different style in terms of parameter like colour, font size etc
Also there can be millions of characters in a text editor file,but total number of different styles used will be just from a few 100's to a few 1000's
So it makes sense to share character style as constant intrinsic state and reduce memory footprint of program
State Design Pattern
State is a behavioural design pattern and it is used when an object has to change its behaviour as its internal state changes.
For example an ATM machine may be in different states like idle, money withdrawal in progress, validating pin etc and will respond to keyboard button presses differently in each state. The same keyboard can be used to input user's PIN as well as mobile number, depending on the current state of ATM.
another example is of a Cab booking System. Ride can be in states like: Ride Requested, Ongoing, Finished, Cancelled which will have multiple states and in each of these states calculation for a common behaviour like timeTakenToReachDestaination() will be different.
Practice Problem
Design an elevator management System with multiple lifts
https://codezym.com/question/11
A lift in an elevator system can be in one of three states. Moving Up, Moving Down and Idle And in each state it will behave differently in taking decisions like whether to stop on a floor, add a new request or not etc
So we will use state design pattern to solve this problem.
Design an elevator management System with multiple lifts
https://codezym.com/question/11
A lift in an elevator system can be in one of three states. Moving Up, Moving Down and Idle And in each state it will behave differently in taking decisions like whether to stop on a floor, add a new request or not etc
So we will use state design pattern to solve this problem.
Multi-Threading
In LLD interviews, after design patterns, discussion moves on to multi-threading. For these question you have to use synchronization, locks and thread safe data structures.
If you are using Java then you have access to thread safe data structures like AtomicInteger, ConcurrentHashMap, ConcurrentLinkedDequeue etc.
Below are Practice Problems
6. Design a Hit Counter/Webpage Visits Counter
https://codezym.com/question/6
This is an easy question, yet it gives you a taste of basic data structures to use in a multithreaded environment. This makes it an ideal question to put you in preparation mode for your low-level design interviews.
https://codezym.com/question/6
This is an easy question, yet it gives you a taste of basic data structures to use in a multithreaded environment. This makes it an ideal question to put you in preparation mode for your low-level design interviews.
4. Design an order and inventory management system https://codezym.com/question/4
An inventory management system of a simple e-commerce platform, needs to have the capability of handling sellers, products and orders.
Inventory is number of items of a particular product in a seller's warehouse.
An inventory management system of a simple e-commerce platform, needs to have the capability of handling sellers, products and orders.
Inventory is number of items of a particular product in a seller's warehouse.
3. Design a customer issue resolution system https://codezym.com/question/3 Customer support is nothing but a group of agents with skills for solving a set of issues.
Customer Issues can be classified into multiple types like order delayed, Payment Related etc. A customer issue resolution system is used to assign customer's issues to agents. Watch below YouTube video for a detailed explanation.
Customer Issues can be classified into multiple types like order delayed, Payment Related etc. A customer issue resolution system is used to assign customer's issues to agents. Watch below YouTube video for a detailed explanation.
Best of Luck
Prashant Priyadarshi
Founder, CodeZym
Founder, CodeZym