LLD interview expectations - System Design Exercise

Design: Low-Level Design Interview Preparation

Covers typical LLD interview expectations including design approach, communication, and coding style. Does not cover specific coding problems or system design at large scale.

Functional Requirements

FR1: Understand common expectations in LLD interviews

FR2: Identify key components and design principles to demonstrate

FR3: Prepare to explain design decisions clearly

FR4: Show ability to write clean, modular, and scalable code

FR5: Demonstrate knowledge of design patterns and best practices

Non-Functional Requirements

NFR1: Focus on clarity and simplicity in design

NFR2: Use industry best practices

NFR3: Designs should be scalable and maintainable

NFR4: Time-limited interview setting (typically 30-45 minutes)

Think Before You Design

Questions to Ask

❓ Question 1

❓ Question 2

❓ Question 3

❓ Question 4

❓ Question 5

❓ Question 6

Key Components

Classes and objects representing entities

Interfaces and abstractions

Methods and their responsibilities

Data structures used internally

Error handling and validation

Modularity and separation of concerns

Design Patterns

Factory pattern for object creation

Singleton for shared resources

Observer for event handling

Strategy for interchangeable algorithms

Decorator for extending functionality

MVC or layered architecture for separation

Reference Architecture

LLD Interview Preparation
+-------------------------+
| Understand Requirements |
+-----------+-------------+
            |
            v
+-------------------------+
| Identify Key Entities    |
+-----------+-------------+
            |
            v
+-------------------------+
| Define Classes & Methods |
+-----------+-------------+
            |
            v
+-------------------------+
| Apply Design Patterns    |
+-----------+-------------+
            |
            v
+-------------------------+
| Write Clean, Modular Code|
+-------------------------+

Components

Requirement Analysis

N/A

Understand problem scope and constraints

Entity Modeling

UML or simple class diagrams

Identify main objects and their relationships

Class Design

Object-oriented programming

Define classes, methods, and responsibilities

Design Patterns

Common OOP patterns

Use reusable solutions for common problems

Code Implementation

Programming language of choice

Write modular, readable, and testable code

Communication

Verbal explanation

Explain design decisions clearly to interviewer

Request Flow

1. Start by clarifying requirements with the interviewer

2. Identify key entities and their attributes

3. Define classes and their methods based on responsibilities

4. Choose appropriate design patterns to simplify design

5. Write code focusing on modularity and clarity

6. Explain each step and justify design choices

7. Handle edge cases and error scenarios

8. Iterate based on feedback or questions

Database Schema

Not applicable for general LLD interview expectations; focus is on class design and object relationships rather than database.

Scaling Discussion

Bottlenecks

Overcomplicating design with unnecessary patterns

Poor communication leading to misunderstood requirements

Writing monolithic code without modularity

Ignoring edge cases and error handling

Not considering scalability or maintainability

Solutions

Keep design simple and focused on requirements

Ask clarifying questions frequently

Break down code into small, reusable components

Include error handling and validation in design

Discuss how design can evolve for scalability

Interview Tips

Time: Spend first 5-10 minutes clarifying requirements and asking questions, next 20-25 minutes designing and coding, last 5-10 minutes explaining and refining design.

Clarify requirements before starting design

Explain your thought process clearly

Use simple and modular class structures

Apply relevant design patterns appropriately

Write clean and readable code

Discuss trade-offs and alternatives

Show awareness of scalability and maintainability

Practice

1. What is the main focus of a Low-Level Design (LLD) interview?

easy

A. Choosing cloud providers and infrastructure

B. Estimating system capacity and scaling

C. Designing detailed components and their interactions

D. Writing complex SQL queries

Solution

1. Step 1: Understand LLD interview scope

   LLD interviews focus on designing small parts of a system clearly and in detail.

2. Step 2: Compare options with LLD focus

   Estimating capacity and scaling, choosing cloud providers and infrastructure, and writing database queries relate to other areas, not the main focus of LLD interviews.

3. Final Answer:

   Designing detailed components and their interactions -> Option C

4. Quick Check:

   LLD = detailed component design [OK]

Hint: LLD = detailed design of small system parts [OK]

Common Mistakes:

• Confusing LLD with high-level system design
• Thinking LLD focuses on infrastructure
• Mixing LLD with database query writing

2. Which of the following is the correct way to represent a relationship in a class diagram during an LLD interview?

easy

A. Using arrows to show inheritance and lines for associations

B. Using SQL queries to define relationships

C. Using cloud icons to represent services

D. Using flowcharts to show data flow

Solution

1. Step 1: Identify class diagram conventions

   Class diagrams use arrows for inheritance and lines for associations between classes.

2. Step 2: Eliminate unrelated options

   SQL queries, cloud icons, and flowcharts are not used to represent class relationships in LLD diagrams.

3. Final Answer:

   Using arrows to show inheritance and lines for associations -> Option A

4. Quick Check:

   Class diagram = arrows + lines [OK]

Hint: Class diagrams use arrows and lines for relationships [OK]

Common Mistakes:

• Confusing class diagrams with flowcharts
• Using database or cloud symbols incorrectly
• Ignoring inheritance arrows

3. Consider this simplified class design snippet in an LLD interview:

class User {
  int id;
  String name;
  List<Order> orders;
}

class Order {
  int orderId;
  double amount;
}

What is the relationship between User and Order?

medium

A. Inheritance

B. Aggregation

C. Composition

D. No relationship

Solution

1. Step 1: Analyze the class fields

   User has a list of Orders, meaning User contains Orders but Orders can exist independently.

2. Step 2: Determine relationship type

   Since Orders can exist without User, this is aggregation, not composition or inheritance.

3. Final Answer:

   Aggregation -> Option B

4. Quick Check:

   User "has" Orders = Aggregation [OK]

Hint: List field means aggregation, not inheritance [OK]

Common Mistakes:

• Confusing aggregation with composition
• Thinking it's inheritance due to class names
• Ignoring independence of Order objects

4. You designed a class diagram but forgot to show the multiplicity on associations. What issue might this cause in an LLD interview?

medium

A. Ambiguity about how many objects relate to each other

B. Syntax error in the diagram

C. The system will not compile

D. No impact, multiplicity is optional

Solution

1. Step 1: Understand multiplicity role

   Multiplicity shows how many instances of one class relate to another, clarifying design details.

2. Step 2: Consequence of missing multiplicity

   Without it, the design is unclear about object counts, causing ambiguity.

3. Final Answer:

   Ambiguity about how many objects relate to each other -> Option A

4. Quick Check:

   Missing multiplicity = unclear relationships [OK]

Hint: Multiplicity clarifies object counts in relationships [OK]

Common Mistakes:

• Thinking diagram syntax breaks without multiplicity
• Assuming multiplicity is always optional
• Confusing multiplicity with inheritance

5. During an LLD interview, you are asked to design a simple library system with Books and Members. Which design choice best follows LLD interview expectations?

hard

A. Design a single class Library with all methods and no separation

B. Use complex inheritance hierarchies for Book types without explaining choices

C. Focus only on database schema without class diagrams

D. Create classes Book and Member with clear attributes and a borrowing relationship using aggregation

Solution

1. Step 1: Identify good LLD design practices

   LLD expects clear classes with attributes and simple relationships like aggregation.

2. Step 2: Evaluate options against LLD expectations

   Creating classes Book and Member with clear attributes and a borrowing relationship using aggregation matches best by defining classes and relationships clearly. A single class lacks separation, database schema skips class diagrams, and unexplained complex inheritance overcomplicates without clarity.

3. Final Answer:

   Create classes Book and Member with clear attributes and a borrowing relationship using aggregation -> Option D

4. Quick Check:

   Clear classes + simple relationships = LLD [OK]

Hint: Clear classes + simple relationships = good LLD design [OK]

Common Mistakes:

• Overcomplicating with inheritance
• Ignoring class separation
• Skipping diagrams and explanations