LLD interview expectations - System Design Guide

Problem Statement

Candidates often fail low-level design interviews because they write code without structure or ignore design principles, leading to unmaintainable and buggy solutions.

Solution

The interviewee should demonstrate clear class design, apply SOLID principles, and write modular, reusable code. This approach shows understanding of good software architecture and problem decomposition.

Architecture

Interview

Problem

→Candidate's

↓

SOLID Principles

SOLID Principles

This diagram shows the flow from the interview problem to the candidate's design plan applying SOLID principles and modular code.

Trade-offs

✓ Pros

→

Improves code readability and maintainability.

→

Demonstrates professional software engineering skills.

→

Helps identify design flaws early through clear structure.

✗ Cons

→

Takes more time during the interview to plan and write.

→

May be challenging under strict time constraints.

→

Requires prior practice and understanding of design principles.

Always use in low-level design interviews where code quality and design are evaluated, especially for mid to senior roles.

Not necessary for quick algorithm-only interviews or entry-level coding tests focused solely on syntax.

Real World Examples

Amazon

Amazon evaluates candidates on their ability to design scalable, maintainable classes following SOLID principles during LLD interviews.

Google

Google expects clear modular design and proper use of design patterns in LLD interviews to assess system thinking.

Microsoft

Microsoft uses LLD interviews to check candidates' understanding of object-oriented design and clean code practices.

Code Example

The before code mixes payment types in one method, violating single responsibility. The after code uses an abstract class and separate classes for each payment type, following SOLID principles and making the code modular and extensible.

LLD

### Before: No clear design, everything in one class
class PaymentProcessor:
    def process(self, payment_type, amount):
        if payment_type == 'credit':
            print(f"Processing credit payment of {amount}")
        elif payment_type == 'paypal':
            print(f"Processing PayPal payment of {amount}")

### After: Applying SOLID principles with separate classes
from abc import ABC, abstractmethod

class PaymentMethod(ABC):
    @abstractmethod
    def pay(self, amount):
        pass

class CreditCardPayment(PaymentMethod):
    def pay(self, amount):
        print(f"Processing credit payment of {amount}")

class PayPalPayment(PaymentMethod):
    def pay(self, amount):
        print(f"Processing PayPal payment of {amount}")

class PaymentProcessor:
    def __init__(self, method: PaymentMethod):
        self.method = method

    def process(self, amount):
        self.method.pay(amount)

# Usage
processor = PaymentProcessor(CreditCardPayment())
processor.process(100)

Alternatives

Algorithm-focused interview

Focuses on solving coding problems with efficient algorithms rather than design structure.

Use when: When the role is primarily about algorithmic problem solving or entry-level coding.

System design interview

Focuses on high-level architecture and system components rather than detailed class design.

Use when: When the role involves designing large-scale distributed systems.

Summary

LLD interviews test your ability to write clean, modular, and maintainable code.

Applying SOLID principles and clear class design is essential to succeed.

Good design shows your understanding of software engineering beyond syntax.

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