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.