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⏱️ Guide: Low-Level Design Delivery Framework

📝 Overview

The Low-Level Design (LLD) delivery framework is a structured, step-by-step methodology for tackling object-oriented design interviews within a constrained 35-minute timeframe. It guides candidates through requirements gathering, entity relationship mapping, class API design, implementation, and extensibility—without getting bogged down in edge cases too early.

This process helps you maintain steady pacing, isolate responsibilities cleanly, and produce code that reflects disciplined, production-grade thinking.

Why This Matters?

  • Core Skill: Enables you to convert ambiguous problem statements into clear specifications and well-encapsulated class structures.
  • Interview Relevance: LLD interviews move fast; poor time management is a common failure mode.
  • Real-world Use: Mirrors actual engineering workflows—scoping, modeling, API design, and implementation.
  • Common Mistake: Jumping straight into coding and getting lost in edge cases before defining structure.

🧠 Core Concepts

🔑 Step 1: Requirements

Definition: Turning a minimal prompt into a concrete specification.

Key Idea: Ask questions across four themes:

  • Primary capabilities
  • Rules and completion
  • Error handling
  • Scope boundaries

Example: In Tic Tac Toe, define win conditions, draw conditions, and invalid input handling.

When to Use: First ~5 minutes.

🔑 Step 2: Entities and Relationships

Definition: Extracting core nouns to define system structure.

Key Idea: An entity:

  • Maintains changing state, or
  • Enforces rules

If it’s just data, it’s likely a field—not a class.

Example: Game (orchestrator), Board, and Player.

When to Use: ~3 minutes after requirements.

🔑 Step 3: Class Design

Definition: Defining state (data) and behavior (methods).

Key Ideas:

  • Work top-down from the orchestrator
  • Follow “Tell, Don’t Ask” (encapsulation)

Example:

  • Game tracks current player
  • makeMove() defines behavior

When to Use: ~10–15 minutes.

🔑 Step 4: Implementation

Definition: Writing core logic for key methods.

Key Idea:

  • Start with the happy path
  • Then handle edge cases

Example: Implement move logic in a board game, then add validations.

When to Use: ~10 minutes.

🔑 Step 5: Extensibility

Definition: Adapting the design for new requirements.

Key Idea: Show that your system can evolve cleanly without hacks.

Example: Add undo functionality using a command history stack.

When to Use: Final ~5 minutes.

🏗️ Mental Models & Intuition

Think of:

  • System Design → City map
  • Low-Level Design → Blueprint of a building

You focus on how internal components interact—not large-scale traffic.

💡 Rule of Thumb: Keep rules with the entity that owns the data. If one class constantly queries another for decisions, encapsulation is weak.

⏱️ Execution Strategy

Requirements (~5 min):

  • Lock scope
  • Define what’s out of scope

Entities (~3 min):

  • Identify orchestrator and supporting classes

Class Design (~10–15 min):

  • Define state + behavior
  • Build APIs top-down

Implementation (~10 min):

  • Write core logic
  • Trace execution step-by-step

Extensibility (~5 min):

  • Handle “what if” scenarios

🗣️ Communication Tips

  • Guide, don’t fight: Follow interviewer direction but maintain structure
  • Ask preferences: Code vs pseudocode vs discussion
  • Think out loud: Show reasoning clearly
  • Avoid formal UML: Use simple class sketches instead

🧩 Patterns & Techniques

Entity Filtering Technique

Rule:

  • Has state or rules → Class
  • Just data → Field

Prevents over-engineering.

Verification Tracing

What: Mentally simulate execution

Why: Catches logical bugs early

How:

  • Start from initial state
  • Apply operations step-by-step
  • Track transitions and outcomes

⚙️ Practical Examples

Tic Tac Toe Mapping

Requirement: Players alternate moves

  • State: Players, board, turn
  • Behavior: makeMove(), getCurrentPlayer()

Requirement: Game ends on win/draw

  • State: Game status, winner
  • Behavior: getGameState(), getWinner()

Real-World Example

Elevator System:

  • Start with ElevatorController (entry point)
  • Then design Elevator for movement logic

Focus on how components interact rather than isolated logic.

⚖️ Trade-offs

Approach Pros Cons
Formal UML Structured, academic Slow, rarely used in interviews
Simple Diagrams Fast, flexible Less detailed
Pseudocode Clear logic focus May not satisfy coding-heavy interviews

🎤 Interview Focus

  • State vs Behavior clarity
  • Ability to trace execution
  • Clear scope boundaries
  • Avoid premature coding

🚀 How to Apply

  1. Clarify Requirements Define capabilities, rules, and scope

  2. Identify Entities Filter nouns into classes vs fields

  3. Design APIs Top-Down Start with orchestrator

  4. Implement + Trace Build core logic and validate

  5. Handle Extensions Show flexibility in design