High Level and Low Level Design

Two levels of system design that every engineer must master — high-level design decides what to build and why, low-level design decides exactly how to build it.

HLD vs LLD - The Architect's Tale

🎯 Challenge 1: The House Building Dilemma

Imagine this scenario: You want to build your dream house. You meet with your builder and say: "I want a 3-bedroom house with a modern kitchen."

Pause and think: Would you expect the builder to start pouring concrete immediately? Or would you need something in between?

The Two-Level Reality: Just like building a house needs both an architectural blueprint AND detailed construction plans, software systems need:

• High-Level Design (HLD) - The "what" and "why"
• Low-Level Design (LLD) - The "how" in detail

Key insight: You can't jump from idea to implementation without both levels of design!

🖼️ Interactive Exercise: The Restaurant Analogy

Scenario: You're opening a restaurant. Let's see the difference between HLD and LLD thinking:

High-Level Design (HLD) Thinking:

Question: "What kind of restaurant are we building?"

Your HLD answers:

✓ Italian restaurant

✓ Seats 50 customers

✓ Has kitchen, dining area, bar

✓ Takes reservations online

✓ Accepts credit cards

✓ Open 5pm-11pm

Low-Level Design (LLD) Thinking:

Question: "HOW exactly will online reservations work?"

Your LLD answers:

✓ Use PostgreSQL database with 'reservations' table

✓ Table has columns: id, customer_name, phone, date, time, party_size

✓ API endpoint: POST /api/reservations

✓ Validation: Check if time slot available before confirming

✓ Send confirmation email using SendGrid API

✓ Handle double-booking with database transaction locks

Mental Model:

• HLD = The restaurant floor plan showing "Kitchen here, tables there, bar over here"
• LLD = The detailed electrical wiring diagram showing "outlet here, wire gauge 14AWG, circuit breaker 20A"

🎨 Visual Comparison: Building a Social Media App

Let's design a Twitter-like app to see both levels in action:

HIGH-LEVEL DESIGN (30,000 foot view)

Components: User Service, Tweet Service, Database Data Flow: User → Posts Tweet → Stored in DB Tech Stack: Node.js backend, React frontend, MongoDB

HLD answers:

• What are the main components? (User Service, Tweet Service, Database)
• How do they interact? (API calls between services)
• What technologies? (Node.js, MongoDB, React)

LOW-LEVEL DESIGN (Ground level view)

Just for the "Post Tweet" feature:

LLD answers:

• Exact function signatures
• Data validation logic
• Error handling
• Database schema with indexes
• Algorithm for each operation
• Edge cases handling

🚨 Common Misconception: "Can't I Just Code Without All This Design?"

You might think: "This seems like extra work. Why not just start coding?"

The Reality Check:

Imagine building IKEA furniture:

• Without HLD: You don't look at the picture of the finished product → Build random parts → End up with a chair that looks like a table
• Without LLD: You see the picture but skip the step-by-step instructions → Parts don't fit → Screws in wrong places → Wobbly furniture

Real-world comparison:

No HLD = Building a house without a floor plan Result: Bathroom ends up in the middle of the living room

No LLD = Having the floor plan but no construction details Result: You know WHERE the bathroom goes, but not HOW to install the plumbing, what pipe sizes, what materials

The Sweet Spot:

1. HLD first → Make sure you're building the RIGHT thing
2. LLD next → Make sure you're building it the RIGHT way
3. Code last → Implementation is now straightforward

🎮 Decision Game: Which Level of Design?

For each statement, identify if it's HLD or LLD:

A. "We'll use Redis for caching frequently accessed data"

B. "The cache key will be user:{userId}:profile with TTL of 3600 seconds"

C. "The system will have three microservices: Auth, Orders, and Payments"

D. "The Order service will use a state machine with states: PENDING → CONFIRMED → SHIPPED → DELIVERED"

E. "We'll use PostgreSQL for persistent storage"

F. "The users table will have a B-tree index on email column and a composite index on (last_name, first_name)"

Think about each one... What level of detail is being specified?

Answers revealed:

• A: HLD → Mentions technology choice, but not implementation details
• B: LLD → Specific key format, exact TTL value
• C: HLD → System architecture, component breakdown
• D: LLD → Exact state transitions and values
• E: HLD → Database technology choice
• F: LLD → Specific index types and columns

Mental Model:

• If your manager or client could understand it → HLD
• If only developers need this level of detail → LLD

📊 The Complete Picture: HLD vs LLD

LLD (Engineer)

🔑 Key Takeaway: The Two-Level Strategy

When designing a system:

1. Start with HLD

   • Sketch the big picture
   • Identify major components
   • Choose technologies
   • Define interactions
   • Get stakeholder buy-in

2. Drill down to LLD

   • Design each component in detail
   • Specify algorithms
   • Define data structures
   • Plan error handling
   • Ready for coding

Real-world parallel:

• Planning a road trip: HLD = Route from NYC to LA, stop in Chicago and Denver
• LLD = Exact highways (I-80 West), exit numbers (Exit 126B), gas stations every 200 miles