High-Level Design
🏗️ Explain Like I'm 5: Building a Lemonade Stand Empire
Imagine you want to build not just one lemonade stand, but a whole chain of stands across the city! How do you design it?
📋 Step 1: Planning
- • Where will the stands be? (Near parks, playgrounds, busy streets)
- • How many workers do you need? (Cashier, lemon squeezer, cup filler)
- • Where do you store lemons? (Warehouse? Each stand?)
✏️ Step 2: Draw the Big Picture
- • Central lemon warehouse (sends lemons to all stands)
- • Each stand has: cashier, lemon squeezer, ice machine
- • Delivery trucks move lemons from warehouse to stands
- • Money collected goes back to main office
📚 The Lesson:
This is High-Level Design! You draw boxes showing the main parts (warehouse, stands, trucks, office) and arrows showing how things flow between them. You don't worry about tiny details yet - just the big picture!
What is High-Level Design?
High-Level Design (HLD) is creating the overall architecture of a system, showing major components and how they interact. It's like creating a blueprint before building a house - you show where rooms go, but not where every nail goes!
Why is it Important?
- ✓ Provides a roadmap for the entire development team
- ✓ Helps identify major technical challenges early
- ✓ Makes communication easier between teams
- ✓ Allows evaluation of different architectural approaches
- ✓ Critical for system design interviews!
Key Components of HLD
1. Client/User Interface
How users interact with the system (web app, mobile app, CLI)
Examples: React web app, iOS/Android app, REST API client
2. Load Balancer
Distributes incoming traffic across multiple servers
Examples: Nginx, HAProxy, AWS ALB, Cloudflare Why? Prevents any single server from being overwhelmed
3. Application Servers
Handle business logic and process user requests
Examples: Node.js servers, Java Spring Boot, Python Django Pattern: Typically multiple servers for redundancy
4. Databases
Store and retrieve persistent data
SQL (PostgreSQL, MySQL) for structured data, NoSQL (MongoDB, Cassandra) for flexible schemas Pattern: Primary-Replica setup for read scaling
5. Cache Layer
Stores frequently accessed data in memory for fast retrieval
Examples: Redis, Memcached Benefit: Reduces database load by 70-90%
6. Message Queue
Enables asynchronous communication between services
Examples: RabbitMQ, Apache Kafka, AWS SQS Use Case: Background jobs, event processing, decoupling services
7. CDN (Content Delivery Network)
Serves static content from servers closest to users
Examples: CloudFront, Cloudflare, Akamai Benefit: Reduces latency by 60-80%
Common Architecture Patterns
Monolithic Architecture
All components in one application
Pros: Simple to develop, easy to deploy, straightforward debugging
Cons: Hard to scale, one bug can crash entire system, technology locked-in
Microservices Architecture
Application split into small, independent services
Pros: Independent scaling, technology flexibility, fault isolation
Cons: Complex deployment, network overhead, distributed system challenges
Layered (N-Tier) Architecture
Organized into layers: Presentation, Business Logic, Data Access
Pros: Clear separation of concerns, reusable layers, easier testing
Cons: Can become bloated, performance overhead from layer crossing
HLD Design Process
Step 1: Clarify Requirements
Understand functional and non-functional requirements. Ask about users, scale, latency needs.
Step 2: Identify Main Components
List major building blocks: API Gateway, Application Servers, Databases, Cache, etc.
Step 3: Define Data Flow
Show how data moves between components with arrows. Read path vs Write path.
Step 4: Choose Technologies
Select appropriate tech stack: SQL vs NoSQL, REST vs GraphQL, etc.
Step 5: Address Non-Functional Requirements
Show how you achieve scalability, reliability, security, performance.
Step 6: Draw the Diagram
Create clear boxes and arrows. Label everything. Keep it simple!
Example: Twitter-like System HLD
Let's design a simplified Twitter with posts, timeline, and following.
Major Components:
- • API Gateway - Routes requests to appropriate services
- • Auth Service - Handles login, tokens, permissions
- • Tweet Service - Creates, edits, deletes tweets
- • Timeline Service - Generates user timelines
- • Follow Service - Manages follower relationships
- • Notification Service - Sends notifications for likes, follows
Databases:
- • PostgreSQL - Users, relationships (structured data)
- • Cassandra - Tweets, timelines (high write throughput)
- • Redis - Timeline cache, session storage
Data Flow Example - Posting a Tweet:
Best Practices
Common Mistakes to Avoid
System Design Interview Tips
Tools for Creating HLD Diagrams
- •Draw.io (diagrams.net) - Free, web-based
- •Lucidchart - Professional diagramming tool
- •Excalidraw - Hand-drawn style diagrams
- •Miro - Collaborative whiteboard
- •Paper & Pen - Still works in interviews!