High-Level Design (HLD) Interview Prep

A collection of system design documents for interview preparation. Each topic is organized in its own directory with comprehensive documentation, diagrams, and quick revision notes.

📚 Topics

Topic	Difficulty	Key Concepts	Quick Revision
Payment System	Hard	Idempotency, Event Sourcing, CDC, Financial Integrity	⚡ Cheatsheet
Rate Limiter	Medium-Hard	Token Bucket, Redis Sharding, Lua Scripts, Fail-Closed	⚡ Cheatsheet
Ad Click Aggregator	Hard	Stream Processing, Kafka, Flink, Reconciliation, Hot Shards	⚡ Cheatsheet
Web Crawler	Hard	Multi-stage Pipeline, robots.txt, DNS Caching, Crawler Traps	⚡ Cheatsheet
Stock Broker	Hard	Redis Pub/Sub, SSE, Order Consistency, Cleanup Jobs	⚡ Cheatsheet

🗂️ Repository Structure

hld/
├── README.md                    # This index file
├── payment-system/
│   ├── README.md                # Full detailed design
│   └── CHEATSHEET.md            # 5-min quick revision
├── rate-limiter/
│   ├── README.md
│   └── CHEATSHEET.md
├── ad-click-aggregator/
│   ├── README.md
│   └── CHEATSHEET.md
├── web-crawler/
│   ├── README.md
│   └── CHEATSHEET.md
├── stock-broker/
│   ├── README.md
│   └── CHEATSHEET.md
└── [future-topic]/
    ├── README.md
    └── CHEATSHEET.md

📖 How to Use

1. Full Study: Read the comprehensive README.md for each topic
2. Quick Revision: Use CHEATSHEET.md for 5-minute pre-interview refresh
3. Diagrams: Embedded as Mermaid (viewable on GitHub)
4. Good vs Bad: Each cheatsheet shows recommended vs anti-patterns

🎯 Interview Levels

Each document includes expectations for:

• Mid-level: Core functionality, basic security awareness
• Senior: Drive conversation, identify edge cases, propose robust solutions
• Staff+: Deep expertise, product thinking, handle complex trade-offs