Imagine a chain where every link is strong individually. Even if one link has issues, the chain doesn't completely break.

Your system components need the same individual resilience:

🚀 Scalability Questions:

• "What if 1 million users hit my web server?"
• "How does my database handle millions of feeds?"
• "Can my aggregator process feeds for millions of users in parallel?"

🛡️ Fault Tolerance Questions:

• "What if my web server crashes?"
• "What if my database goes down?"
• "What if the aggregator service fails?"

✅ Availability Questions:

• "Is there any single point of failure?"
• "If one component dies, does the entire system go down?"
• "How quickly can we recover from failures?"

Golden Rule: No component should be a single point of failure.

Key Insight: "A system is only as strong as its weakest component - make sure none are weak."

Imagine you're building a house. You could keep adding rooms, floors, swimming pools, helicopter pads, underground bunkers... there's literally no end to what you could add.

System design has the same problem - it's infinitely buildable. You can keep adding:

• More features
• More components
• More services
• More complexity

But here's the million-dollar question: When do you stop?

Without clear evaluation criteria, you might either:

1. Under-design: Draw one box labeled 'Facebook' and call it done
2. Over-design: Create 50 microservices for a simple blog

Key Insight: "Good system design isn't about drawing the most boxes - it's about drawing the right boxes with clear purpose."

Imagine a restaurant where the chef also takes orders, the waiter also cooks, and the cashier also serves food. Chaos, right?

Your system components need mutually exclusive responsibilities - like a well-organized restaurant:

Feed System Example:

🌐 Web Server: "I serve HTTP requests to users. That's MY job."

• Handles user requests
• Fetches feed from database
• Sends response back

⚙️ Generator: "I create candidate feeds. That's MY job."

• Pulls from post service
• Pulls from follow service
• Stores in temporary database

🎯 Aggregator: "I create final feeds. That's MY job."

• Processes temporary data
• Filters low-quality content
• Creates final feed for database

Key Insight: "When responsibilities overlap, bugs multiply and debugging becomes a nightmare."

You wouldn't build a house without knowing about plumbing, electricity, foundation, and roofing, right?

Similarly, every system component needs these four technical details figured out:

🗄️ Database & Caching

• What data does this component store?
• How will it cache frequently accessed data?

📈 Scaling & Fault Tolerance

• How does it handle increased load?
• What happens when something breaks?

⚡ Asynchronous Processing

• What tasks can run in the background?
• How do we handle long-running operations?

🔄 Communication

• How do components talk to each other?
• REST APIs? Message queues? Direct database calls?

Example: Web Server might use HTTP for communication, Redis for caching, but no async processing needed.

Key Insight: "Technical details separate thoughtful design from random box-drawing."