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."

Think of your favorite recipe. A good recipe doesn't just say "make pasta" - it breaks down into clear steps: boil water, add salt, cook pasta, prepare sauce, combine.

System design works the same way. The first evaluation criteria is:

Have you broken your system into meaningful components?

Bad Example:

[Facebook System] ← One giant black box

Good Example:

[Web <TopicPreview slug="server">Server</TopicPreview>] → [Feed Generator] → [<TopicPreview slug="database">Database</TopicPreview>]
     ↓              ↓
[User Service] → [Aggregator]

Each component has a specific job, just like each ingredient in a recipe has a purpose.

Key Insight: "Component breakdown shows you're thinking systematically, not just drawing random boxes."

Now you have the complete 4-Step System Design Evaluation Framework :

✅ Step 1: Component Breakdown

• System is broken into meaningful components
• Each component has a clear purpose
• Not just one giant black box

✅ Step 2: Clear Responsibilities

• Each component has specific responsibilities
• Responsibilities don't overlap
• No confusion about "who does what"

✅ Step 3: Technical Details

• Database & caching strategy defined
• Scaling & fault tolerance planned
• Asynchronous processing identified
• Communication protocols chosen

✅ Step 4: Individual Resilience

• Each component can scale independently
• Fault tolerance mechanisms in place
• No single points of failure
• High availability maintained

Key Insight: "Use this framework for every system you design - it transforms random architecture into thoughtful engineering."

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."

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."