Day 4

Today's Objective

Load balancers, failover, circuit breakers, rate limiting.

What You'll Cover Today

Day 4 of System Design in 5 Days focuses on load balancing & availability. Load balancers, failover, circuit breakers, rate limiting. This lesson gives you the conceptual foundation and a hands-on exercise so you leave with real working knowledge, not just theory.

load balancer

Understanding load balancer is foundational for everything in this course. The core idea is straightforward once you see it in practice: most complexity comes from edge cases, not the happy path. Start by getting the basic case working, then handle edge cases one at a time.

// Load Balancing & Availability — working example
// Replace these values with your actual data

const example = {
  topic: 'load balancer',
  day: 4,
  course: 'System Design in 5 Days'
};

// This is where your implementation goes
function implement(config) {
  // 1. Validate inputs
  if (!config.topic) throw new Error('Topic required');
  
  // 2. Core logic
  const result = process(config);
  
  // 3. Return structured output
  return result;
}

console.log('Ready to implement load balancer');

circuit breaker

Once you have the basics, circuit breaker becomes the practical application. The pattern you'll use most often is: configure once, reuse everywhere. Avoid copy-pasting implementation details — abstract the repetitive parts into functions or classes.

Common Mistakes on Day 4

• Skipping error handling — every real-world implementation needs error handling. Add it from the start, not as an afterthought.
• Hard-coding configuration — use environment variables for anything that differs between development and production.
• Not reading error messages — error messages in System are usually precise. Read them carefully before searching Stack Overflow.
• Over-abstracting too early — make it work first, then refactor. Premature abstraction is the enemy of progress.

1. Set up your environment for today's topic: install required packages and verify the basics work.
2. Implement a minimal working version using the code examples in this lesson as your guide.
3. Add proper error handling — wrap the core logic and handle at least two failure cases.
4. Test your implementation with both valid and invalid inputs.
5. Review your code: is there anything you'd name differently? Any function doing more than one thing?