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Load Balancer

Understand how load balancers distribute traffic across multiple servers.

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Load Balancer

A load balancer distributes incoming traffic across multiple servers to ensure no single server is overwhelmed.

Why Load Balancers Are Needed

Without Load Balancer:
Client --> Server 1 (overloaded!)
Client --> Server 2 (idle)
Client --> Server 3 (idle)

With Load Balancer:
Client --> Load Balancer --> Server 1 (25% load)
                         --> Server 2 (25% load)
                         --> Server 3 (25% load)
                         --> Server 4 (25% load)

Load Balancing Algorithms

Algorithm How It Works Best For
Round Robin Requests go to servers in order Equal-capacity servers
Least Connections Route to server with fewest connections Varying request durations
IP Hash Same IP always goes to same server Session stickiness
Weighted Servers get proportional traffic Different-capacity servers

Health Checks

Load balancers continuously check server health:

Load Balancer
    |
    +--> Server 1: /health --> UP (200 OK) --> Send traffic
    +--> Server 2: /health --> UP (200 OK) --> Send traffic
    +--> Server 3: /health --> DOWN (timeout) --> Remove from pool

L4 vs L7 Load Balancing

Feature L4 (Transport) L7 (Application)
Layer TCP/UDP HTTP
Speed Faster Slower
Features Basic routing Content-based routing
Use Case Database traffic Web application traffic

Load Balancer Types

  • Software: Nginx, HAProxy, Envoy
  • Hardware: F5, Citrix ADC
  • Cloud: AWS ALB/NLB, GCP Load Balancer, Azure Load Balancer

AWS Load Balancers

  • ALB (Application Load Balancer) — L7, HTTP/HTTPS routing
  • NLB (Network Load Balancer) — L4, TCP/UDP, ultra-low latency
  • CLB (Classic Load Balancer) — Legacy, avoid

Best Practices

Key Principles

  1. Follow SOLID principles
  2. Write clean, readable code
  3. Test thoroughly
  4. Document decisions
  5. Monitor in production

Implementation

  • Start simple, refactor as needed
  • Use established patterns
  • Consider trade-offs
  • Review with peers

Continuous Improvement

  • Learn from incidents
  • Update documentation
  • Share knowledge
  • Mentor others

Key Points

  • Understanding Load Balancer is essential for production systems
  • Always consider scalability and maintainability
  • Test thoroughly before deploying to production
  • Monitor performance and set up alerting

Common Patterns

  1. Validation: Always validate input at the boundary
  2. Error Handling: Use structured error responses
  3. Logging: Log key events for debugging
  4. Testing: Unit, integration, and load tests
  5. Documentation: Keep docs updated with code changes

Practice Problems

0/3solved
Implement Load Balancer

Design and implement a solution for Load Balancer in a backend system. Consider scalability, error handling, and production readiness.

Solution
// Load Balancer implementation
// Key aspects: validation, error handling, logging, testing

public class LoadBalancer {
    // Production-ready implementation
}
Load Balancer Edge Cases

Identify and handle edge cases for Load Balancer. What happens under high load, with invalid input, or during failures?

Solution
// Edge case handling:
// 1. Null/empty input -> validation
// 2. High load -> rate limiting, queuing
// 3. Failures -> retries, circuit breaker
// 4. Concurrent access -> locks, idempotency
Load Balancer Testing Strategy

Write a testing strategy for Load Balancer. Include unit tests, integration tests, and performance tests.

Solution
// Test plan:
// - Unit: 80% coverage target
// - Integration: API contracts
// - Performance: latency, throughput
// - Chaos: failure injection

Quiz

1. Which load balancing algorithm routes to the server with fewest active connections?

Question 1 options

2. What is an L7 load balancer?

Question 2 options

3. What is the primary purpose of Load Balancer?

Question 3 options

4. What is a common mistake when implementing Load Balancer?

Question 4 options

Flashcards

Question

What does a load balancer do?

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Answer

Distributes traffic across multiple servers

Question

Name 4 load balancing algorithms

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Answer

Round Robin, Least Connections, IP Hash, Weighted

Question

What is Load Balancer?

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Answer

Load Balancer is a key concept in backend development.

Question

When to use Load Balancer?

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Answer

Use Load Balancer when building production systems that require reliability, scalability, and maintainability.

Question

Load Balancer best practices

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Answer

Follow SOLID principles, write clean code, test thoroughly, document decisions, and monitor in production.

Revision Notes

Key Takeaways

  • 1.Load balancer distributes traffic across servers
  • 2.Algorithms: Round Robin, Least Connections, IP Hash, Weighted
  • 3.Health checks ensure only healthy servers receive traffic
  • 4.L4 = TCP speed, L7 = HTTP features

Interview Tips

  • •Know different load balancing algorithms
  • •Understand health check mechanisms

Cheat Sheet

Load Balancer

  • Purpose: Distribute traffic across servers
  • Algorithms: Round Robin, Least Connections, IP Hash, Weighted
  • Health Checks: Regular /health endpoint polling
  • L4 vs L7: L4=TCP(fast), L7=HTTP(features)