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Load Balancing

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1. What is Load Balancing?

Load balancing is the process of distributing network traffic across multiple servers to ensure no single server is overwhelmed. It improves availability, reliability, and performance.

2. Types of Load Balancers

Type Layer Description
L4 (Transport) TCP/UDP Routes based on IP and port
L7 (Application) HTTP Routes based on content, headers, URL

L4 vs L7 Load Balancers

Feature L4 L7
Speed Faster Slightly slower
Intelligence Basic Content-aware
SSL Termination Usually no Yes
URL Routing No Yes
WebSocket Support Limited Full

3. Load Balancing Algorithms

Static Algorithms

Algorithm Description Use Case
Round Robin Distribute sequentially Equal capacity servers
Weighted Round Robin Consider server weights Unequal capacity
IP Hash Hash client IP to server Session persistence

Dynamic Algorithms

Algorithm Description Use Case
Least Connections Route to server with fewest connections Long-lived connections
Weighted Least Connections Consider weights + connections Variable capacity
Least Response Time Route to fastest responding Latency-sensitive apps
Resource Based Based on server resources (CPU, RAM) Heterogeneous servers

4. Health Checks

Load balancers continuously check server health to avoid routing to failed servers.

Health Check Types:
1. TCP Check - Can establish connection?
2. HTTP Check - Does /health return 200?
3. Custom Script - Run specific health validation
Parameter Typical Value
Interval 5-30 seconds
Timeout 2-5 seconds
Unhealthy Threshold 2-3 failures
Healthy Threshold 2-3 successes

5. Session Persistence (Sticky Sessions)

Ensure requests from the same client go to the same server.

Methods

  • Cookie-based: Insert cookie with server ID
  • IP-based: Hash client IP
  • URL-based: Encode session in URL
Pros Cons
Session data locality Uneven load distribution
Simpler app design Server failure loses sessions
Cache efficiency Scaling challenges

6. Reverse Proxy

A reverse proxy sits between clients and servers, forwarding requests to backend servers.

Client → Reverse Proxy → Server 1
                       → Server 2
                       → Server 3

Reverse Proxy Benefits

  • SSL/TLS termination
  • Caching static content
  • Compression
  • Security (hide server IPs)
  • Rate limiting

Load Balancer vs Reverse Proxy

Feature Load Balancer Reverse Proxy
Primary Purpose Distribute load Security, caching
Health Checks Yes Optional
SSL Termination Some Yes
Caching No Yes

7. High Availability Setup

Avoid single point of failure in load balancers.

        Internet
           ↓
  ┌─────────────────┐
  │  DNS (Multiple) │
  └────────┬────────┘
           ↓
  ┌─────────────────┐
  │ LB 1 ←───→ LB 2 │  (Active-Passive or Active-Active)
  │   (Heartbeat)   │
  └────────┬────────┘
           ↓
    ┌──────┼──────┐
    ↓      ↓      ↓
  App1   App2   App3

8. Popular Load Balancers

Software Type License
NGINX L7 Open Source/Commercial
HAProxy L4/L7 Open Source
AWS ALB L7 Cloud (AWS)
AWS NLB L4 Cloud (AWS)
Google Cloud LB L4/L7 Cloud (GCP)
Traefik L7 Open Source

9. DNS Load Balancing

Distribute traffic at DNS level by returning different IPs.

example.com → 1.2.3.4 (30% of time)
           → 5.6.7.8 (30% of time)
           → 9.10.11.12 (40% of time)
Pros Cons
Simple to implement No health checks (basic)
Geographic routing DNS caching issues
No extra infrastructure Limited control

10. Global Server Load Balancing (GSLB)

Route users to the nearest or best-performing data center.

User (India) → GSLB → Asia DC
User (USA) → GSLB → US DC
User (Europe) → GSLB → EU DC