Load Balancing

Load balancing distributes incoming network traffic across multiple servers to ensure no single server bears too much load.

Why Load Balancing?

• High Availability: No single point of failure
• Scalability: Handle more traffic by adding servers
• Performance: Optimal resource utilization
• Flexibility: Maintenance without downtime

Types of Load Balancers

Layer 4 (Transport Layer)

Distributes traffic based on IP and TCP/UDP port info. Fastest, but no content awareness.

Layer 7 (Application Layer)

Can inspect HTTP headers, cookies, URL paths. More flexible routing decisions.

Algorithms

Round Robin

Distributes requests sequentially across servers:

Request 1 → Server A
Request 2 → Server B
Request 3 → Server C
Request 4 → Server A (cycle repeats)

Weighted Round Robin

Assigns more traffic to more powerful servers.

Least Connections

Routes to the server with the fewest active connections.

IP Hash

Routes based on client IP for session persistence.

Consistent Hashing

Used in distributed caching (e.g., Redis Cluster). Minimizes redistribution when adding/removing nodes.

Health Checks

Load balancers continuously monitor server health:

GET /health → 200 OK (Server is healthy)
GET /health → 503 (Server is unhealthy, remove from pool)

Real-World Example

Client → CDN → Load Balancer → [Server Pool]
                    ↓
              ┌─────┼─────┐
              ▼     ▼     ▼
           App-1  App-2  App-3
              │     │     │
              └─────┼─────┘
                    ▼
              Database (Primary)
                    │
              Database (Replica)

Popular Solutions

Solution	Type	Use Case
Nginx	Software L7	Web applications
HAProxy	Software L4/L7	High performance
AWS ALB	Cloud L7	AWS deployments
AWS NLB	Cloud L4	Ultra-low latency

Design Tip: Always design for failure. Use multiple load balancers in active-passive or active-active configurations.