Load Balancing

// L4 load balancer — TCP level
// Client → LB (picks backend by IP:port) → Backend
// Fast, simple, but no content awareness

// L7 load balancer — HTTP level (NGINX example)
upstream api_servers {
    least_conn;                    # algorithm
    server api1.internal:8080;
    server api2.internal:8080;
    server api3.internal:8080;
}

upstream static_servers {
    server cdn1.internal:80;
    server cdn2.internal:80;
}

server {
    listen 443 ssl;

    # Path-based routing (L7 feature)
    location /api/ {
        proxy_pass http://api_servers;
    }
    location /static/ {
        proxy_pass http://static_servers;
    }
}

// Algorithm comparison
//
// Algorithm           | Best For                    | Weakness
// ──────────────────────────────────────────────────────────────
// Round Robin         | Equal servers, short reqs   | Ignores server load
// Weighted Round Robin| Mixed-capacity servers      | Static weights
// Least Connections   | Varying request durations   | Slightly more overhead
// IP Hash             | Session affinity needed     | Uneven if few clients
// Consistent Hashing  | Caches, sharded services    | More complex

// AWS ALB target group with health check
resource "aws_lb_target_group" "api" {
  name     = "api-targets"
  port     = 8080
  protocol = "HTTP"
  vpc_id   = var.vpc_id

  health_check {
    path                = "/health"
    interval            = 15
    timeout             = 5
    healthy_threshold   = 2
    unhealthy_threshold = 3
  }
}

// Multi-tier load balancing
//
//  Users (worldwide)
//    │
//    ▼
// DNS Load Balancer (Route 53 — latency-based routing)
//    │                    │
//    ▼                    ▼
// US-East Region       EU-West Region
//    │                    │
//    ▼                    ▼
// ALB (L7)             ALB (L7)
//  ┌─┼─┐               ┌─┼─┐
//  ▼ ▼ ▼               ▼ ▼ ▼
// App App App         App App App

// Route 53 latency-based record
resource "aws_route53_record" "api" {
  zone_id        = var.zone_id
  name           = "api.example.com"
  type           = "A"
  set_identifier = "us-east-1"
  latency_routing_policy {
    region = "us-east-1"
  }
  alias {
    name    = aws_lb.us_east.dns_name
    zone_id = aws_lb.us_east.zone_id
  }
}