Load Balancers

A single server is both a bottleneck and a single point of failure. A Load Balancer solves this by sitting in front of a fleet of targets (EC2 instances, containers, Lambda functions, IPs), distributing incoming traffic across them, and continuously health-checking targets so traffic never routes to an unhealthy one.

• An Elastic Load Balancer is a regional resource, but it operates by placing nodes into multiple AZs you select — you must enable at least one subnet per AZ you want it to serve.
• Application Load Balancer (ALB) operates at Layer 7 (HTTP/HTTPS) — routes by host, path, headers; ideal for microservices and container-based apps.
• Network Load Balancer (NLB) operates at Layer 4 (TCP/UDP/TLS) — ultra-low latency, millions of requests/sec, and supports a static IP per AZ.

graph LR
Internet(("Internet")) --> ALB["Application Load Balancer"]
ALB --> T1["Target: EC2 AZ-a"]
ALB --> T2["Target: EC2 AZ-b"]
ALB -.->|"health check"| T1
ALB -.->|"health check"| T2

Load balancers bill an hourly charge plus Load Balancer Capacity Units (LCUs) based on actual usage (new connections, active connections, bandwidth, and for ALBs, rule evaluations). There’s no per-target charge — cost scales with traffic, not fleet size.

• ALB understands HTTP semantics: path-based routing (/api/* vs /images/*), host-based routing, and native WebSocket/HTTP2 support — the default choice for modern web apps and microservices.
• NLB is the choice when you need a static IP (or Elastic IP) per AZ, extreme performance, or non-HTTP protocols — common exam trigger words are “static IP” and “millions of requests per second.”
• The older Classic Load Balancer (CLB) is legacy — the exam expects you to know it exists but to always prefer ALB/NLB for new designs.
• Load balancers require at least two AZs/subnets enabled to be considered highly available — a single-subnet ALB is a common exam “what’s wrong with this design” trap.
• A Gateway Load Balancer (GWLB) is a separate, third type used for deploying third-party virtual appliances (firewalls, IDS/IPS) transparently in the traffic path.