AWS Elastic Load Balancer for Developers

When your app starts to grow, one server won’t cut it. You need to balance traffic across multiple servers, handle traffic spikes, and stay online even if some parts fail. That’s where AWS Elastic Load Balancer (ELB) comes in.

What Is ELB?

Elastic Load Balancer (ELB) is a managed AWS service that automatically spreads incoming traffic across multiple targets like EC2 instances, containers, or IPs. Its a traffic controller for your webapps.

When to Use ELB:

Scale your app across multiple EC2 instances
Handle sudden surges in traffic
Increase fault tolerance and uptime

How ELB Works

Traffic Distribution: When a request comes to your app, ELB distributes it to healthy targets based on the routing rules and health checks.
Health Checks: It continuously checks if your targets are healthy, and only routes traffic to healthy instances.
Auto Scaling Integration: ELB integrates with Auto Scaling to automatically scale the number of instances in your pool as traffic increases or decreases.

Key Concepts in ELB (Quick Reference)

Concept	Description
Target Group	A group of targets (EC2 instances, Lambda functions, etc.) that the load balancer routes traffic to.
Listener	A process that checks for connection requests (e.g., HTTP on port 80).
Routing Rules	Determines how requests are forwarded based on conditions (e.g., host-based routing).
SSL Termination	ELB can handle the SSL handshake for secure communication, offloading the EC2 instances.

Types of Load Balancers

Type	Best For	Layer
Application Load Balancer (ALB)	HTTP/HTTPS apps with smart routing (e.g., path-based)	Layer 7
Network Load Balancer (NLB)	High performance TCP/UDP traffic	Layer 4
Classic Load Balancer (CLB)	Legacy or simple use cases	Layer 4 & 7

Scaling with ELB

Elasticity is the key benefit of ELB. Whether you are running a few EC2 instances or hundreds of them, ELB can handle massive fluctuations in traffic while maintaining high availability.

As traffic increases, the load balancer adds instances to the target group.
If traffic decreases, ELB gracefully removes instances.

ELB + Auto Scaling Group = True Elasticity - It lets you add or remove EC2 instances automatically based on demand and keep traffic flowing smoothly, even during heavy load or failure.

Setting Up an Application Load Balancer (ALB)

Step 1: Create a Target Group

Go to the EC2 > Target Groups section.
Choose targets (like EC2 instances).
Set a health check path (e.g., /health).

Step 2: Create the ALB

Choose Internet-facing (for public apps).
Add listeners (usually HTTP 80 or HTTPS 443).
Attach to at least two subnets in different availability zones.
Assign a security group to allow traffic.

Step 3: Register Targets

Add your EC2 instances to the target group.

Step 4: Test It!