What Is AWS Load Balancer (ELB)?
AWS Elastic Load Balancing (ELB) is a managed service that automatically distributes incoming traffic across multiple resources such as EC2 instances, containers, IP addresses, or Lambda functions. It ensures applications are highly available, fault-tolerant, and scalable.
How It Works
- Round-robin: Distributes requests in a circular manner to each target
- Least connections: Routes to the target with fewest active connections
- Weighted distribution: Distributes traffic based on assigned weights
ELB includes health checks for each target, ensuring traffic is only sent to healthy resources.
Types of AWS Load Balancers
- Classic Load Balancer (CLB): Oldest version, supports HTTP/HTTPS and TCP at Layer 4 and Layer 7. Mainly for legacy applications
- Application Load Balancer (ALB): Operates at Layer 7, ideal for web apps and microservices. Supports URL-based routing, WebSocket, HTTP/2, and custom routing rules
- Network Load Balancer (NLB): Operates at Layer 4 for high-throughput, low-latency traffic. Handles TCP/UDP with static IP addresses
- Gateway Load Balancer (GLB): Routes traffic through third-party virtual appliances like firewalls and intrusion detection systems
Setting Up AWS Load Balancer
- Log in to AWS Console and navigate to EC2 dashboard
- Select Load Balancers under Load Balancing section
- Choose ALB or NLB based on traffic needs
- Configure VPC, availability zones, and listeners (HTTP/HTTPS/TCP)
- Create target groups (EC2, containers, IP, Lambda)
- Configure health checks for target groups
- Set security groups and SSL/TLS certificates
- Review and launch
Key Features
- Automatic Scaling: Adds more backend resources as traffic increases
- Health Checks: Monitors resource health, stops routing to unhealthy targets
- SSL/TLS Termination: Offloads encryption from backend servers
- Sticky Sessions: Routes user requests consistently to the same server
- Cross-Zone Load Balancing: Distributes traffic across multiple availability zones
AWS Service Integrations
- EC2 Instances: Register instances as targets for automatic traffic distribution
- Auto Scaling Groups: Launch new instances when traffic increases, load balancer routes automatically
- Amazon ECS/Kubernetes: Distribute traffic to containers based on defined rules
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Use Cases
- Web Applications: Handle varying traffic levels with high availability
- Real-Time Applications: Low-latency routing for gaming, IoT, and financial services
- Microservices: Traffic distribution to individual containers in ECS/EKS
- API Gateways: Even distribution across backend services during traffic spikes
Best Practices
- Configure SSL/TLS certificates for encrypted communication
- Always set up health checks to avoid unhealthy instance routing
- Monitor with Amazon CloudWatch and set up alarms
- Enable access logging to capture traffic details
Conclusion
AWS Load Balancer (ELB) is a powerful tool for ensuring high availability, scalability, and fault tolerance. By distributing traffic efficiently and integrating with AWS services like Auto Scaling, ECS, and CloudWatch, it manages traffic for applications of all sizes — from simple HTTP applications to sophisticated multi-tiered containerized architectures.
