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What is Elastic Load Balancer?

Learn What is Elastic Load Balancer? with simple explanations, real-time examples, interview tips and practical use cases.

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Elastic Load Balancer (ELB) is a fully managed AWS service that automatically distributes incoming application traffic across multiple targets such as:

  • EC2 instances
  • Containers
  • IP addresses
  • Lambda functions
Simple Definition: Elastic Load Balancer (ELB) is an AWS service that distributes traffic across multiple servers to improve: availability, scalability, fault tolerance, and application performance.

Why Load Balancers are Needed

Imagine an application running on a single EC2 instance. 

Users
   |
Single EC2 Instance

Problems:

  • Single point of failure
  • Cannot handle high traffic
  • Poor scalability
  • Downtime risk

Solution Using ELB

                Users
                   |
          Elastic Load Balancer
          /        |         \
      EC2-1     EC2-2      EC2-3

ELB distributes requests across multiple servers.

Main Responsibilities of ELB

Feature Purpose
Traffic Distribution Spread requests across targets
High Availability Reduce downtime
Health Checks Detect unhealthy instances
Auto Scaling Integration Support dynamic scaling
SSL Termination Handle HTTPS traffic

How ELB Works Internally

Client Request
       |
DNS Resolution
       |
Elastic Load Balancer
       |
Health Check Validation
       |
Forward Request to Healthy Target
       |
Application Response

Core Components of ELB

  • Listener
  • Target Group
  • Health Checks
  • Routing Rules
  • Availability Zones

1. Listener

Listener defines: 

Protocol + Port

Examples

HTTP : 80
HTTPS : 443
TCP : 3306

2. Target Group

A target group contains backend targets:

  • EC2 instances
  • Containers
  • IP addresses
  • Lambda functions

Architecture

Load Balancer
      |
Target Group
      |
-------------------
|        |        |
EC2-1   EC2-2   EC2-3

3. Health Checks

ELB continuously checks: 

Whether backend servers are healthy

Example

GET /health

Health Check Flow

ELB
 |
Health Check
 |
Healthy Instance → Traffic Sent
Unhealthy Instance → Traffic Stopped

Benefits

  • Automatic failure handling
  • Improved availability

Types of Elastic Load Balancers

Type Layer Use Case
Application Load Balancer (ALB) Layer 7 HTTP/HTTPS applications
Network Load Balancer (NLB) Layer 4 High-performance TCP/UDP traffic
Gateway Load Balancer (GWLB) Layer 3/4 Security appliances
Classic Load Balancer (CLB) Legacy Older AWS applications

1. Application Load Balancer (ALB)

ALB works at: 

OSI Layer 7 (Application Layer)

Supports

  • HTTP
  • HTTPS
  • Path-based routing
  • Host-based routing
  • Microservices

Architecture

Users
   |
ALB
   |
---------------------------------
|               |               |
/api         /payment       /orders
 |               |               |
Service-A     Service-B     Service-C

Use Cases

  • Microservices
  • Web applications
  • Kubernetes ingress

2. Network Load Balancer (NLB)

NLB works at: 

OSI Layer 4 (Transport Layer)

Supports

  • TCP
  • UDP
  • TLS

Features

  • Ultra-low latency
  • Millions of requests per second
  • Static IP support

Use Cases

  • Gaming
  • Real-time systems
  • Financial applications

3. Gateway Load Balancer (GWLB)

GWLB is used for:

  • Firewalls
  • Intrusion detection systems
  • Security appliances

Architecture

Internet Traffic
       |
Gateway Load Balancer
       |
Security Appliances
       |
Application Servers

4. Classic Load Balancer (CLB)

Legacy load balancer supporting:

  • Basic HTTP
  • TCP traffic

AWS recommends: 

ALB or NLB instead of CLB

Elastic Load Balancer and Auto Scaling

ELB integrates tightly with: 

Auto Scaling Groups (ASG)

Architecture

Users
   |
ELB
   |
Auto Scaling Group
   |
---------------------
|        |          |
EC2      EC2       EC2

Benefits

  • Automatic traffic balancing
  • Automatic scaling
  • Self-healing infrastructure

ELB Across Multiple Availability Zones

ELB supports: 

Multi-AZ architecture

Architecture

                 ELB
               /     \
         AZ-1         AZ-2
        /   \         /   \
     EC2   EC2     EC2   EC2

Benefits

  • High availability
  • Disaster resilience
  • Fault tolerance

SSL Termination

ELB can handle: 

HTTPS encryption/decryption

Benefits

  • Reduced backend CPU usage
  • Simplified certificate management

Sticky Sessions

ELB supports: 

Session persistence

Same user requests go to same backend server.

Use Cases

  • Legacy session-based applications

ELB Monitoring

ELB integrates with:

  • CloudWatch
  • CloudTrail
  • AWS WAF

Important Metrics

  • Request count
  • Latency
  • Healthy host count
  • HTTP 5xx errors

Security Features

  • SSL/TLS support
  • AWS WAF integration
  • DDoS protection using AWS Shield
  • Security group integration

Production Best Practices

  • Use ALB for web applications
  • Enable multi-AZ deployment
  • Configure health checks properly
  • Use HTTPS everywhere
  • Enable access logs
  • Monitor CloudWatch metrics

Common Production Problems

  • Improper health check configuration
  • Backend timeout issues
  • SSL certificate expiration
  • High latency
  • Unhealthy targets

Real-World Production Architecture

Internet Users
       |
CloudFront CDN
       |
Application Load Balancer
       |
Auto Scaling Group
       |
Microservices / EC2 / Containers
       |
RDS Database

Advantages of ELB

  • High availability
  • Automatic scaling
  • Fault tolerance
  • Managed infrastructure
  • Improved performance

Disadvantages

  • Additional cost
  • Configuration complexity
  • Latency overhead in some cases

Interview Answer

Elastic Load Balancer (ELB) is an AWS managed service that automatically distributes incoming traffic across multiple backend targets such as EC2 instances, containers, IP addresses, or Lambda functions.

ELB improves:

  • Availability
  • Scalability
  • Fault tolerance
  • Application performance

AWS provides multiple ELB types:

  • Application Load Balancer (ALB)
  • Network Load Balancer (NLB)
  • Gateway Load Balancer (GWLB)
  • Classic Load Balancer (CLB)

ELB also supports:

  • Health checks
  • SSL termination
  • Auto Scaling integration
  • Multi-AZ deployments

Quick Summary Table

ELB Type Layer Main Use Case
ALB Layer 7 Web applications
NLB Layer 4 High-performance TCP/UDP
GWLB Layer 3/4 Security appliances
CLB Legacy Older applications

Useful Internal Links

Final Conclusion

Elastic Load Balancer (ELB) is a critical AWS networking service used to build:

  • Highly available applications
  • Scalable cloud architectures
  • Fault-tolerant systems

ELB automatically distributes traffic, removes unhealthy instances, integrates with Auto Scaling, and supports modern cloud-native architectures.

Modern production AWS environments heavily rely on:

  • ALB for microservices
  • NLB for ultra-high performance
  • GWLB for security architectures

Understanding ELB deeply is essential for AWS architects, DevOps engineers, cloud engineers, and system administrators.

Why this AWS question is important?

This interview question helps candidates understand real-time backend development concepts, practical problem solving, coding fundamentals, system design basics and production-ready application behavior.

Practice this question carefully for Java backend roles, Spring Boot developer interviews, microservices interviews, company interviews and full-stack developer preparation.

About the Author

Naresh Kumar is a Senior Java Backend Engineer with experience building enterprise applications using Java, Spring Boot, Microservices, Docker, Kubernetes and Cloud technologies.