← Back to Questions
AWS

Explain about Application Load Balancer ?

Learn Explain about Application Load Balancer ? with simple explanations, real-time examples, interview tips and practical use cases.

Updated: •

Application Load Balancer (ALB) is a modern AWS load balancing service designed for:

  • HTTP applications
  • HTTPS applications
  • Microservices
  • Containerized workloads
  • Cloud-native architectures
Simple Definition: Application Load Balancer (ALB) is a Layer 7 AWS load balancer that intelligently distributes HTTP and HTTPS traffic across multiple backend targets based on application-level routing rules.

Why ALB is Needed

Modern applications often contain:

  • Microservices
  • REST APIs
  • Containers
  • Multiple backend services

These applications require:

  • Intelligent traffic routing
  • Scalability
  • High availability
  • Fault tolerance

Without ALB

Users
   |
Single EC2 Instance
   |
Application Failure Risk

Problems:

  • Single point of failure
  • No scalability
  • Poor fault tolerance
  • Difficult traffic management

With ALB

                 Users
                    |
        Application Load Balancer
         /            |           \
     Service-A    Service-B    Service-C

ALB intelligently distributes traffic across backend services.

OSI Layer

Layer 7 — Application Layer

ALB understands:

  • HTTP requests
  • URLs
  • Headers
  • Hostnames
  • Query parameters

Main Responsibilities of ALB

Feature Purpose
Traffic Distribution Balance incoming requests
Path-Based Routing Route by URL path
Host-Based Routing Route by domain name
SSL Termination Handle HTTPS traffic
Health Checks Detect unhealthy targets

High-Level ALB Architecture

Internet Users
        |
Application Load Balancer
        |
------------------------------------------------
|                     |                        |
Target Group-A    Target Group-B         Target Group-C
|                     |                        |
EC2 / ECS         Microservices          Lambda Functions

How ALB Works Internally

1. Client sends HTTP/HTTPS request
2. DNS resolves ALB endpoint
3. ALB receives request
4. Listener evaluates routing rules
5. Request routed to target group
6. Healthy target processes request
7. Response returned to client

Core Components of ALB

  • Listener
  • Listener Rules
  • Target Groups
  • Health Checks
  • Availability Zones

1. Listener

Listener defines: 

Protocol + Port

Examples

HTTP : 80
HTTPS : 443

Architecture

Users
   |
Listener : HTTPS 443
   |
Routing Rules

2. Listener Rules

Rules define: 

How requests are routed

Types of Routing

  • Path-based routing
  • Host-based routing
  • Header-based routing
  • Query-string routing

3. Path-Based Routing

ALB can route requests based on URL paths.

Example

/api       → API Service
/orders    → Order Service
/payment   → Payment Service

Architecture

Users
   |
ALB
   |
------------------------------------------------
|                     |                        |
/api               /orders               /payment
 |                    |                      |
API Service       Order Service        Payment Service

Benefits

  • Microservices support
  • Centralized routing
  • Efficient traffic management

4. Host-Based Routing

Requests can be routed using hostnames.

Example

api.company.com      → API Service
admin.company.com    → Admin Portal
shop.company.com     → E-Commerce Service

Architecture

Users
   |
ALB
   |
------------------------------------------------
|                     |                        |
api.company.com   admin.company.com   shop.company.com

5. Target Groups

Target groups contain:

  • EC2 instances
  • ECS containers
  • IP addresses
  • Lambda functions

Architecture

ALB
 |
Target Group
 |
------------------------
|          |           |
EC2-1     EC2-2      EC2-3

6. Health Checks

ALB continuously checks: 

Backend target health

Example

GET /health

Health Check Flow

ALB
 |
Health Check
 |
Healthy Target → Traffic Allowed
Unhealthy Target → Traffic Blocked

Benefits

  • Automatic failure detection
  • Improved availability
  • Fault tolerance

ALB and Auto Scaling

ALB integrates with: 

Auto Scaling Groups (ASG)

Architecture

Users
   |
ALB
   |
Auto Scaling Group
   |
---------------------------------
|              |               |
EC2           EC2            EC2

Benefits

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

ALB with ECS and Kubernetes

ALB is widely used with:

  • Amazon ECS
  • EKS Kubernetes clusters

Kubernetes Architecture

Internet
    |
ALB Ingress Controller
    |
Kubernetes Services
    |
Pods

ALB with Lambda

ALB supports: 

Serverless architectures

Architecture

Users
   |
ALB
   |
Lambda Functions

SSL Termination

ALB can terminate: 

HTTPS SSL/TLS traffic

Benefits

  • Backend CPU optimization
  • Centralized certificate management
  • Simplified HTTPS handling

WebSocket Support

ALB supports: 

WebSocket connections

Use Cases

  • Chat applications
  • Real-time dashboards
  • Live notifications

ALB Security Features

  • AWS WAF integration
  • AWS Shield integration
  • Security groups
  • HTTPS support

Monitoring ALB

ALB integrates with:

  • CloudWatch
  • CloudTrail
  • AWS X-Ray

Important Metrics

  • Request count
  • Target response time
  • HTTP 5xx errors
  • Healthy host count

Production Best Practices

  • Use HTTPS everywhere
  • Enable multi-AZ deployment
  • Configure proper health checks
  • Use path-based routing for microservices
  • Enable access logs
  • Integrate with Auto Scaling

Common Production Problems

  • Misconfigured health checks
  • Incorrect routing rules
  • SSL certificate expiration
  • Backend timeout issues
  • Unhealthy targets

Production-Grade ALB Architecture

Internet Users
       |
CloudFront CDN
       |
AWS WAF
       |
Application Load Balancer
       |
------------------------------------------------
|                     |                        |
Microservice-A    Microservice-B         Microservice-C
       |
Auto Scaling Groups
       |
RDS / DynamoDB / S3

Advantages of ALB

  • Advanced Layer 7 routing
  • Excellent microservices support
  • Cloud-native integration
  • High availability
  • Managed infrastructure

Disadvantages of ALB

  • Supports only HTTP/HTTPS
  • Slightly higher latency than NLB
  • Complex routing configuration in large systems

Real-World Example

E-Commerce Platform

Users
   |
ALB
   |
------------------------------------------------
|                     |                        |
/products          /orders               /payments
 |                    |                      |
Product Service   Order Service        Payment Service

Interview Answer

Application Load Balancer (ALB) is a Layer 7 AWS load balancer designed for HTTP and HTTPS traffic.

It intelligently routes requests based on:

  • URL paths
  • Hostnames
  • Headers
  • Query strings

ALB is widely used in:

  • Microservices architectures
  • Containerized applications
  • Kubernetes environments
  • Modern cloud-native systems

ALB supports:

  • Target groups
  • Health checks
  • SSL termination
  • Auto Scaling integration
  • AWS WAF integration

Quick Summary Table

Feature ALB Capability
OSI Layer Layer 7
Protocols HTTP/HTTPS
Path Routing Yes
Host Routing Yes
Microservices Support Excellent
Container Support Excellent

Useful Internal Links

Final Conclusion

Application Load Balancer (ALB) is a critical AWS networking service used in modern cloud-native architectures.

It provides:

  • Intelligent Layer 7 routing
  • High availability
  • Microservices traffic management
  • Container orchestration integration
  • Scalable cloud networking

Modern production systems heavily rely on ALB for:

  • Microservices
  • Kubernetes ingress
  • REST APIs
  • Cloud-native applications

Understanding ALB deeply is essential for AWS architects, DevOps engineers, cloud engineers, and infrastructure professionals.

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.