Scaling Spring Boot with Auto Scaling Group (ASG) + Elastic Load Balancer (ELB)

Deploying a Spring Boot app on multiple EC2 instances with an Application Load Balancer (ALB) works great ✅ But what happens when traffic spikes? Or if one EC2 instance crashes?

👉 That’s where Auto Scaling Groups (ASG) step in.

By the end of this guide, you’ll know how to:

• Create an AMI of your Spring Boot app
• Build a Launch Template
• Configure an Auto Scaling Group (ASG)
• Integrate it with your Application Load Balancer (ALB)
• Auto-scale your application 🚀

👉 Flow: Browser → ELB → ASG → EC2 Instances running Spring Boot

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🔑 Pre-requisite

From the previous blog, you should already have:

✅ 2 EC2 instances with Spring Boot JAR deployed on port 8080
✅ Application Load Balancer (ALB) setup with a Target Group

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📦 Step 1: Create an AMI (Golden Image)

We’ll use one existing EC2 as a base image (AMI).

1. Go to EC2 Dashboard → Instances
2. Select an EC2 instance where your Spring Boot app is running
3. From Actions → Image and Templates → Create Image
4. Name it: springboot-ami
5. Click Create AMI

⏳ Wait 8–10 minutes. This AMI will be the blueprint for new EC2s in the ASG.

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📝 Step 2: Create a Launch Template

Launch Template = Recipe to spin up new EC2s.

1. Go to EC2 → Auto Scaling Groups → Create Auto Scaling Group

2. Under Launch Template, click Create new launch template

3. Fill details:

   • Name: springboot-template
   • AMI: Select My AMIs → springboot-ami
   • Instance type: t3.micro
   • Key Pair: Same as before
   • Security Group: Must allow 22 (SSH), 80 (HTTP), 8080 (Spring Boot)

4. Add User Data (to auto-start the app):

#!/bin/bash
cd /home/ubuntu
sudo nohup java -jar app.jar > app.log 2>&1 &

5. Click Create Launch Template ✅

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⚙️ Step 3: Create Auto Scaling Group (ASG)

1. Back to Auto Scaling Group creation
2. Name: springboot-asg
3. Select the Launch Template you just created
4. Click Next

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🌐 Step 4: Configure Network Settings

• VPC → Default VPC
• Subnets → Select available ones (where ALB exists)
• Leave defaults → Next

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🔗 Step 5: Integrate with Load Balancer

1. On Load Balancer settings:

   • Attach to existing Load Balancer
   • Select your Spring Boot Target Group (8080)
   • Enable ALB health checks for reliability

2. Click Next

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📊 Step 6: Set Group Size & Scaling Policy

Define scaling behavior:

• Desired capacity → 2 (start with 2 instances)
• Minimum capacity → 1
• Maximum capacity → 3

Add a scaling policy:

• Policy type → Target tracking
• Metric → ALB Request Count per Target
• Target Group → springboot-tg
• Target Value → 50 (scale out if >50 requests per instance)

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✅ Step 7: Review & Create

• Double-check everything
• Click Create Auto Scaling Group

Your ASG is now active 🎉

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🔍 Step 8: Verify Setup

• Go to EC2 → Instances → You’ll see new instances launched by ASG
• Open ELB → Target Group → Targets → Should show healthy instances
• Try terminating one instance manually → ASG will auto-recreate it!

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🧪 Step 9: Load Test Your Setup

To simulate traffic for 5 mins:

ab -n 1000 -c 50 http://<ALB_DNS_NAME>/api/adapt/welcome

(Using ApacheBench or any load testing tool)

🎯 ASG will launch new EC2s automatically if traffic increases.

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🎯 Final Result

You’ve successfully:

✅ Created an AMI for your Spring Boot app
✅ Configured a Launch Template
✅ Built an Auto Scaling Group (ASG)
✅ Integrated ASG with your Load Balancer
✅ Enabled self-healing + scaling 🚀

Now your app can handle failures and traffic surges without manual effort 💪

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