Building Scalable Web Apps on AWS: My Hands-On Journey with ALBs, ASGs, and Custom VPCs

Posted at 5 AM, after another late-night deep dive into the cloud.

After learning from a painful mistake last night (read my previous post if you're curious), I promised myself to rebuild everything from scratch today, this time the right way.

And I did. I combined my learnings from the past two days: Application Load Balancers and Auto Scaling Groups into one working mini project.

🛠️ What I Built

To keep it structured, here’s what the setup looked like:

VPC Setup: Created a custom VPC with 3 public and 3 private subnets (spread across multiple Availability Zones for high availability).
Internet & NAT Gateways:
- Attached an Internet Gateway (IGW) to allow public access to public subnets.
- Deployed one NAT Gateway per public subnet to route internet traffic for the private subnets securely.
Routing & Security:
- Created Route Tables and associated them with respective subnets.
- Configured Security Groups for public-facing and internal components.
Flask App & Launch Template:
- Set up a simple Flask server running on port 8080.
- Created a custom AMI from this EC2 instance.
- Used it to define an EC2 Launch Template.
Target Group & Load Balancer:
- Created a Target Group pointing to instances in the private subnets on port 8080.
- Deployed an Application Load Balancer (ALB) in the public subnets and connected it to the target group.
Auto Scaling Group (ASG):
- Configured the ASG using the launch template and attached it to the target group.
- This enabled instances to scale in and out automatically based on demand.
Bastion Host for Debugging:
- Deployed a Bastion EC2 instance in a public subnet with a public IP.
- Used it to SSH into the private EC2 instances for validation and debugging.

✅ Final Result

The Application Load Balancer started successfully routing traffic to EC2 instances launched by the Auto Scaling Group. Each response was coming from a different instance depending on the load, a satisfying sight after a night of building.

🌙 Reflection

Compared to yesterday, this build felt cleaner and more intentional. Every service was tested step-by-step before stitching them together. A reminder that in the cloud (and in life), skipping fundamentals can cost you time, but every mistake brings clarity.