Highly Available Multi-Tier Web Application on AWS

The problem

A single-server application is one failure away from an outage, and it keeps its database credentials and its database on the same public box.

Network

• A custom VPC on 10.0.0.0/16 spanning two Availability Zones.
• Public subnets carry only the load balancer and NAT. The web and data tiers sit in private subnets with no inbound route from the internet.
• Security groups chain rather than open: the load balancer reaches the web tier, and only the web tier reaches the database.

Application tier

• EC2 User Data bootstraps the LAMP stack and the application on first boot, so an instance joins the fleet with no manual step.
• The load balancer health check hits /health.html, so a half-booted instance never takes traffic.
• An Auto Scaling group with CPU target tracking adds and removes instances without intervention.

Data and access

• RDS MySQL in isolated subnets, with credentials held in AWS Secrets Manager instead of a config file.
• Administrative access runs through SSM Session Manager, so there is no SSH port and no key material to lose.

Decisions

Session Manager instead of a bastion host

It removes an internet-facing instance, the SSH port and the key rotation problem, and every session is logged.

Secrets Manager instead of environment files

The credential never lands in the AMI, the repository or the User Data script.

The result

• Losing an instance, or an entire Availability Zone, no longer takes the application down.
• The database is unreachable from the internet, and no credential is stored on disk.