Introduction

A Rails-based EC2 AMI is a custom machine image that already contains the operating system, Ruby runtime, application dependencies, and server tooling needed to boot a Rails instance quickly. The value of the AMI is consistency: every EC2 instance launched from it starts from the same known base instead of reinstalling everything on each deployment.

What to Bake Into the AMI

A practical Rails AMI usually includes:

• a patched base operating system
• Ruby and Bundler
• system packages such as git, nginx, and database client libraries
• an app server such as Puma
• a systemd unit or startup script
• log and temp directories with correct permissions

What you usually should not bake into the AMI:

• production secrets
• mutable user uploads
• environment-specific database credentials

Those belong in a secret manager, instance profile, or runtime configuration.

A Typical Build Workflow

The usual flow is:

1. launch a clean EC2 instance
2. install Ruby, Bundler, Node or JavaScript runtime pieces, and system packages
3. deploy the Rails application or a release artifact
4. verify that the app boots correctly
5. create an AMI from that verified instance

On the instance, a simplified setup might look like this:

1sudo apt-get update
2sudo apt-get install -y ruby-full build-essential nginx libpq-dev
3bundle config set deployment true
4bundle install
5RAILS_ENV=production bundle exec rails assets:precompile

The exact package commands depend on the base image, but the shape is the same.

Make Boot Behavior Explicit

A custom AMI is only useful if instances launched from it know how to start the app. A systemd service is a common way to do that.

1[Unit]
2Description=Puma for my Rails app
3After=network.target
4
5[Service]
6WorkingDirectory=/srv/myapp/current
7ExecStart=/usr/local/bin/bundle exec puma -C config/puma.rb
8Restart=always
9Environment=RAILS_ENV=production
10
11[Install]
12WantedBy=multi-user.target

That gives each instance a repeatable startup path after boot.

Bake the App or Pull It on Boot

There are two common strategies.

Bake the app into the AMI:

• fastest startup
• highly repeatable
• requires a new AMI for each application release

Bake only the runtime into the AMI and pull the app on boot:

• slower startup
• easier frequent app releases
• more moving parts during instance launch

For autoscaling groups that need predictable warm-up, baking the app into the AMI is often worth the extra image-management work.

Test the Image Before Trusting It

After creating the AMI, launch a fresh instance from it and verify:

• the app server starts automatically
• environment variables are injected correctly
• reverse proxy configuration works
• the app can reach the database, cache, and other dependencies
• logs are written where you expect

This test instance is where you catch mistakes such as hard-coded hostnames, missing migrations, or wrong file permissions.

Common Pitfalls

The most common mistake is storing secrets directly inside the image. That makes rotation and environment isolation harder.

Another issue is baking mutable data such as uploads into the AMI. Machine images are for immutable system state, not changing application content.

A third pitfall is creating the AMI before startup scripts, permissions, and service definitions are fully tested on a fresh boot.

Summary

• A Rails EC2 AMI gives you a repeatable server base for launching Rails instances.
• Bake the operating system, runtime, dependencies, and startup logic into the image.
• Keep secrets and mutable data out of the AMI.
• Decide whether to bake the app itself into the image or pull it during boot.
• Always test a new instance launched from the AMI before using it in production.