Docker deployment in Ruby on Rails - Performance & Optimization

Performance: Docker deployment

MEDIUM IMPACT

Docker deployment affects the initial page load speed and server response time by controlling how the Rails app and its dependencies are packaged and started.

Deploying a Rails app with Docker for fast startup and minimal resource use

Ruby on Rails

FROM ruby:3.1-slim
RUN apt-get update && apt-get install -y --no-install-recommends nodejs yarn
WORKDIR /app
COPY Gemfile Gemfile.lock ./
RUN bundle install --without development test
COPY . .
CMD ["rails", "server", "-b", "0.0.0.0"]

Uses slim base image, installs only needed packages, caches bundle install, excludes dev/test gems to reduce image size and speed startup.

📈 Performance GainReduces image size by 300MB, container starts 5 seconds faster

Deploying a Rails app with Docker for fast startup and minimal resource use

Ruby on Rails

FROM ruby:3.1
RUN apt-get update && apt-get install -y nodejs
COPY . /app
WORKDIR /app
RUN bundle install
CMD ["rails", "server", "-b", "0.0.0.0"]

Installs unnecessary packages at build time and copies entire source including development files, causing large image size and slow startup.

📉 Performance CostAdds 500MB+ to image size, blocks container start for 10+ seconds

Performance Comparison

Pattern	Image Size	Startup Time	Resource Usage	Verdict
Full Ruby image with all packages	Large (700MB+)	Slow (10+ sec)	High CPU & Memory	[X] Bad
Slim Ruby image with cached dependencies	Smaller (400MB)	Faster (5 sec)	Lower CPU & Memory	[OK] Good

Rendering Pipeline

Docker deployment impacts server startup and response time, which affects when the browser receives HTML to start rendering.

→Server Startup

→Network Response

→Browser Rendering

⚠️ BottleneckServer Startup time inside the container

Core Web Vital Affected

LCP

Docker deployment affects the initial page load speed and server response time by controlling how the Rails app and its dependencies are packaged and started.

Optimization Tips

1Use minimal base images to reduce Docker image size.

2Cache dependencies like gems to avoid reinstalling on every build.

3Exclude development and test dependencies from production images.

Performance Quiz - 3 Questions

What Docker deployment practice helps reduce Rails app startup time?

A Using a slim base image and caching bundle install

B Installing all development and test gems in the image

C Copying the entire source code before installing dependencies

D Running bundle install on every container start

DevTools: Network

How to check: Open DevTools Network tab, reload page, and observe Time to First Byte (TTFB) and content download times.

What to look for: Long TTFB indicates slow server response, which can be caused by slow Docker container startup.

Practice

1. What is the main purpose of using Docker for deploying a Rails application?

easy

A. To package the app and its environment together for consistent deployment

B. To write Rails code faster

C. To replace the Rails framework

D. To automatically fix bugs in the app

Solution

1. Step 1: Understand Docker's role

   Docker packages the app with all dependencies so it runs the same everywhere.

2. Step 2: Compare options

   Options B, C, and D describe unrelated or incorrect uses of Docker.

3. Final Answer:

   To package the app and its environment together for consistent deployment -> Option A

4. Quick Check:

   Docker deployment = packaging app + environment [OK]

Hint: Docker bundles app + environment for consistent runs [OK]

Common Mistakes:

• Thinking Docker speeds up coding
• Confusing Docker with Rails framework
• Believing Docker fixes app bugs automatically

2. Which line correctly exposes port 3000 in a Dockerfile for a Rails app?

easy

A. EXPOSE port 3000

B. EXPOSE '3000'

C. EXPOSE:3000

D. EXPOSE 3000

Solution

1. Step 1: Recall Dockerfile syntax for exposing ports

   The correct syntax is simply EXPOSE followed by the port number without quotes or extra words.

2. Step 2: Evaluate options

   EXPOSE 3000 matches the correct syntax. Options A, B, and D have invalid syntax.

3. Final Answer:

   EXPOSE 3000 -> Option D

4. Quick Check:

   Expose port syntax = EXPOSE 3000 [OK]

Hint: Use EXPOSE followed by port number without quotes [OK]

Common Mistakes:

• Adding quotes around port number
• Writing extra words like 'port'
• Using colon instead of space

3. Given this Dockerfile snippet for a Rails app:

FROM ruby:3.1
WORKDIR /app
COPY . .
RUN bundle install
EXPOSE 3000
CMD ["rails", "server", "-b", "0.0.0.0"]

What happens when you run this container and visit http://localhost:3000 on your host machine?

medium

A. You can access the Rails app running inside the container

B. The app will not be accessible because the port is not exposed

C. The app will crash because of missing files

D. The server will bind only to localhost inside the container

Solution

1. Step 1: Analyze Dockerfile commands

   The Dockerfile exposes port 3000 and runs Rails server bound to 0.0.0.0, allowing external access.

2. Step 2: Understand port exposure and binding

   Exposing port 3000 and binding to 0.0.0.0 lets the host access the app via localhost:3000.

3. Final Answer:

   You can access the Rails app running inside the container -> Option A

4. Quick Check:

   Expose + bind 0.0.0.0 = accessible app [OK]

Hint: Bind Rails to 0.0.0.0 and expose port to access app [OK]

Common Mistakes:

• Thinking EXPOSE alone publishes port to host
• Binding server only to localhost inside container
• Assuming missing files cause crash without evidence

4. You wrote this Dockerfile for your Rails app:

FROM ruby:3.1
WORKDIR /app
COPY . .
RUN bundle install
EXPOSE 3000
CMD ["rails", "server"]

Why might your app not be accessible from your host machine at http://localhost:3000?

medium

A. Because the port 3000 is not exposed in the Dockerfile

B. Because the Rails server is not bound to 0.0.0.0, so it listens only inside the container

C. Because the WORKDIR is incorrect

D. Because bundle install was not run

Solution

1. Step 1: Check server binding in CMD

   The CMD runs 'rails server' without '-b 0.0.0.0', so it binds to localhost inside container only.

2. Step 2: Understand port exposure and binding

   Even though port 3000 is exposed, binding only to localhost prevents external access from host machine.

3. Final Answer:

   Because the Rails server is not bound to 0.0.0.0, so it listens only inside the container -> Option B

4. Quick Check:

   Missing bind 0.0.0.0 = no host access [OK]

Hint: Always bind Rails server to 0.0.0.0 for external access [OK]

Common Mistakes:

• Assuming EXPOSE publishes port to host automatically
• Thinking WORKDIR affects network access
• Believing bundle install affects accessibility

5. You want to deploy a Rails app with Docker and ensure the app reloads code changes during development inside the container. Which Dockerfile and command setup is best?

hard

A. Copy code once in Dockerfile and run: rails server without binding

B. Expose port 3000 but do not bind Rails server to 0.0.0.0

C. Use a volume to mount your app code and run: rails server -b 0.0.0.0

D. Run Rails server with rails server -b localhost and no volume

Solution

1. Step 1: Understand code reload needs

   For code changes to reflect immediately, the app code must be mounted as a volume, not copied once.

2. Step 2: Ensure external access

   Binding Rails server to 0.0.0.0 allows access from host machine.

3. Step 3: Evaluate options

   Use a volume to mount your app code and run: rails server -b 0.0.0.0 mounts code and binds correctly. Others miss volume or binding, causing no reload or no access.

4. Final Answer:

   Use a volume to mount your app code and run: rails server -b 0.0.0.0 -> Option C

5. Quick Check:

   Volume + bind 0.0.0.0 = live reload + access [OK]

Hint: Mount code volume and bind server to 0.0.0.0 for dev [OK]

Common Mistakes:

• Copying code only once, missing live reload
• Not binding server to 0.0.0.0
• Binding server to localhost disables host access