Debugging Rails App With Docker Compose

How to use Byebug in a dockerized rails app

Photo by Charles Deluvio on Unsplash

Context

Have you ever faced an issue where you want to debug a live session, went ahead and attach a byebug in the code, the app just gave up and paused itself until the end of time unless you restart the server?

Step 1: Setup your environment

• Install Docker on your system.
• Feel free to use my minimal app if you don’t have one already once Docker is installed.
• You can go straight to Step 4 if you are using the minimal app.

Step 2: Modify docker compose file

• Add this to your docker-compose.yml under networks in web service:

web:
  ...
  stdin_open: true
  tty: true

What this does is allowing us to start an interactive shell in a container using docker compose so we are able to enter byebug commands such as continue , step and frame .

Step 3: Create Database Instances

You’ll need to create databases inside of your container to be able to visit any API:

• Spin up database and rails app in the container in detached mode so we can progress in the same terminal session: docker-compose up -d
• Go to the app running inside our container: docker-compose exec web bash
• Create databases for our app: rails db:create
• You can exit from the bash session once the database is created by simply typing exit

Step 4: Attach your terminal to the container

Attach your terminal to the container: docker attach [container_id]

You may find your container_id by using: docker container ls

At this point you should be able to see every input, output and errors from our running container.

Let’s try to attach a byebug in your codebase for instance:

minimal-rails-app/app/controllers/health_controller.rb

Then you can try visiting the API (or http://127.0.0.1/health/check in my example)

Your app should pause when the program execution reach the byebug line and now you can enter byebug commands in the attached docker process:

/health/check API example

You can always detach from this Docker process by entering the escape sequence: Ctrl+P then Ctrl+Q.

Summary

This is a slick solution for someone who prefer to debug a live session even though I always debug my app via test cases with RSpec so I don’t have to worry about overheads such as request timeout and mocking failure response from a certain module that the API is depending on all that kind of stuff.