Ismail HakimMaret 2025

Remote Debugging Ruby Sinatra Application using Docker and VSCode

Ruby might not always be the loudest voice in the room these days, but it’s still a solid favorite for developers who love clean syntax and rapid development. As of 2025, Ruby ranks in the top 20 most loved languages according to the Stack Overflow Developer Survey — proof that its charm hasn’t worn off. And while Rails often steals the spotlight, Sinatra continues to quietly power thousands of lightweight apps, APIs, and internal tools around the world.

Simple, fast, and incredibly flexible, Sinatra is still a go-to for developers who want to build web apps without all the extra noise. Whether you’re prototyping an idea, building a microservice, or just learning the ropes of web development in Ruby, Sinatra makes it easy to get started.

In this blog, we’re diving into a key skill every Ruby (and Sinatra) developer should have in their toolkit: debugging. Let’s set up rdbg and walk through how to trace, break, and inspect your Ruby code like a pro — even in a minimal Sinatra app.

Quick Break

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Firing Up VSCode for Ruby Remote Debugging

On our successful remote debugging, we used VSCode rdbg Ruby Debugger by Koichi Sasada (https://marketplace.visualstudio.com/items?itemName=KoichiSasada.vscode-rdbg).

To connect our remote debugger in VSCode to the debugee inside the docker container, we can configure .vscode/launch.json file.

{
  "version": "0.2.0",
  "configurations": [
    {
      "name": "Remote Debug Sinatra",
      "type": "rdbg",
      "request": "attach",
      "debugPort": "localhost:12345",
      "localfsMap": "/app/:${workspaceFolder}/app/",
    }
  ]
}

Hovering these variables in VSCode, will help you to restructure key:value required. VSCode will also mark incorrect key:value combinations during your restructuring effort.

`rdbg` For Remote Debugging

First of all, make sure all of your code is in the testing environment. Then, you can include this line inside Gemfile, or install it on the testing environment.

gem 'debug'

To execute remote debugging, you can utilize rdbg. It is the built-in debugger for Ruby, introduced in Ruby 3.1. It gives you a powerful, interactive way to inspect and control your Ruby code while it’s running.

# inside Dockerfile

RUN rdbg --open --host "0.0.0.0" --port 12345 -- /your/ruby/file.rb

EXPOSE 12345

Finally you can build and run the docker container. Don’t forget to expose the debugging port.

docker build -t sinatra_debug . 
docker run -p 80:80 -p 12345:12345 -it sinatra_debug

Testing

Remote debugging testing.

Our remote debugger has successfully set a breakpoint.

Source Code Navigation.

The variables are clearly visible to us.

Closing

Debugging might not be the flashiest part of development, but it’s where the real understanding happens. With `rdbg` in your toolkit, you’ve got a powerful way to dig into your Ruby or Sinatra app, fix bugs faster, and even spot potential security issues before they become problems. Whether you’re building something new or hunting down a weird edge case, solid debugging skills will always have your back.

Happy coding — and happy bug hunting!

Remote Debugging Ruby Sinatra Application using Docker and VSCode was originally published in Cyberkarta on Medium, where people are continuing the conversation by highlighting and responding to this story.