As we mentioned many times, at FastRuby.io we like to use the Dual-Boot technique during upgrades to quickly test the same code with the current and the next version of what we are upgrading. We usually talk about dual-booting Rails versions but this can be used to upgrade Ruby itself too. We have to make some changes to adapt the technique, and we’ll explain the basic changes in this article.

Starting in Rails 7.1, Docker files are added by default in new applications, but Docker has been popular for Rails development for many years before that. At FastRuby.io, we use the Dual-Boot technique when we work on upgrades, and using that approach when an application uses Docker requires some extra steps to keep a great development experience.

You just finished upgrading your app to the latest Rails version and you made the decision to never stay behind and always be ready to upgrade as soon as possible… But how can you do that?

In this article we’ll explain how the Dual-Boot technique can be used to test the app against Rails’ main branch, to catch problems and warnings before a new version is released.

Gems are a central part in a Rails application, they help us add new functionality to our apps so we don’t have to reinvent the wheel, but also allows us to extract code to better organize the codebase and to share logic between multiple apps. In many cases, we have custom made gems, and we need to ensure they will work properly with the two Rails versions we run when we use the Dual Boot technique during upgrades. But… How do you dual boot the gems?

One of the steps we recommend taking when doing an upgrade for any Rails version is to dual boot the application with your current Rails version and your next rails version.

This is important because it allows you to quickly run the test suite for both versions, having dual booting available allows you to debug and also revert to your current version in a much simpler fashion.

However, sometimes changes that you make for the new version of Rails may not be compatible with your current version of Rails. This means that you will need to use a few different techniques to get both versions to be able to use the dual booting and run smoothly.

In this article I will explain how you can dual boot your application in your local environment and your continuous integration (CI) service. I hope that this will help you get ready for the next stable release of Rails.