Simplexity Quest

The mystery of Rails’ lib/ folder 📚


Update 2024-02-19: With the release of packwerk 3.2.0, the instructions in this article now work as originally intended again. The article has been simplified to reflect this.

In Ruby on Rails applications, one of the directories that come with the default structure is lib/. What is it for? How should it be used? And why should you care?

Railway tracks, a big Ruby, and a lot of books. Generated with GPT-4.

Used well, lib/ is a very powerful tool that can declutter an application, reduce cognitive load and improve developer productivity. But it’s often not used well.

The official Rails guides say it’s supposed to contain “extended modules for your application”. I don’t think that’s a useful definition. I’m not even sure what it means.

Another idea that I’ve heard often is that the actual application should live in lib/. The earliest source I could find is a 2011 blog post by Corey Haines:

Rails is not your application. It might be your views and data source, but it’s not your application. Put your app in a Gem or under lib/.

I’m sorry to be the bearer of bad news, but if you’re using Rails as intended, it is indeed an integral part of your application. That’s what distinguishes frameworks from libraries.

If you’re using libraries, you’re following the Hollywood Principle: Don’t call us, we’ll call you. You have control over the application. If you’re using a framework, the framework calls you. In other words, you’re just filling in the blanks in a pre-defined structure.

And it’s especially pronounced in Rails thanks to the Active Record pattern. Active Record is all about mixing data access logic (Rails!) with domain objects (your application!).

So, what is lib for?

You can divide the code in your application into two categories: code that is specific to your application’s core domain, and code that is more generic.

DHH, the creator of Rails, stated:

lib/ is intended to be for non-app specific library code that just happens to live in the app for now (usually pending extraction into open source or whatever). Everything app specific that’s part of the domain model should live in app/models (that directory is for POJOs as much as ARs).

Code that’s not specific to your application’s core domain may concern responsibilities like HTTP clients, authentication and authorization, (generic) serialization and deserialization, and so on. For these concerns, we usually don’t write the code ourselves - we use libraries, gems. Almost always those are third party gems from

But what if what we need is not available in a well maintained library? What if the functionality we need is so small that it doesn’t warrant introducing an external dependency? Maybe it’s something that we can easily write ourselves?

You may want the code to live in the same repository as your application, even though it’s not specific to your application’s domain. Luckily, Rails is prepared for this scenario. That’s why app/ and lib/ folders exist!

Library code within your repository is still library code. So it should go into lib/.

It’s right there in the name!

💡 lib/ is for libraries.


The main advantage of separating library code from your application instead of just lumping it in with all of your other stuff is the reduction of cognitive load. Ideally, you can look at the library code and understand it without having to know any of the details of your application. The less you as a developer need to know to do your work the faster you can get things done, delivering value to your users and checking items off your to-do list.

Robert Martin writes in Clean Architecture:

Software Architecture is the art of drawing lines that I call boundaries. Those boundaries separate software elements from one another, and restrict those on one side from knowing about those on the other.

Let’s do some Software Architecture! 🧑‍💼

If software element A doesn’t know about element B, that also means you can understand element A without having to understand element B. In practice, “A doesn’t know about B” means that A doesn’t have source code dependencies (AKA static dependencies) on B.

Remember, we want to be able to understand our libraries without having to understand or know any details of the application. Therefore, the code in lib/ should not have dependencies on the code in app/.

Dependency diagram - app/ depends on lib/, but the inverse is not true.

💡 The application depends on its libraries, but libraries never depend on the application.

Complication: Autoloading

In Rails versions before 7.1, lib/ is not autoloaded, which means that if you want to use code from lib/ in your application, you need to explicitly require it. While in theory this is a good thing because it makes a dependency explicit, requiring a file explicitly will still add its contents to the global namespace. That means requiring a file once, in one place, will make it available everywhere, and implicit dependencies on it will creep in. So the advantage of explicit dependencies doesn’t really exist in Rails apps in practice. Also, without autoloading, the code in lib/ will not be reloaded after making changes locally, which is inconvenient.

But you don’t want to autoload all of lib/ either. There’s likely a lot of code in there that your application doesn’t need to run in a production environment, like rake tasks. Autoloading it in development implies eagerloading in production, which would slow down application startup, use up additional memory, and potentially cause bugs in production.

One good workaround has been proposed by prominent Rails contributor and autoloading expert Xavier Noria:

The best practice […] is to move that code to app/lib. Only the Ruby code you want to reload, tasks or other auxiliary files are OK in lib.

Any folder under app/ will be autoloaded by default, including app/lib/.

In Rails 7.1, a new default was introduced to autoload lib/, specifically omitting the subfolders lib/assets/, lib/tasks/ and lib/generators/, which means you don’t need Xavier’s workaround anymore.

So, going forward I will assume that the library code your application relies on is autoloaded - either it is in app/lib/ or it is in lib/ and autoloaded via Rails 7.1’s config.autoload_lib.

💡 Your application libraries should be autoloaded.

Making it so

Because local libraries and application code are versioned and tested together, it is easy to accidentally break the boundary by introducing a dependency on the application to the code in lib/. Most of the time, this will be a call to a model class. In a small, well aligned team, you can probably avoid this erosion for a while. But as the team grows and the application ages, at some point boundary violations will creep in.

The problem with an incomplete boundary is that you can’t trust it. You can’t be sure that the code you’re looking at is independent of the application; you have to check every time, and your cognitive load increases. Incomplete boundaries will also erode more quickly.

But I have good news for you: Because your library code is autoloaded, you can use a nifty little tool called packwerk1 to enforce the boundary.

Enforcing your library boundary with packwerk is easy. Start with

bundle add packwerk --group "development, test"
bundle binstub packwerk
bin/packwerk init

As part of the initialization process, packwerk will place a package.yml file in the root folder of your application, which defines the root package. All your code that is not explicitly in another package is in the root package.

⚠️ In the following, replace lib/ with app/lib/ if you’re using the app/lib/ setup.

To enforce a boundary between application code and library code, we just drop another package.yml file into the lib/ folder:

# lib/package.yml

enforce_dependencies: true
dependencies: []

This tells packwerk that lib/ is a package, code inside of it should respect the declared dependencies, and we don’t want it to depend on any other package (not even the root package).

💡 Run bin/packwerk validate to validate packwerk’s configuration files, which encompass packwerk.yml and all of the package.ymls you may have.

Now, if we run bin/packwerk check, we get a list of all the violations of the boundary we just defined. Ideally, this list is empty. Maybe there are a few entries there that you can easily rectify. But more likely, you have a long list of violations that you’ll want to fix one by one.

To facilitate this, packwerk has a ratcheting mode where it accepts existing violations but complains about new ones. You can record existing violations by executing bin/packwerk update-todo. This will create to-do files for each package with violations. Check them into your version control system.

To get the most out of packwerk you will want to add a step to your CI system that executes bin/packwerk validate (to validate the configuration) and bin/packwerk check (to enforce the boundaries).


Rails’ lib/ folder is more than just a directory. It’s a manifestation of a fundamental software engineering principle. By understanding and applying the concept of architectural boundaries, developers can create more maintainable, scalable, and understandable applications.

For many Rails developers, this is a new skill to master, which can seem daunting. However, the lib/ folder is a great place to start, and packwerk can guide you on your way.

Screenshot from the 1986 video game The Legend of Zelda. An old man is depicted saying: “It’s dangerous to go alone! Take this.”, and we can see a sword in the middle of the screen.

Further Reading

  1. I developed the idea and core functionality of packwerk in 2020 during my time at Shopify, supported and inspired by a lot of very smart people around me. Together with the team we later polished the tool and open sourced it, and it has since found significant adoption in the Ruby on Rails community.↩︎