Last modified on Wed 24 Feb 2021

Every dependency in your application has the potential to bloat your app, to destabilize your app, to inject odd behaviour via monkeypatching or buggy native code.

When you are considering adding a dependency to your Rails app, it’s a good idea to do a quick sanity check, in order of preference:

Do I really need this at all? Kill it.

Can I implement the required minimal functionality myself? Own it.

If you need a gem:

Does the gem have a native extension? Look for pure ruby alternatives.

Does the gem transitively pull in a lot of other gems? Look for simpler alternatives.

Gems with native extensions can destabilize your system; they can be the source of mysterious bugs and crashes. Avoid gems which pull in more dependencies than their value warrants.

-- Kill your dependencies

Why should you bother with updates?

Convincing clients that we need to spend time regularly updating dependencies is sometimes challenging. The application code already works and it doesn't need changing to keep functioning. Properly updating a dependency takes not only development time (research and execution) but also warrants testing of affected features on all environments. From the client's naive point of view there's no added benefit. It is our job to communicate the following points clearly and take care of overall project health.

Identifying outdated dependencies

The Ruby ecosystem is quite large and active, so being up to date on all your dependency changes can be a challenge. Manual options like checking the Ruby security mailing lists and Github repositiories have their place, but they require developers to put in extra time and effort. Luckily the community has developed some great automated tools to help you ease the load.

Bundle outdated

Bundler already includes a handy tool which can be used to get a list of outdated gems in your project.

    Outdated gems included in the bundle:
      * aws-partitions (newest 1.367.0, installed 1.338.0)
      * blueprinter (newest 0.25.1, installed 0.25.0) in groups "default"
      * brakeman (newest 4.9.1, installed 4.9.0) in groups "development"

You should have a periodic reminder in your calendar to run bundle outdated and triage the result. You can't always be on the edge, but you should at least be aware how far behind you are. This will help you estimate the time needed to update your dependencies and fit them in your schedule.


Running bundle-audit give you a list of all the dependencies in your project that contain publicly known security vulnerabilities. We mandate this check to be executed before each commit (using overcommit or lefthook). This gives developers a clear message -

Triage and deal with the problem immediately, then continue your work.

Once a vulnerability is made public and added to the Ruby Advisory Database you don't have much time to spare. By the time that you get the warning the vulnerability is probably already being exploited in the wild. Arrange with your project manager to get this sorted immediately.


Can you imagine having a co-worker who would check your dependencies for updates, summarise the changelogs and open a PR for each new one each week? You're in luck, Dependabot does exactly that.

To trigger Dependabot you will have to add a file named dependabot.yml to the .github directory, located in the root of the project. The configuration should loosely follow the following format

version: 2
  - package-ecosystem: "bundler"
    directory: "/"
      interval: "weekly"
    open-pull-requests-limit: 2

  - package-ecosystem: "npm"
    directory: "/"
      interval: "weekly"
    open-pull-requests-limit: 2

We recommend that you adjust the limit of weekly pull requests based on the circumstance of your project. Dependencies get updated rapidly, and you won't always have the time sift through all the updates in the same week.

Updating dependencies

Dependency upgrades should be treated no differently than a feature request. They should be reviewed, deployed to staging, checked, and then finally deployed to production and monitored. You will seldomly understand the workings of all your downstream dependencies, so you should in fact take even more care when reviewing dependency updates.

The key to reducing dependency upgrade risk is making small steps. You need to understand what's being changed and how it's going to affect your project.

It may take a bit more planning to pull off, but by finding a way to avoid disrupting the rest of the business, housekeeping activities like keeping your dependencies up-to-date can be done over the course of everyday development—hopefully helping the team avoid ever again falling several years behind on a major dependency like Rails.

-- 3 keys to upgrading Rails

The following steps can serve as a guide to help you on your path:

  1. Check the dependency policy regarding versioning. Is the dependency using Semantic versioning? What version number changes include breaking API changes?
  2. Survey the changelog for breaking changes. It is usually located in the CHANGELOG.md file, placed at the root level of the source code. Always go through all the intermediate steps between the current and future dependency version tag. You probably won't be familiar with all the phrases and terminology, so take your time. Be especially careful when the major version is bumped.
  3. When in doubt, look at the source code diffs. Commit messages and associated PRs usually tell most of the story, but you may still have to dig into the commit content sometimes.
  4. Check for any open issues related to the new version.
  5. Make an overview of all the places in the codebase that are affected by the dependency.
  6. Apply the required code changes and bump the dependency version (make sure to use the --conservative bundler flag to only update the necessary dependencies)
  7. Run tests and verify the behaviour locally.
  8. Identify & remedy deprecation warnings.
  9. Deploy to a live environment and have one of your colleagues check the behaviour. In most cases it would be too expensive to run a full project smoke test, so focus on the areas that are most likely to be affected by the dependency.
  10. If all goes well then deploy to production and monitor for regressions.

Updating Rails

Rails is probably one of your biggest project dependencies. While it may look like a big blob of magic, Rails is in essence a composition of multiple opinionated (omakase) frameworks. Each of the individual functionalities (object-relation mapping, view templating, email sending, ...) is handled by its own gem:

actioncable, actionmailbox, actionmailer, actionpack, actiontext, actionview, activejob, activemodel, activerecord, activestorage, activesupport

Upgrading Rails is thus much more challenging and tends to hurt because of the big surface area. You're essentially upgrading 11 gems at one time, which are themselves dependencies to a lot of your project's dependencies.

Your primary source of information should be the Rails upgrade docs. Read about the relevant changes once, then read again. An ounce of prevention is worth a pound of cure. Don't start playing error whack-a-mole without fully understanding the framework changes.

Important Rails upgrade tips

There are a couple of important notes you should bear in mind besides following all the steps in the Rails upgrade docs in the correct order.

Updating one minor version at a time

You should always update one minor version of Rails at a time. Each new minor version increment can introduce important deprecation warnings to warn you of changes in the next minor/major release. Skipping minor versions can lead to unexpected, hard to diagnose errors. It may seem like you're going slower one minor version at a time, but you'll be faster (and safer) in the long run. Minimising the complexity and risk of code changes is always the better option.

Configuration file changes

Updating Rails configuration files updated is sometimes tricky. The interactive update tool rails app:update is helpful enough to guide you through the process with well established options like overwrite, show diff and run merge tool. While this process will run smoothly for most apps, you will run into merge conflicts with apps that have a lot of custom edits in the configuration files. Approach each file change separately and make sure you do this step right. Missing a line could cause hours of troubleshooting. You should use Rails diff to get a better (cleaner) view at the before and after state of the configuration files.

Dual booting rails

Upgrading a major version of Rails is no small task. It usually takes a couple of days, and there is a big probability of unexpected problems cropping up. You may be using a dependency that hasn't been updated for the next Rails version yet. If you'll get unlucky you'll have to wait for a patch or even change your dependency. What should have been a just a short Rails upgrade could now turn into weeks. Your Rails upgrade branch was fresh at first, but now it's a month behind main and you're in merge hell. Long-lived Rails upgrade branches and feature freezes are no fun at all.

One of the proposed novel strategies for battling a long running upgrade is dual booting Rails. The idea is that your Rails application is bootable with a pair of Gemfile.lock dependency sets - the current and the next one. This allows you merge the necessary changes to the main branch one small PR at a time, as frequently as you would like!

Going down this road requires some more setup. There are a few gems like Next Rails and bootboot that can help you get started. The only thing you will need to add to your code base is a version condition helper, to achieve version specific behaviour -

if MyApplication.before_rails_6?

You will be able to keep track of your update progress with an additional CI check that will use the new dependencies with the existing codebase.