Gems development guidelines

GitLab uses Gems as a tool to improve code reusability and modularity in a monolithic codebase.

We extract libraries from our codebase when their functionality is highly isolated and we want to use them in other applications ourselves or we think it would benefit the wider community.

Extracting code to a gem also ensures that the gem does not contain any hidden dependencies on our application code.

Gems should always be used when implementing functionality that can be considered isolated, that are decoupled from the business logic of GitLab and can be developed separately.

The best place in a Rails codebase with opportunities to extract new gems is the lib/ folder.

Our lib/ folder is a mix of code that is generic/universal, GitLab-specific, and tightly integrated with the rest of the codebase.

In order to decide whether to extract part of the codebase as a Gem, ask yourself the following questions:

Is this code generic or universal that can be done as a separate and small project?
Do I expect it to be used internally outside of the Monolith?
Is this useful for the wider community that we should consider releasing as a separate component?

If the answer is Yes for any of the questions above, you should strongly consider creating a new Gem.

You can always start by creating a new Gem in the same repository and later evaluate whether to migrate it to a separate repository, when it is intended to be used by a wider community.

WARNING: To prevent malicious actors from name-squatting the extracted Gems, follow the instructions to reserve a gem name.

Advantages of using Gems

Using Gems can provide several benefits for code maintenance:

Code Reusability - Gems are isolated libraries that serve single purpose. When using Gems, a common functions can be isolated in a simple package, that is well documented, tested, and re-used in different applications.
Modularity - Gems help to create isolation by encapsulating specific functionality within self-contained library. This helps to better organize code, better define who is owner of a given module, makes it easier to maintain or update specific gems.
Small - Gems by design due to implementing isolated set of functions are small. Small projects are much easier to comprehend, extend and maintain.
Testing - Using Gems since they are small makes much faster to run all tests, or be very through with testing of the gem. Since the gem is packaged, not changed too often, it also allows us to run those tests less frequently improving CI testing time.

Gem naming

Gems can fall under three different case:

unique_gem: Don't include gitlab in the gem name if the gem doesn't include anything specific to GitLab
existing_gem-gitlab: When you fork and modify/extend a publicly available gem, add the -gitlab suffix, according to Rubygems' convention
gitlab-unique_gem: Include a gitlab- prefix to gems that are only useful in the context of GitLab projects.

Examples of existing gems:

y-rb: Ruby bindings for yrs. Yrs "wires" is a Rust port of the Yjs framework.
activerecord-gitlab: Adds GitLab-specific patches to the activerecord public gem.
gitlab-rspec and gitlab-utils: GitLab-specific set of classes to help in a particular context, or re-use code.

In the same repo

When extracting Gems from existing codebase, put them in gems/ of the GitLab monorepo

That gives us the advantages of gems (modular code, quicker to run tests in development). and prevents complexity (coordinating changes across repositories, new permissions, multiple projects, etc.).

Gems stored in the same repository should be referenced in Gemfile with the path: syntax.

WARNING: To prevent malicious actors from name-squatting the extracted Gems, follow the instructions to reserve a gem name.

Create and use a new Gem

You can see example adding a new gem: !121676.

Pick a good name for the gem, by following the Gem naming convention.
Create the new gem in gems/<name-of-gem> with bundle gem gems/<name-of-gem> --no-exe --no-coc --no-ext --no-mit.
Remove the .git folder in gems/<name-of-gem> with rm -rf gems/<name-of-gem>/.git.
Edit gems/<name-of-gem>/README.md to provide a simple description of the Gem.

Edit gems/<name-of-gem>/<name-of-gem>.gemspec and fill the details about the Gem as in the following example:

# frozen_string_literal: true

require_relative "lib/name/of/gem/version"

Gem::Specification.new do |spec|
  spec.name = "<name-of-gem>"
  spec.version = Name::Of::Gem::Version::VERSION
  spec.authors = ["group::tenant-scale"]
  spec.email = ["engineering@gitlab.com"]

  spec.summary = "Gem summary"
  spec.description = "A more descriptive text about what the gem is doing."
  spec.homepage = "https://gitlab.com/gitlab-org/gitlab/-/tree/master/gems/<name-of-gem>"
  spec.license = "MIT"
  spec.required_ruby_version = ">= 3.0"
  spec.metadata["rubygems_mfa_required"] = "true"

  spec.files = Dir['lib/**/*.rb']
  spec.require_paths = ["lib"]
end

Update gems/<name-of-gem>/.rubocop.yml with:
```
inherit_from:
  - ../config/rubocop.yml
```

Configure CI for a newly added Gem:

Add gems/<name-of-gem>/.gitlab-ci.yml:

include:
  - local: gems/gem.gitlab-ci.yml
    inputs:
      gem_name: "<name-of-gem>"

To .gitlab/ci/gitlab-gems.gitlab-ci.yml add:

include:
  - local: .gitlab/ci/templates/gem.gitlab-ci.yml
    inputs:
      gem_name: "<name-of-gem>"

Reference Gem in Gemfile with:

gem '<name-of-gem>', path: 'gems/<name-of-gem>'

Specifying dependencies for the Gem

It is important to note that while the gem has its own Gemfile, in the actual application the top-level Gemfile for the monolith GitLab will be used instead of the individual Gemfile sitting in the directory of the gem.

This means we should be aware that the Gemfile for the gem should not use any versions of dependencies which might be conflicting with the top-level Gemfile, and we should try to use the same dependencies if possible.

An example of this is Rack. If the monolith is using Rack 2 and we're in the process of upgrading to Rack 3, all gems we develop should also be tested against Rack 2, optionally also with Rack 3 if a separate Gemfile is used in CI. See an example here.

Note that this does not limit to just Rack, but any dependencies.

Examples of Gem extractions

The gitlab-utils is a Gem containing as of set of class that implement common intrinsic functions used by GitLab developers, like strong_memoize or Gitlab::Utils.to_boolean.

The gitlab-database-schema-migrations is a potential Gem containing our extensions to Rails framework improving how database migrations are stored in repository. This builds on top of Rails and is not specific to GitLab the application, and could be generally used for other projects or potentially be upstreamed.

The gitlab-database-load-balancing similar to previous is a potential Gem to implement GitLab specific load balancing to Rails database handling. Since this is rather complex and highly specific code maintaining its complexity in a isolated and well tested Gem would help with removing this complexity from a big monolithic codebase.

The gitlab-flipper is another potential Gem implementing all our custom extensions to support feature flags in a codebase. Over-time the monolithic codebase did grow with the check for feature flags usage, adding consistency checks and various helpers to track owners of feature flags added. This is not really part of GitLab business logic and could be used to better track our implementation of Flipper and possibly much easier change it to dogfood GitLab Feature Flags.

The activerecord-gitlab is a gem adding GitLab specific Active Record patches. It is very well desired for such to be managed separately to isolate complexity.

Other potential use cases

The gitlab-ci-config is a potential Gem containing all our CI code used to parse .gitlab-ci.yml. This code is today lightly interlocked with GitLab the application due to lack of proper abstractions. However, moving this to dedicated Gem could allow us to build various adapters to handle integration with GitLab the application. The interface would for example define an adapter to resolve includes:. Once we would have a gitlab-ci-config Gem it could be used within GitLab and outside of GitLab Rails and GitLab CLI.

In the external repo

In general, we want to think carefully before doing this as there are severe disadvantages.

Gems stored in the external repository MUST be referenced in Gemfile with version syntax. They MUST be always published to RubyGems.

Examples

At GitLab we use a number of external gems:

Potential disadvantages

Gems - even those maintained by GitLab - do not necessarily go through the same code review process as the main Rails application. This is particularly critical for Application Security.
Requires setting up CI/CD from scratch, including tools like Danger that support consistent code review standards.
Extracting the code into a separate project means that we need a minimum of two merge requests to change functionality: one in the gem to make the functional change, and one in the Rails app to bump the version.
Integration with gitlab-rails requiring a second MR means integration problems may be discovered late.
With a smaller pool of reviewers and maintainers compared to gitlab-rails, it may take longer to get code reviewed and the impact of "bus factor" increases.
Inconsistent workflows for how a new gem version is released. It is currently at the discretion of library maintainers to decide how it works.
Promotes knowledge silos because code has less visibility and exposure than gitlab-rails.
We have a well defined process for promoting GitLab reviewers to maintainers. This is not true for extracted libraries, increasing the risk of lowering the bar for code reviews, and increasing the risk of shipping a change.
Our needs for our own usage of the gem may not align with the wider community's needs. In general, if we are not using the latest version of our own gem, that might be a warning sign.

Potential advantages

Faster feedback loops, since CI/CD runs against smaller repositories.
Ability to expose the project to the wider community and benefit from external contributions.
Repository owners are most likely the best audience to review a change, which reduces the necessity of finding the right reviewers in gitlab-rails.

Create and publish a Ruby gem

The project for a new Gem should always be created in gitlab-org/ruby/gems namespace:

Determine a suitable name for the gem. If it's a GitLab-owned gem, prefix the gem name with gitlab-. For example, gitlab-sidekiq-fetcher.
Locally create the gem or fork as necessary.
Publish an empty 0.0.1 version of the gem to rubygems.org to ensure the gem name is reserved.
Add the gitlab_rubygems user as owner of the new gem by running:
```
gem owner <gem-name> --add gitlab_rubygems
```
Optional. Add some or all of the following users as co-owners:
Optional. Add any other relevant developers as co-owners.
Visit https://rubygems.org/gems/<gem-name> and verify that the gem was published successfully and gitlab_rubygems is also an owner.
Create a project in the gitlab-org/ruby/gems group (or in a subgroup of it):
1. Follow the instructions for new projects.
2. Follow the instructions for setting up a CI/CD configuration.
3. Use the gem-release CI component to release and publish new gem versions by adding the following to their .gitlab-ci.yml:
```
include:
  - component: gitlab.com/gitlab-org/components/gem-release/gem-release@~latest
```
  This job will handle building and publishing the gem (it uses a gitlab_rubygems Rubygems.org API token inherited from the gitlab-org/ruby/gems group, in order to publish the gem package), as well as creating the tag, release and populating its release notes by using the Generate changelog data API endpoint.
  
  For instructions for when and how to generate a changelog entry file, see the dedicated Changelog entries page. To be consistent with the GitLab project, Gem projects could also define a changelog YAML configuration file at .gitlab/changelog_config.yml with the same content as in the gitlab-styles gem.
4. To ease the release process, you could also create a .gitlab/merge_request_templates/Release.md MR template with the same content as in the gitlab-styles gem (make sure to replace gitlab-styles with the actual gem name).
5. Follow the instructions for publishing a project.

Notes: In some cases we may want to move a gem to its own namespace. Some examples might be that it will naturally have more than one project (say, something that has plugins as separate libraries), or that we expect users outside GitLab to be maintainers on this project as well as GitLab team members. The latter situation (maintainers from outside GitLab) could also apply if someone who currently works at GitLab wants to maintain the gem beyond their time working at GitLab.

The `vendor/gems/`

The purpose of vendor/ is to pull into GitLab monorepo external dependencies, which do have external repositories, but for the sake of simplicity we want to store them in monorepo:

The vendor/gems/ MUST ONLY be used if we are pulling from external repository either via script, or manually.
The vendor/gems/ MUST NOT be used for storing in-house gems.
The vendor/gems/ MAY accept fixes to make them buildable with GitLab monorepo
The gems/ MUST be used for storing all in-house gems that are part of GitLab monorepo.
The RubyGems MUST be used for all externally stored dependencies that are not in gems/ in GitLab monorepo.

Handling of an existing gems in `vendor/gems`

For in-house Gems that do not have external repository and are currently stored in vendor/gems/:
- For Gems that are used by other repositories:
  - We will migrate it into its own repository.
  - We will start or continue publishing them via RubyGems.
  - Those Gems will be referenced via version in Gemfile and fetched from RubyGems.
- For Gems that are only used by monorepo:
  - We will stop publishing new versions to RubyGems.
  - We will not pull from RubyGems already published versions since there might be applications dependent on those.
  - We will move those gems to gems/.
  - Those Gems will be referenced via path: in Gemfile.
For vendor/gems/ that are external and vendored in monorepo:
- We will maintain them in the repository if they require some fixes that cannot be or are not yet upstreamed.
- It is expected that vendored gems might be published by third-party.
- Those Gems will not be published by us to RubyGems.
- Those Gems will be referenced via path: in Gemfile, since we cannot depend on RubyGems.

Reserve a gem name

We reserve a gem name as a precaution before publishing any public code that contains a new gem, to avoid name-squatters taking over the name in RubyGems.

To reserve a gem name, follow the steps to Create and publish a Ruby gem, with the following changes:

Use 0.0.0 as the version.
Include a single file lib/NAME.rb with the content raise "Reserved for GitLab".
Perform the build and publish, and check https://rubygems.org/gems/ to confirm it succeeded.