Guidelines for reusing abstractions

As GitLab has grown, different patterns emerged across the codebase. Service classes, serializers, and presenters are just a few. These patterns made it easy to reuse code, but at the same time make it easy to accidentally reuse the wrong abstraction in a particular place.

Why these guidelines are necessary

Code reuse is good, but sometimes this can lead to shoehorning the wrong abstraction into a particular use case. This in turn can have a negative impact on maintainability, the ability to easily debug problems, or even performance.

An example would be to use ProjectsFinder in IssuesFinder to limit issues to those belonging to a set of projects. While initially this may seem like a good idea, both classes provide a very high level interface with very little control. This means that IssuesFinder may not be able to produce a better optimised database query, as a large portion of the query is controlled by the internals of ProjectsFinder.

To work around this problem, you would use the same code used by ProjectsFinder, instead of using ProjectsFinder itself directly. This allows you to compose your behaviour better, giving you more control over the behaviour of the code.

To illustrate, consider the following code from IssuableFinder#projects:

return @projects = project if project?

projects =
  if current_user && params[:authorized_only].presence && !current_user_related?
  elsif group
    finder_options = { include_subgroups: params[:include_subgroups], only_owned: true } group, current_user: current_user, options: finder_options).execute
  else current_user).execute

@projects = projects.with_feature_available_for_user(klass, current_user).reorder(nil)

Here we determine what projects to scope our data to, using three different approaches. When a group is specified, we use GroupProjectsFinder to retrieve all the projects of that group. On the surface this seems harmless: it is easy to use, and we only need two lines of code.

In reality, things can get hairy very quickly. For example, the query produced by GroupProjectsFinder may start out simple. Over time more and more functionality is added to this (high level) interface. Instead of only affecting the cases where this is necessary, it may also start affecting IssuableFinder in a negative way. For example, the query produced by GroupProjectsFinder may include unnecessary conditions. Since we're using a finder here, we can't easily opt-out of that behaviour. We could add options to do so, but then we'd need as many options as we have features. Every option adds two code paths, which means that for four features we have to cover 8 different code paths.

A much more reliable (and pleasant) way of dealing with this, is to simply use the underlying bits that make up GroupProjectsFinder directly. This means we may need a little bit more code in IssuableFinder, but it also gives us much more control and certainty. This means we might end up with something like this:

return @projects = project if project?

projects =
  if current_user && params[:authorized_only].presence && !current_user_related?
  elsif group
      .owned_groups(subgroups: params[:include_subgroups])

@projects = projects.with_feature_available_for_user(klass, current_user).reorder(nil)

This is just a sketch, but it shows the general idea: we would use whatever the GroupProjectsFinder and ProjectsFinder finders use under the hoods.

End goal

The guidelines in this document are meant to foster better code reuse, by clearly defining what can be reused where, and what to do when you can not reuse something. Clearly separating abstractions makes it harder to use the wrong one, makes it easier to debug the code, and (hopefully) results in fewer performance problems.


Now let's take a look at the various abstraction levels available, and what they can (or cannot) reuse. For this we can use the following table, which defines the various abstractions and what they can (not) reuse:

Abstraction Service classes Finders Presenters Serializers Model instance method Model class methods Active Record Worker
Controller Yes Yes Yes Yes Yes No No No
Service class Yes Yes No No Yes No No Yes
Finder No No No No Yes Yes No No
Presenter No Yes No No Yes Yes No No
Serializer No Yes No No Yes Yes No No
Model class method No No No No Yes Yes Yes No
Model instance method No Yes No No Yes Yes Yes Yes
Worker Yes Yes No No Yes No No Yes


Everything in app/controllers.

Controllers should not do much work on their own, instead they simply pass input to other classes and present the results.

Grape endpoint

Everything in lib/api.

Service classes

Everything that resides in app/services.


Everything in app/finders, typically used for retrieving data from a database.

Finders can not reuse other finders in an attempt to better control the SQL queries they produce.


Everything in app/presenters, used for exposing complex data to a Rails view, without having to create many instance variables.


Everything in app/serializers, used for presenting the response to a request, typically in JSON.

Model class methods

These are class methods defined by GitLab itself, including the following methods provided by Active Record:

  • find
  • find_by_id
  • delete_all
  • destroy
  • destroy_all

Any other methods such as find_by(some_column: X) are not included, and instead fall under the "Active Record" abstraction.

Model instance methods

Instance methods defined on Active Record models by GitLab itself. Methods provided by Active Record are not included, except for the following methods:

  • save
  • update
  • destroy
  • delete

Active Record

The API provided by Active Record itself, such as the where method, save, delete_all, etc.


Everything in app/workers.

The scheduling of Sidekiq jobs using SomeWorker.perform_async, perform_in, etc. Directly invoking a worker using should be avoided at all times in application code, though this is fine to use in tests.