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Last updated: 12 Jun 2019

content-publisher: 11. Moving business logic out of controllers

Date: 2019-06-03


Over time the controller classes for Content Publisher have become increasingly complex as the application grew in size and functionality. This has led to action methods that are long, difficult to understand and expensive to test. Looking towards the future it can be anticipated that there will be more functionality in controllers, such as access limiting, concurrent editing protection and invalid state handling. Therefore, there is an expectation that controllers would become an increasing pain point for the application due to their complexity.

This is a common scenario for a growing Rails application. A customary approach to address this is to distinguish between business and rendering logic in an action (where business logic is the action the user requested, such as changing application state, and rendering logic is the process of building the response to be returned to the user) and to move business logic outside of the controller action to another class.

Within the Content Publisher team we have been evaluating a number of patterns and tools for creating business logic classes. We came to the following conclusions:

  • Service objects: This is a pattern already used in Content Publisher, however it is used to perform distinct application tasks rather than being coupled to particular controller actions. We felt the services directory had become a mess in Content Publisher, with little consistency between service objects.
  • Dry-transaction gem: This provides a nice interface for managing responses through ruby blocks. However dry-transaction requires a strict adherence to a provided DSL which can make a transaction class very different from a plain old Ruby object.
  • Interactor gem: This is a simple, relatively free-form pattern (no DSL) that uses a context object for basic control flow. It has a disadvantage that the input and output are not particularly clear.
  • Trailblazer operation: This gem provides a DSL for creating classes to perform a business operation. It works best within a Rails application that has embraced the collection of Trailblazer utilities.
  • Publishing API command pattern: This is an effective pattern at moving logic out of controllers. However, we felt it had become convoluted and inflexible due to domain logic coupling.


Content Publisher will make use of the Interactor gem as the tool to implement classes for performing controller business logic. The logic within controller actions will be focused on producing a HTTP response.

Interactors for controller actions are intended to be coupled to a particular action and not be reused outside that context. This is to maximise clarity (in purpose and naming) and to minimise the logic, options and outcomes of an interactor.

Interactors should be created for all controller actions that mutate application state (typically POST/PUT/PATCH/DELETE requests). Only in cases where an action is very simple should an interactor not be used.

For controller actions that don't mutate application state (such as GET requests) interactor classes may be created, if beneficial in abstracting complexity. In most cases these types of requests have low amounts of code and so use of this pattern would not be advantageous.

Interactors are stored in the app/interactors directory, within here there are directories created for each controller to store the interactors for each action, in a pattern similar to views. Each interactor class name is suffixed with "Interactor", to make their purpose clear. As an example, given a create action on a DocumentsController there is a corresponding interactor to create a document, Documents::CreateInteractor, which is stored in app/interactors/documents.

The Interactor gem was chosen over writing our own framework for handling business logic, as per the Publishing API command approach, for the following reasons:

  • a bespoke problem is not being solved, so it shouldn't require a bespoke solution;
  • authoring a new pattern is something that requires documentation, maintenance and iteration;
  • It’s harder to bikeshed on decisions made externally.

We felt that compared to other community gems, such as dry-transaction and Trailblazer, Interactor offered a lower learning curve, greater flexibility and a lower dependency overhead. We also felt that as the gem is quite simple, using it would not preclude us from building, or changing to, a different approach were our needs to change or we found a problem with the gem.




Many of the controller actions in Content Publisher have been refactored to make use of the Interactor gem. This has resulted in an increase in the number of classes of the application while the size and complexity of controller classes have decreased.

Conventions have been established on a consistent approach to writing interactors. These have focused on: