Divergent Change

A class suffers from Divergent Change when it changes for multiple reasons.

Symptoms

• You can’t easily describe what the class does in one sentence.
• The class is changed more frequently than other classes in the application.
• Different changes to the class aren’t related to each other.

Example

# app/controllers/summaries_controller.rb
class SummariesController < ApplicationController
  def show
    @survey = Survey.find(params[:survey_id])
    @summaries = @survey.summarize(summarizer)
  end

  private

  def summarizer
    case params[:id]
    when 'breakdown'
      Breakdown.new
    when 'most_recent'
      MostRecent.new
    when 'your_answers'
      UserAnswer.new(current_user)
    else
      raise "Unknown summary type: #{params[:id]}"
    end
  end
end

This controller has many reasons to change:

• Control flow logic related to summaries, such as authentication.
• Any instance in which a summarizer strategy is added or changed.

Solutions

• Extract class to move one cause of change to a new class.
• Move method if the class is changing because of methods that relate to another class.
• Extract validator to move validation logic out of models.
• Introduce form object to move form logic out of controllers.
• Use convention over configuration to eliminate changes that can be inferred by a convention, such as a name.

Prevention

You can prevent divergent change from occurring by following the single responsibility principle. If a class has only one responsibility, it has only one reason to change.

Following the open/closed principle limits future changes to classes, including divergent change.

Following composition over inheritance will make it easier to create small classes, preventing divergent change.

If a large portion of the class is devoted to instantiating subclasses, try following the dependency inversion principle.

You can use churn to discover which files are changing most frequently. This isn’t a direct relationship, but frequently changed files often have more than one responsibility and thus more than one reason to change.