Mutable Global State (ARCH6187)

Key ruby.quality.mutable-global-state · Severity warning · Category quality

Global variables and mutable class variables (@@) create hidden dependencies and race conditions, making it impossible to track who mutates the state.

Incorrect

class Counter
  @@count = 0

  def self.increment
    @@count += 1
  end
end

Correct

class Counter
  def initialize
    @count = 0
  end

  def increment
    @count += 1
  end
end

Fix

Replace shared mutable state with instance state, or move it into an injected, encapsulated object.