This module is used to encapsulate access to thread local variables.

Instead of polluting the thread locals namespace:

Thread.current[:connection_handler]

you define a class that extends this module:

module ActiveRecord
  class RuntimeRegistry
    extend ActiveSupport::PerThreadRegistry

    attr_accessor :connection_handler
  end
end

and invoke the declared instance accessors as class methods. So

ActiveRecord::RuntimeRegistry.connection_handler = connection_handler

sets a connection handler local to the current thread, and

ActiveRecord::RuntimeRegistry.connection_handler

returns a connection handler local to the current thread.

This feature is accomplished by instantiating the class and storing the instance as a thread local keyed by the class name. In the example above a key “ActiveRecord::RuntimeRegistry” is stored in Thread.current. The class methods proxy to said thread local instance.

If the class has an initializer, it must accept no arguments.

Methods
E
I
Class Public methods
extended(object)

Source: show | on GitHub 

# File activesupport/lib/active_support/per_thread_registry.rb, line 37
def self.extended(object)
  object.instance_variable_set '@per_thread_registry_key', object.name.freeze
end
Instance Public methods
instance()

Source: show | on GitHub 

# File activesupport/lib/active_support/per_thread_registry.rb, line 41
def instance
  Thread.current[@per_thread_registry_key] ||= new
end