Thread-safe Singleton pattern trong Ruby implement thế nào?

Ruby có module Singleton trong stdlib — include nó để tạo class chỉ có đúng 1 instance, thread-safe:

require 'singleton'

class AppConfig
  include Singleton

  attr_accessor :debug, :log_level

  def initialize
    @debug     = false
    @log_level = :info
  end
end

config = AppConfig.instance
config.debug = true

AppConfig.instance.equal?(config)  # => true — cùng object
AppConfig.new                       # => NoMethodError — private

Singleton module dùng Mutex nội bộ để đảm bảo instance chỉ được tạo 1 lần dù nhiều thread cùng gọi lần đầu.

Ứng dụng: config object, connection pool (1 instance), logger dùng chung, registry global. Lưu ý: Singleton là global state — khó test; cân nhắc dependency injection thay thế trong code production.

Ruby ships the Singleton module in its standard library — include it to create a class with exactly one instance, thread-safe out of the box:

require 'singleton'

class AppConfig
  include Singleton

  attr_accessor :debug, :log_level

  def initialize
    @debug     = false
    @log_level = :info
  end
end

config = AppConfig.instance
config.debug = true

AppConfig.instance.equal?(config)  # => true — same object
AppConfig.new                       # => NoMethodError — private

Singleton uses an internal Mutex to ensure instance is created only once even when multiple threads call it simultaneously.

Use cases: config objects, shared connection pools, global loggers, registries. Note: Singleton is global state — hard to test; consider dependency injection in production code.