• Refer to Services and dependency injection in the Drupal 8 documentation at https://www.drupal.org/docs/8/api/services-and-dependency-injection/services-and-dependency-injection-in-drupal-8
• Refer to the Symfony service container documentation at
  http://symfony.com/doc/current/book/service_container.html
• Refer to the Symfony Dependency Injection component documentation at
  http://symfony.com/doc/current/components/dependency_injection/introduction.html