Drupal 6 had Messaging & Notification
Drupal 7 has Message-subscribe & Message-notify

Message-subscribe is a new module in the Message stack, that finally ties everything together to give us the ability to subscribe to content, and send notifications.

If you know Message you already know that some development skills, or knowing Rules really really good are needed. This post will go over the main concepts and flow of the notification process.

Message-subscribe, just like Message module has zero knowledge about your business logic - it is up to you to implement the Message creation. What Message subscribe gives you, though, is one simple function your implementing module will call -- and that’s it. It will take care of it from there.

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