The Message Stack appears to be a set of 3 Drupal contributed modules, that can be used in a Drupal 7 or 8 site to implement messaging in a Drupal site.
Here are some details about it (from that same link):
The modules have been written with a pluggable architecture that supports customization and extensibility. Each module includes implementation examples that give a hint of the potential use cases and integration with other popular Drupal modules.
I wonder what this is all about, ie:
- What's the purpose of each of these modules?
- How do its modules work together?
- Where can I find examples of others who have been using the Message Stack already?