Use Message notify for email notifications

Commons 3.x has relatively simple email notifications requirements. They are related to the activity stream requirements in that the stream and email notifications share a similar purpose (informing community members about new activity on a site). As a result, it would be ideal to present the settings for email subscriptions and stream subscriptions to updates on specific threads through a unified UI.

The requirements are described in more detail in the above Google Doc matrix, but essentially they come down to:

- Group members can subscribe to content in a group or mute all notifications for a group
- Community members can subscribe to new comments on a thread without commenting on the thread.
- Provide a unified UI for managing email subscription to a group, email subscription to an individual thread

Other kinds of site activity will display in the site activity stream, but we're keeping email notifications simple.

I see 3 general approaches here:

1) Use Comment notify + simple OG content subscriptions and build a unified UI, probably specific to Commons

This would require us to implement #288726: Allow users to subscribe without posting a comment and then provide a unified UI for pulling in thread-specific subscriptions from Comment notify and the thread-specific activity stream subscriptions. It seems likely that this UI would end up being implemented in a way that is specific to Commons, or at least specific to those particular contrib modules.

2) Extend Message notify module

We will likely use the Message module to power the Activity streams in Commons.

By using Message notify for email notifications, we would have a shared architecture for both types of notifications (email and stream), which may simplify the process of providing a unified end-user UI for managing user subscriptions and generally making these related features work together smoothly.

3) Push the Notifications and Messaging modules towards stable Drupal 7 releases

http://drupal.org/project/messaging http://drupal.org/project/notifications both have a substantial Drupal 6 install base, but a limited Drupal 7 install base and a non-trivial amount of bugs in the 7.x branch, which suggests that the effort to update these modules to Drupal 7 is large.

Beyond that, we'd still need to provide an integrated UI. My experience with working with the Drupal 6 versions of Messaging and Notifications is that they are architecturally complex. Also, Messaging and Notifications haven't historically been developed with a focus on large site scalability (eg http://drupal.org/node/1070164 http://drupal.org/node/868446).

4) Use Subscriptions module and implement a custom UI

I'm not particularly familiar with this module beyond recalling that it predates Messaging Notifications. Subscriptions has a smaller site install base (around ~5,000 sites) than Messaging and Notifications. It seems like the need to implement a unified UI for email and activity stream notifications still exists with this solution.

I favor what I see as the opportunity to build on the simplest architecture and ideally, also one that's highly generalizable. So, I prefer approaches 2 and 1, in that order.

Whichever solution we implement, I'd like us to use a system like http://drupal.org/project/queue_mail that uses the Drupal queue system for sending mail.