Yea, an HA Proxy would make a lot of sense with something like this. As you scale up the number of Interference services for SD, you can increase the number of background workers that are available on the Ruby side to take advantage of the additional servers. You can probably set up HA Proxy to balance the load which would help simplify the infrastructure. By default, I think it does Round Robin which would route each task to a different server in turns. 

The logic that I meant was for sending out the emails. Typically, instead of looping over all of the users, you would check if the user has unsubscribed from the email, if there was any kind of time preference, using batch finds instead of looping through every user, etc.

I hold command and click and drag up to select multiple cursors for the number of items that I copied. You have to do something similar for the items you copy.

You're correct that indexing a low cardinality column like the Role may not be beneficial. 

While it may be annoying with Active Storage, it's probably best to let it handle the folder/file structure within S3. This isn't something that one should be touching in ideal worlds.