Architecting a system for reminder emails

Question

I'm building a reservation system in Rails 4.2 where I need to send a set of emails to users at predefined intervals (for example that they have an upcoming reservation, feedback after it's done, a link to change/cancel an existing reservation, etc.). I've looked around and found this and this, but I'm trying to decide between the approaches.

I see two main ways of building this system out.

1. Using a queue system like delayed_job. Whenever someone makes a reservation, we queue up all the emails for the correct time when they should be sent.

   Pro: One queue for all emails. Automatic retry logic.

   Con: Thousands of emails will eventually get queued in the system. Need to dequeue whenever someone cancels a reservation (dependent: destroy emails related to it might be pretty easy). Somewhat more complex logic around what time we need the emails to go out.

2. cron + rake task that runs at some predefined interval (hourly? every fifteen minutes?) and checks for the emails that need to go out. It runs a query like "Find all reservations that are three days from now", and then sends out all emails.

   Pro: Put everything into application logic, reduce the amount of state we need to keep track of.

   Con: Need to keep track of which emails have been sent, which is conceptually similar to whatever jobs table we already have created above.

Answer 1

Second approach is better (use heroku scheduler if on heroku), queues are more for "run as soon as possible" than "run at this particular datetime"