`erlcron` 🕥

[nelsonic]

Need to make time to explore this package: https://github.com/erlware/erlcron

Looks to include most of the use-cases we would need to run a "reminders" type app.
However, it is still subject to process/server failure ...
i.e. if the process dies your reminder dies with it. 💀

So ... maybe we need a hybrid approach:

1. Define the event / reminder in a DSL that can easily be understood by a human and converted to SQL.
2. Save the event / reminder to a distributed Postgres (or other reliable DB)
3. Load the event / reminder records into a process-based cron when the server starts.
4. All new event / reminder records are first inserted into Postgres for persistence/availability
   and then loaded into an Erlang process.

The open questions are:
A. How do we know that a reminder is only loaded on one server/instance to avoid duplication?
B. Does erlcron understand time-zones? (it appears to, how can we test it to avoid bugs?)
C. What happens to an event / reminder that is meant to trigger when the server/instance is offline?