Hi all

Need for exactly monthly limiting

I am learning Elixir (and Phoenix and LiveView) by creating a simple app that would fetch weather from external service and “creatively” present it (think “today is a bit warmer that yesterday” compared to “today is +17”).

At some point I’ll make the app public. Certainly for a hobby project I don’t want to pay much for the weather API and have to stay within free or cheap pricing tiers that is my Elixir backend should call external API no more than e.g. 1000 times per calendar month.

Precision/reliability needs

• I will probably be fine with limiter being not super-precise, so it’s fine if limiting mechanism e.g. allows sometimes for 1050 per month and if it’s per 30 days from some arbitrary day, not exactly within a calendar month
• On the other hand I won’t like if somebody uses monthly quota in one minute, better to leave some calls for the tomorrow visitors though good to allow small bursts.

So the ideal limiter would simultaneously apply something like:

• no more than 1000 calls within 30 days or within a calendar month
• no more than 100 calls a day
• no more than 50 calls an hour
• no more than 10 calls a minute
• store whatever it needs in Postgres (okay to store it in-memory for a while, but syncs to DB are a must)
  • it’s a hobby project under active development, I don’t want to figure e.g. how to install and use Redis on gigalixir or fly.io while Postgres I am already using anyway
  • I don’t want to store state in-memory only - service is very likely to be restarted often. Actually in case of in-memory state even one restart mid-month could by accident overrun quota 2 times

What are the options?

I figured several options for proceeding and would love to get some advice from the more experienced folks.

1. Hammer library (with a bit of massaging for combining the buckets) seems to allow for the rate limiting rules I am into. It doesn’t have a Postgres backend option, but maybe one is easy to write
2. Create an own rate limiter/counter saving the log of requests (or the final counter) starting from mix phx.gen.context so the context would query call log table to tell the client how many calls are still allowed within month/day/hour?
   1. And if I’ll want to be “precise” over “fast” then maybe wrap it into a GenServer guaranteed to be run in 1 copy only
3. Locate some other library that does approximately what I want. Maybe there’s something else besides Hammer that I just failed to locate so far.
4. Something completely different?

What would you do in this kind of a situation?