Rails Memcache Store and expires_in problems

Question

[25] pry(main)> Rails.cache.fetch("my_key", :expires_in => 1.year) do
[25] pry(main)*   (Time.now.to_date).to_s
[25] pry(main)* end
=> "2013-11-01"
[28] pry(main)> Rails.cache.fetch("my_key")
=> nil

I cant understand the above behavior. The cache does not have this key before this, or rather it is nil.

This works fine if I remove the expires_in option.

This is in the production version of my app which uses memcached

 # Use a different cache store in production
  config.cache_store = :mem_cache_store, <DNS NAME>

This also works on my local which I expect is the filesystem based caching.

Edit: ah, 1.year is too long perhaps.. It works with 1.day. Is this a bug or is this documented somewhere?

Edit: It appears that 1.month is the max in a duration form. But this still cant be accepted behavior Memcache maximum key expiration time

@Beerlington I doubt choice of console would make a difference.. But do you know a easy way to switch? I use `pry-rails` :( — Karthik T, Nov 01 '13 at 11:06
@Beerlington just to verify, are you checking with memcached, because it appears to be the behavior it is designed with, since one other person on the linked answer also had the silent failure like mine — Karthik T, Nov 01 '13 at 11:07
@Beerlington and this failed in the app itself, that is what brought it to my attention — Karthik T, Nov 01 '13 at 11:08

score 2 · Accepted Answer · edited May 23 '17 at 12:09

From a few sources listed below, it appears that this is the way memcached behaves, and nothing to do with rails itself. If, as @Beerlington says, Rails 4.0 doesnt have this, it could be sanity checking the values before passing it to memcached.

So, the answer,

The maximum expiry duration that memcached accepts is 1 month. This is hard coded in its code. If you give a duration longer than 1 month, it will look like it wrote to cache, but it is actually dropped. If you need a longer expiry for some reason, you can always choose to provide an exact time it should expire, i.e the 1.year.from_now style. This will accept longer expiry durations if given this way, and I did not come across any max (perhaps 2038 :P, or not).

Sources

Edit: It isnt dropped exactly.. it is considered as a unix timestamp, and since it is in the past, it isnt saved. — Karthik T, Nov 21 '13 at 01:04

Rails Memcache Store and expires_in problems

1 Answers1