Warn on boot if maxmemory policy is not 'noeviction'

[odlp]

This PR proposes logging a warning when the CLI boots if the maxmemory policy is not 'noeviction'.

Inspired by Nate Berkopec's tweet the other day (I've also had clients who had this misconfiguration):

Running Sidekiq from a Redis datastore with a volatile eviction policy is probably the most common Sidekiq error I see in the wild w/clients. Switch to noeviction, you want Sidekiq to fail very loudly when you run out of memory, not silently drop jobs.

[odlp]

Test failure on Ruby 2.5 appears to be unrelated. It's a temporal assertion which failed (CI running slow?):

Failure:
API::with an empty database#test_0024_can enumerate retries [/home/runner/work/sidekiq/sidekiq/test/test_api.rb:484]:
Expected |1606907616.4180453 - 1606907616.3701653| (0.047879934310913086) to be <= 0.02.

rails test /home/runner/work/sidekiq/sidekiq/test/test_api.rb:472

[mperham]

There will be a lot of these in development. I'd suggest you only print this if env != "development".

[odlp]

Taking a look at redis.conf in various versions, I believe noeviction is the default:

• 4.0
• 5.0
• 6.0

But appreciate that it's a somewhat meaningless check in development, apart from the educational value. I'll add the additional condition 👍

[mperham]

Good catch, if it’s the default in the brew formula then I’m ok with it as is.

[odlp]

It appears that the Homebrew formulae don't modify maxmemory_policy.

Here's 4.0:
https://github.com/Homebrew/homebrew-core/blob/fdb83fcfb482e5ed1f1c3c442a85b99223fcabeb/Formula/redis%404.0.rb#L27-L31

Here's 6.0:
https://github.com/Homebrew/homebrew-core/blob/fdb83fcfb482e5ed1f1c3c442a85b99223fcabeb/Formula/redis.rb#L30-L34

[mperham]

Thank you, great idea!

[Dantemss]

I have a question about why this warning is necessary when using volatile-* policies:
If the maxmemory policy is volatile-* then keys without a TTL will never be evicted...
So I imagine those policies would work fine unless Sidekiq was setting a TTL on important keys or if the client somehow managed to configure redis-rb to add a TTL to all keys.
I've seen that configuration option on the Rails cache side, but I don't think redis-rb itself supports it.
So what would be causing those keys to expire?