MemCacheStore's move to dalli as a backend broke :race_condition_ttl support

[mje113]

Sending dalli an expires_in integer that matches the Entry instance's expires_at will invalidate the cached value in Memcache at the same time the Entry is expired. This breaks the ability for Cache#fetch to determine if the :race_condition_ttl has occurred since the value will never make it back from Memcached.

[mje113]

Sorry for no test coverage, but this is pretty hard to test.

[jwinter]

You can see where the additional 5.minutes used to be added to the expires_in sent to memcached here:

rails/activesupport/lib/active_support/cache/mem_cache_store.rb

Line 142 in a02b40a

# Set the memcache expire a few minutes in the future to support race condition ttls on read

Without that padding, there will never be a window where both your entry is expired (via its expired_at) and memcached still has its value, so you'll never be able to serve the cached value to the other readers.

[guilleiguaran]

The removal of race_condition_ttl was discussed here: http://github.com/rails/rails/pull/6903/files#r1078311

[mje113]

What if someone is relying on race_condition_ttl to work correctly when upgrading to 4.0? Perhaps the removal of the functionality should be pointed out--or if it's agreed that it's up to the user to implement their own race_condition_ttl functionality, shouldn't it also be yanked from ActiveSupport::Cache#fetch?

Finally, if ActiveSupport::Cache isn't keeping its own tabs on the expires_in, and modifying that value to avoid multiple concurrent fetches on a key miss, why wrap the cached value in an Entity instance at all? The overhead with the extra marshaling and compressing (Dalli also will marshal and compress the value sent to it) just doesn't seem worth it.

[fxn]

race_condition_ttl is provided by the cache store, see https://github.com/rails/rails/blob/master/activesupport/lib/active_support/cache.rb#L23, and we recently had a thread about this feature and the ability to distinguish between an unset key and a cached nil value. Those two are the ones that justify having ActiveSupport::Cache::Entry objects serialized.

The outcome of the discussion was that we want to support them, and @bdurand provided a patch that made serialized entry objects much lighter. If someone wants to store raw values in a different cache store implementation at the cost of losing those features, he can do it in a plugin.

So, we need to bring those 5 minutes back.

[whatbird]

FWIW: there is currently a pull request on Dalli to support :race_condition_ttl

petergoldstein/dalli#277