On Rails 5.2, when compression is enabled (which it is by default),
the actual value being written to the underlying storage is actually
_bigger_ than the uncompressed raw value.
This is because the `@marshaled_value` instance variable (typically)
gets serialized with the entry object, which is then written to the
underlying storage, essentially double-storing every value (once
uncompressed, once possibly compressed).
This regression was introduced in #32254.
Compression has long been available, but opt-in and at a 16kB threshold.
It wasn't enabled by default due to CPU cost. Today it's cheap and
typical cache data is eminently compressible, such as HTML or JSON
fragments.
Compression dramatically reduces Memcached/Redis mem usage, which means
the same cache servers can store more data, which means higher hit
rates.
To disable compression, pass `compress: false` to the initializer.
Split up the caching tests as prep for adding a new cache store. Slices
the mega test/caching_test.rb into behavior modules, concrete store
tests, and cross-cutting store tests.
Considering moving cache store behavior modules into lib/ so they may be
used for acceptance testing by third parties.
