Global variable vs. constant vs. class instance variable for global cache

Question

In general which is better for a global cache: global variable, constant, or class instance variable?

Here is an example of each:

module Foo
  $FOO_CACHE = {}
  def self.access_to_cache
    $FOO_CACHE
  end
end

module Foo
  CACHE = {}
  def self.access_to_cache
    CACHE
  end
end

module Foo
  @cache = {}
  def self.access_to_cache
    @cache
  end
end

Answer 1

This is ultimately pretty subjective, but I’ll address each option one-by-one:

• Global variable: no …because putting a global variable inside a module (or a class, or anything for that matter) doesn’t make much sense, it’s going to be in scope everywhere anyway. Besides the fact that if you can use something other than a global variable, you should always do so.

• Constant: no …because the cache is not constant! While Ruby doesn't enforce that constants can’t change, that doesn’t mean you should do it. There’s a reason they’re called constants.

• Class instance variable: yes …because it’s the only one here that makes any sense (though the name might not, technically here it’s a module instance variable, but that’s being rather pedantic). This is the only one of the three that both makes semantic sense to modify and is encapsulated by some scope.