For Rails, why does it matter that the Gem Devise is Rack-based?

Question

As far as I know, Rack just sits in between Rails and the Webserver (like Mongrel or Webrick), to act as an "adapter" so that the choice of webserver is not important.

So why does Devise is known to be Rack-based and it is good? If Rack is transparent, doesn't it not matter whether the authentication system is Rack based or non-Rack based? (or how does it relate to Rack at all?)

Answer 1

It just means that the gem does not depend on a particular implementation (such as Rails) of the Rack interface, just that it conforms to it. This is good because you can use the gem with other Rack-based frameworks such as Sinatra.