title: What is the "env" variable in Rack middleware?
body: <p>I know a Rack middleware filter is a Ruby class with an initialize and a call method.  I know the call method takes an "env" argument. Something like this:</p>
  
  <pre><code>class MyFilter
    def initialize(app)
    end
  
    def call(env)
    end
  end
  </code></pre>
  
  <p>My question is: what exactly is the "env" argument sent to "call"?  Is this the same as the Rails ENV environment (ie. development, testing, production).</p>
  
  <p>Thanks!</p>