What is the "env" variable in Rack middleware?

Question

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:

class MyFilter   def initialize(app)   end    def call(env)   end end 

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).

Thanks!

Answer 1

env is just a hash. Rack itself and various middlewares add values into it.

To understand what the various keys are in the hash, check out the Rack Specification.

And here is a sample env hash:

{   "GATEWAY_INTERFACE" => "CGI/1.1",   "PATH_INFO" => "/index.html",   "QUERY_STRING" => "",   "REMOTE_ADDR" => "::1",   "REMOTE_HOST" => "localhost",   "REQUEST_METHOD" => "GET",   "REQUEST_URI" => "http://localhost:3000/index.html",   "SCRIPT_NAME" => "",   "SERVER_NAME" => "localhost",   "SERVER_PORT" => "3000",   "SERVER_PROTOCOL" => "HTTP/1.1",   "SERVER_SOFTWARE" => "WEBrick/1.3.1 (Ruby/2.0.0/2013-11-22)",   "HTTP_HOST" => "localhost:3000",   "HTTP_USER_AGENT" => "Mozilla/5.0 (Macintosh; Intel Mac OS X 10.9; rv:26.0) Gecko/20100101 Firefox/26.0",   "HTTP_ACCEPT" => "text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8",   "HTTP_ACCEPT_LANGUAGE" => "zh-tw,zh;q=0.8,en-us;q=0.5,en;q=0.3",   "HTTP_ACCEPT_ENCODING" => "gzip, deflate",   "HTTP_COOKIE" => "jsonrpc.session=3iqp3ydRwFyqjcfO0GT2bzUh.bacc2786c7a81df0d0e950bec8fa1a9b1ba0bb61",   "HTTP_CONNECTION" => "keep-alive",   "HTTP_CACHE_CONTROL" => "max-age=0",   "rack.version" => [1, 2],   "rack.input" => #<StringIO:0x007fa1bce039f8>,   "rack.errors" => #<IO:<STDERR>>,   "rack.multithread" => true,   "rack.multiprocess" => false,   "rack.run_once" => false,   "rack.url_scheme" => "http",   "HTTP_VERSION" => "HTTP/1.1",   "REQUEST_PATH" => "/index.html" } 

And to make it easier to use, checkout Rack::Request which makes it easier to access the values inside the env hash.