Less allocated objects on each request

How many? Using `memory_profiler` and a Rails app (codetriage.com), master uses:

```
rack/lib x 7318
```

After this patch, the app uses:

```
rack/lib x 4598
```

Or `(7318 - 4598) / 7318.0 * 100 # => 37.16` % fewer objects __PER REQUEST__.

To do this, I extracted really commonly used strings into top level Rack constants. It makes for a bit of a big diff, but I believe the changes are worth it. 

Running benchmark/ips against the same app, I'm seeing a performance host of `2~4%` across the entire app response. This doesn't just make Rack faster, it will make your app faster.

While we could certainly go overboard and pre-define ALL strings as constants, that would be pretty gnarly to work with. This patch goes after the largest of the low hanging fruit.

36 files changed, 129 insertions, 109 deletions

diff --git a/lib/rack.rb b/lib/rack.rb
index 341514c5..97817bcb 100644
--- a/lib/rack.rb
+++ b/lib/rack.rb
@@ -22,6 +22,16 @@ module Rack
   def self.release
     "1.5"
   end
+  PATH_INFO      = 'PATH_INFO'.freeze
+  REQUEST_METHOD = 'REQUEST_METHOD'.freeze
+  SCRIPT_NAME    = 'SCRIPT_NAME'.freeze
+  QUERY_STRING   = 'QUERY_STRING'.freeze
+  CACHE_CONTROL  = 'Cache-Control'.freeze
+  CONTENT_LENGTH = 'Content-Length'.freeze
+  CONTENT_TYPE   = 'Content-Type'.freeze
+
+  GET  = 'GET'.freeze
+  HEAD = 'HEAD'.freeze
 
   autoload :Builder, "rack/builder"
   autoload :BodyProxy, "rack/body_proxy"
diff --git a/lib/rack/auth/abstract/handler.rb b/lib/rack/auth/abstract/handler.rb
index 214df629..c657691e 100644
--- a/lib/rack/auth/abstract/handler.rb
+++ b/lib/rack/auth/abstract/handler.rb
@@ -17,8 +17,8 @@ module Rack
 
       def unauthorized(www_authenticate = challenge)
         return [ 401,
-          { 'Content-Type' => 'text/plain',
-            'Content-Length' => '0',
+          { CONTENT_TYPE => 'text/plain',
+            CONTENT_LENGTH => '0',
             'WWW-Authenticate' => www_authenticate.to_s },
           []
         ]
@@ -26,8 +26,8 @@ module Rack
 
       def bad_request
         return [ 400,
-          { 'Content-Type' => 'text/plain',
-            'Content-Length' => '0' },
+          { CONTENT_TYPE => 'text/plain',
+            CONTENT_LENGTH => '0' },
           []
         ]
       end
diff --git a/lib/rack/auth/digest/request.rb b/lib/rack/auth/digest/request.rb
index 706c651c..01994364 100644
--- a/lib/rack/auth/digest/request.rb
+++ b/lib/rack/auth/digest/request.rb
@@ -7,7 +7,7 @@ module Rack
     module Digest
       class Request < Auth::AbstractRequest
         def method
-          @env['rack.methodoverride.original_method'] || @env['REQUEST_METHOD']
+          @env['rack.methodoverride.original_method'] || @env[REQUEST_METHOD]
         end
 
         def digest?
diff --git a/lib/rack/body_proxy.rb b/lib/rack/body_proxy.rb
index 95a74626..b35167c8 100644
--- a/lib/rack/body_proxy.rb
+++ b/lib/rack/body_proxy.rb
@@ -4,9 +4,14 @@ module Rack
       @body, @block, @closed = body, block, false
     end
 
-    def respond_to?(*args)
-      return false if args.first.to_s =~ /^to_ary$/
-      super or @body.respond_to?(*args)
+    def respond_to?(method_name)
+      case method_name
+      when :to_ary
+        return false
+      when String
+        return false if /^to_ary$/ =~ method_name
+      end
+      super or @body.respond_to?(method_name)
     end
 
     def close
diff --git a/lib/rack/cascade.rb b/lib/rack/cascade.rb
index c2891e5f..6b8f415a 100644
--- a/lib/rack/cascade.rb
+++ b/lib/rack/cascade.rb
@@ -4,7 +4,7 @@ module Rack
   # status codes).
 
   class Cascade
-    NotFound = [404, {"Content-Type" => "text/plain"}, []]
+    NotFound = [404, {CONTENT_TYPE => "text/plain"}, []]
 
     attr_reader :apps
 
diff --git a/lib/rack/chunked.rb b/lib/rack/chunked.rb
index ea221fa9..36c4959d 100644
--- a/lib/rack/chunked.rb
+++ b/lib/rack/chunked.rb
@@ -56,11 +56,11 @@ module Rack
 
       if ! chunkable_version?(env['HTTP_VERSION']) ||
          STATUS_WITH_NO_ENTITY_BODY.include?(status) ||
-         headers['Content-Length'] ||
+         headers[CONTENT_LENGTH] ||
          headers['Transfer-Encoding']
         [status, headers, body]
       else
-        headers.delete('Content-Length')
+        headers.delete(CONTENT_LENGTH)
         headers['Transfer-Encoding'] = 'chunked'
         [status, headers, Body.new(body)]
       end
diff --git a/lib/rack/commonlogger.rb b/lib/rack/commonlogger.rb
index 1c99045e..d2d6dc34 100644
--- a/lib/rack/commonlogger.rb
+++ b/lib/rack/commonlogger.rb
@@ -46,9 +46,9 @@ module Rack
         env['HTTP_X_FORWARDED_FOR'] || env["REMOTE_ADDR"] || "-",
         env["REMOTE_USER"] || "-",
         now.strftime("%d/%b/%Y:%H:%M:%S %z"),
-        env["REQUEST_METHOD"],
-        env["PATH_INFO"],
-        env["QUERY_STRING"].empty? ? "" : "?"+env["QUERY_STRING"],
+        env[REQUEST_METHOD],
+        env[PATH_INFO],
+        env[QUERY_STRING].empty? ? "" : "?"+env[QUERY_STRING],
         env["HTTP_VERSION"],
         status.to_s[0..3],
         length,
@@ -65,7 +65,7 @@ module Rack
     end
 
     def extract_content_length(headers)
-      value = headers['Content-Length'] or return '-'
+      value = headers[CONTENT_LENGTH] or return '-'
       value.to_s == '0' ? '-' : value
     end
   end
diff --git a/lib/rack/conditionalget.rb b/lib/rack/conditionalget.rb
index 88573166..3d4c78aa 100644
--- a/lib/rack/conditionalget.rb
+++ b/lib/rack/conditionalget.rb
@@ -20,14 +20,14 @@ module Rack
     end
 
     def call(env)
-      case env['REQUEST_METHOD']
-      when "GET", "HEAD"
+      case env[REQUEST_METHOD]
+      when GET, HEAD
         status, headers, body = @app.call(env)
         headers = Utils::HeaderHash.new(headers)
         if status == 200 && fresh?(env, headers)
           status = 304
-          headers.delete('Content-Type')
-          headers.delete('Content-Length')
+          headers.delete(CONTENT_TYPE)
+          headers.delete(CONTENT_LENGTH)
           original_body = body
           body = Rack::BodyProxy.new([]) do
             original_body.close if original_body.respond_to?(:close)
diff --git a/lib/rack/content_length.rb b/lib/rack/content_length.rb
index 71bc919b..d0f491d1 100644
--- a/lib/rack/content_length.rb
+++ b/lib/rack/content_length.rb
@@ -16,7 +16,7 @@ module Rack
       headers = HeaderHash.new(headers)
 
       if !STATUS_WITH_NO_ENTITY_BODY.include?(status.to_i) &&
-         !headers['Content-Length'] &&
+         !headers[CONTENT_LENGTH] &&
          !headers['Transfer-Encoding'] &&
          body.respond_to?(:to_ary)
 
@@ -28,7 +28,7 @@ module Rack
           obody.close if obody.respond_to?(:close)
         end
 
-        headers['Content-Length'] = length.to_s
+        headers[CONTENT_LENGTH] = length.to_s
       end
 
       [status, headers, body]
diff --git a/lib/rack/content_type.rb b/lib/rack/content_type.rb
index dd96e959..78ba43b7 100644
--- a/lib/rack/content_type.rb
+++ b/lib/rack/content_type.rb
@@ -20,7 +20,7 @@ module Rack
       headers = Utils::HeaderHash.new(headers)
 
       unless STATUS_WITH_NO_ENTITY_BODY.include?(status)
-        headers['Content-Type'] ||= @content_type
+        headers[CONTENT_TYPE] ||= @content_type
       end
 
       [status, headers, body]
diff --git a/lib/rack/deflater.rb b/lib/rack/deflater.rb
index 9df510bd..1788463b 100644
--- a/lib/rack/deflater.rb
+++ b/lib/rack/deflater.rb
@@ -54,20 +54,20 @@ module Rack
       case encoding
       when "gzip"
         headers['Content-Encoding'] = "gzip"
-        headers.delete('Content-Length')
+        headers.delete(CONTENT_LENGTH)
         mtime = headers.key?("Last-Modified") ?
           Time.httpdate(headers["Last-Modified"]) : Time.now
         [status, headers, GzipStream.new(body, mtime)]
       when "deflate"
         headers['Content-Encoding'] = "deflate"
-        headers.delete('Content-Length')
+        headers.delete(CONTENT_LENGTH)
         [status, headers, DeflateStream.new(body)]
       when "identity"
         [status, headers, body]
       when nil
         message = "An acceptable encoding for the requested resource #{request.fullpath} could not be found."
         bp = Rack::BodyProxy.new([message]) { body.close if body.respond_to?(:close) }
-        [406, {"Content-Type" => "text/plain", "Content-Length" => message.length.to_s}, bp]
+        [406, {CONTENT_TYPE => "text/plain", CONTENT_LENGTH => message.length.to_s}, bp]
       end
     end
 
@@ -138,7 +138,7 @@ module Rack
       # Skip compressing empty entity body responses and responses with
       # no-transform set.
       if Utils::STATUS_WITH_NO_ENTITY_BODY.include?(status) ||
-          headers['Cache-Control'].to_s =~ /\bno-transform\b/ ||
+          headers[CACHE_CONTROL].to_s =~ /\bno-transform\b/ ||
          (headers['Content-Encoding'] && headers['Content-Encoding'] !~ /\bidentity\b/)
         return false
       end
diff --git a/lib/rack/directory.rb b/lib/rack/directory.rb
index 602f2d44..98d66e02 100644
--- a/lib/rack/directory.rb
+++ b/lib/rack/directory.rb
@@ -55,8 +55,8 @@ table { width:100%%; }
 
     def _call(env)
       @env = env
-      @script_name = env['SCRIPT_NAME']
-      @path_info = Utils.unescape(env['PATH_INFO'])
+      @script_name = env[SCRIPT_NAME]
+      @path_info = Utils.unescape(env[PATH_INFO])
 
       if forbidden = check_forbidden
         forbidden
@@ -72,7 +72,7 @@ table { width:100%%; }
       body = "Forbidden\n"
       size = Rack::Utils.bytesize(body)
       return [403, {"Content-Type" => "text/plain",
-        "Content-Length" => size.to_s,
+        CONTENT_LENGTH => size.to_s,
         "X-Cascade" => "pass"}, [body]]
     end
 
@@ -101,7 +101,7 @@ table { width:100%%; }
         @files << [ url, basename, size, type, mtime ]
       end
 
-      return [ 200, {'Content-Type'=>'text/html; charset=utf-8'}, self ]
+      return [ 200, { CONTENT_TYPE =>'text/html; charset=utf-8'}, self ]
     end
 
     def stat(node, max = 10)
@@ -130,7 +130,7 @@ table { width:100%%; }
       body = "Entity not found: #{@path_info}\n"
       size = Rack::Utils.bytesize(body)
       return [404, {"Content-Type" => "text/plain",
-        "Content-Length" => size.to_s,
+        CONTENT_LENGTH => size.to_s,
         "X-Cascade" => "pass"}, [body]]
     end
 
diff --git a/lib/rack/etag.rb b/lib/rack/etag.rb
index fefe671f..88973131 100644
--- a/lib/rack/etag.rb
+++ b/lib/rack/etag.rb
@@ -11,6 +11,7 @@ module Rack
   # used when Etag is absent and a directive when it is present. The first
   # defaults to nil, while the second defaults to "max-age=0, private, must-revalidate"
   class ETag
+    ETAG_STRING = 'ETag'.freeze
     DEFAULT_CACHE_CONTROL = "max-age=0, private, must-revalidate".freeze
 
     def initialize(app, no_cache_control = nil, cache_control = DEFAULT_CACHE_CONTROL)
@@ -28,14 +29,14 @@ module Rack
         body = Rack::BodyProxy.new(new_body) do
           original_body.close if original_body.respond_to?(:close)
         end
-        headers['ETag'] = %(W/"#{digest}") if digest
+        headers[ETAG_STRING] = %(W/"#{digest}") if digest
       end
 
-      unless headers['Cache-Control']
+      unless headers[CACHE_CONTROL]
         if digest
-          headers['Cache-Control'] = @cache_control if @cache_control
+          headers[CACHE_CONTROL] = @cache_control if @cache_control
         else
-          headers['Cache-Control'] = @no_cache_control if @no_cache_control
+          headers[CACHE_CONTROL] = @no_cache_control if @no_cache_control
         end
       end
 
@@ -53,8 +54,8 @@ module Rack
       end
 
       def skip_caching?(headers)
-        (headers['Cache-Control'] && headers['Cache-Control'].include?('no-cache')) ||
-          headers.key?('ETag') || headers.key?('Last-Modified')
+        (headers[CACHE_CONTROL] && headers[CACHE_CONTROL].include?('no-cache')) ||
+          headers.key?(ETAG_STRING) || headers.key?('Last-Modified')
       end
 
       def digest_body(body)
diff --git a/lib/rack/file.rb b/lib/rack/file.rb
index c8f8d0d1..bdae6a66 100644
--- a/lib/rack/file.rb
+++ b/lib/rack/file.rb
@@ -34,11 +34,11 @@ module Rack
     F = ::File
 
     def _call(env)
-      unless ALLOWED_VERBS.include? env["REQUEST_METHOD"]
+      unless ALLOWED_VERBS.include? env[REQUEST_METHOD]
         return fail(405, "Method Not Allowed", {'Allow' => ALLOW_HEADER})
       end
 
-      path_info = Utils.unescape(env["PATH_INFO"])
+      path_info = Utils.unescape(env[PATH_INFO])
       clean_path_info = Utils.clean_path_info(path_info)
 
       @path = F.join(@root, clean_path_info)
@@ -58,19 +58,19 @@ module Rack
 
     def serving(env)
       if env["REQUEST_METHOD"] == "OPTIONS"
-              return [200, {'Allow' => ALLOW_HEADER, 'Content-Length' => '0'}, []]
+              return [200, {'Allow' => ALLOW_HEADER, CONTENT_LENGTH => '0'}, []]
       end
       last_modified = F.mtime(@path).httpdate
       return [304, {}, []] if env['HTTP_IF_MODIFIED_SINCE'] == last_modified
 
       headers = { "Last-Modified" => last_modified }
       mime = Mime.mime_type(F.extname(@path), @default_mime)
-      headers["Content-Type"] = mime if mime
+      headers[CONTENT_TYPE] = mime if mime
 
       # Set custom headers
       @headers.each { |field, content| headers[field] = content } if @headers
 
-      response = [ 200, headers, env["REQUEST_METHOD"] == "HEAD" ? [] : self ]
+      response = [ 200, headers, env[REQUEST_METHOD] == "HEAD" ? [] : self ]
 
       # NOTE:
       #   We check via File::size? whether this file provides size info
@@ -97,7 +97,7 @@ module Rack
         size = @range.end - @range.begin + 1
       end
 
-      response[1]["Content-Length"] = size.to_s
+      response[1][CONTENT_LENGTH] = size.to_s
       response
     end
 
@@ -122,8 +122,8 @@ module Rack
       [
         status,
         {
-          "Content-Type" => "text/plain",
-          "Content-Length" => body.size.to_s,
+          CONTENT_TYPE   => "text/plain",
+          CONTENT_LENGTH => body.size.to_s,
           "X-Cascade" => "pass"
         }.merge!(headers),
         [body]
diff --git a/lib/rack/handler.rb b/lib/rack/handler.rb
index b5fa0268..f11e37e3 100644
--- a/lib/rack/handler.rb
+++ b/lib/rack/handler.rb
@@ -51,7 +51,7 @@ module Rack
         options.delete :Port
 
         Rack::Handler::FastCGI
-      elsif ENV.include?("REQUEST_METHOD")
+      elsif ENV.include?(REQUEST_METHOD)
         Rack::Handler::CGI
       elsif ENV.include?("RACK_HANDLER")
         self.get(ENV["RACK_HANDLER"])
diff --git a/lib/rack/handler/cgi.rb b/lib/rack/handler/cgi.rb
index 15af1ac2..78e135e9 100644
--- a/lib/rack/handler/cgi.rb
+++ b/lib/rack/handler/cgi.rb
@@ -26,7 +26,7 @@ module Rack
                      "rack.url_scheme" => ["yes", "on", "1"].include?(ENV["HTTPS"]) ? "https" : "http"
                    })
 
-        env["QUERY_STRING"] ||= ""
+        env[QUERY_STRING]   ||= ""
         env["HTTP_VERSION"] ||= env["SERVER_PROTOCOL"]
         env["REQUEST_PATH"] ||= "/"
 
diff --git a/lib/rack/handler/fastcgi.rb b/lib/rack/handler/fastcgi.rb
index b26fabc3..5137992b 100644
--- a/lib/rack/handler/fastcgi.rb
+++ b/lib/rack/handler/fastcgi.rb
@@ -59,7 +59,7 @@ module Rack
                      "rack.url_scheme" => ["yes", "on", "1"].include?(env["HTTPS"]) ? "https" : "http"
                    })
 
-        env["QUERY_STRING"] ||= ""
+        env[QUERY_STRING]   ||= ""
         env["HTTP_VERSION"] ||= env["SERVER_PROTOCOL"]
         env["REQUEST_PATH"] ||= "/"
         env.delete "CONTENT_TYPE"  if env["CONTENT_TYPE"] == ""
diff --git a/lib/rack/handler/lsws.rb b/lib/rack/handler/lsws.rb
index 1dee78a3..aec27322 100644
--- a/lib/rack/handler/lsws.rb
+++ b/lib/rack/handler/lsws.rb
@@ -27,7 +27,7 @@ module Rack
           "rack.url_scheme" => ["yes", "on", "1"].include?(ENV["HTTPS"]) ? "https" : "http"
         )
 
-        env["QUERY_STRING"] ||= ""
+        env[QUERY_STRING]   ||= ""
         env["HTTP_VERSION"] ||= env["SERVER_PROTOCOL"]
         env["REQUEST_PATH"] ||= "/"
         status, headers, body = app.call(env)
diff --git a/lib/rack/handler/mongrel.rb b/lib/rack/handler/mongrel.rb
index 20be86b1..ab9891b1 100644
--- a/lib/rack/handler/mongrel.rb
+++ b/lib/rack/handler/mongrel.rb
@@ -78,7 +78,7 @@ module Rack
 
                      "rack.url_scheme" => ["yes", "on", "1"].include?(env["HTTPS"]) ? "https" : "http"
                    })
-        env["QUERY_STRING"] ||= ""
+        env[QUERY_STRING] ||= ""
 
         status, headers, body = @app.call(env)
 
diff --git a/lib/rack/handler/scgi.rb b/lib/rack/handler/scgi.rb
index 40e86fb9..9a465eae 100644
--- a/lib/rack/handler/scgi.rb
+++ b/lib/rack/handler/scgi.rb
@@ -35,9 +35,9 @@ module Rack
         env = Hash[request]
         env.delete "HTTP_CONTENT_TYPE"
         env.delete "HTTP_CONTENT_LENGTH"
-        env["REQUEST_PATH"], env["QUERY_STRING"] = env["REQUEST_URI"].split('?', 2)
+        env["REQUEST_PATH"], env[QUERY_STRING] = env["REQUEST_URI"].split('?', 2)
         env["HTTP_VERSION"] ||= env["SERVER_PROTOCOL"]
-        env["PATH_INFO"] = env["REQUEST_PATH"]
+        env[PATH_INFO] = env["REQUEST_PATH"]
         env["QUERY_STRING"] ||= ""
         env["SCRIPT_NAME"] = ""
 
diff --git a/lib/rack/handler/webrick.rb b/lib/rack/handler/webrick.rb
index 023d8b27..dcbf96ca 100644
--- a/lib/rack/handler/webrick.rb
+++ b/lib/rack/handler/webrick.rb
@@ -79,12 +79,12 @@ module Rack
                    })
 
         env["HTTP_VERSION"] ||= env["SERVER_PROTOCOL"]
-        env["QUERY_STRING"] ||= ""
-        unless env["PATH_INFO"] == ""
+        env[QUERY_STRING] ||= ""
+        unless env[PATH_INFO] == ""
           path, n = req.request_uri.path, env["SCRIPT_NAME"].length
-          env["PATH_INFO"] = path[n, path.length-n]
+          env[PATH_INFO] = path[n, path.length-n]
         end
-        env["REQUEST_PATH"] ||= [env["SCRIPT_NAME"], env["PATH_INFO"]].join
+        env["REQUEST_PATH"] ||= [env["SCRIPT_NAME"], env[PATH_INFO]].join
 
         status, headers, body = @app.call(env)
         begin
diff --git a/lib/rack/head.rb b/lib/rack/head.rb
index 72f3dbdd..f487254a 100644
--- a/lib/rack/head.rb
+++ b/lib/rack/head.rb
@@ -12,7 +12,7 @@ class Head
   def call(env)
     status, headers, body = @app.call(env)
 
-    if env["REQUEST_METHOD"] == "HEAD"
+    if env[REQUEST_METHOD] == HEAD
       [
         status, headers, Rack::BodyProxy.new([]) do
           body.close if body.respond_to? :close
diff --git a/lib/rack/lint.rb b/lib/rack/lint.rb
index 667c34a6..5cdef3f8 100644
--- a/lib/rack/lint.rb
+++ b/lib/rack/lint.rb
@@ -57,7 +57,7 @@ module Rack
       ## and the *body*.
       check_content_type status, headers
       check_content_length status, headers
-      @head_request = env["REQUEST_METHOD"] == "HEAD"
+      @head_request = env[REQUEST_METHOD] == "HEAD"
       [status, headers, self]
     end
 
@@ -278,7 +278,7 @@ module Rack
       check_hijack env
 
       ## * The <tt>REQUEST_METHOD</tt> must be a valid token.
-      assert("REQUEST_METHOD unknown: #{env["REQUEST_METHOD"]}") {
+      assert("REQUEST_METHOD unknown: #{env[REQUEST_METHOD]}") {
         env["REQUEST_METHOD"] =~ /\A[0-9A-Za-z!\#$%&'*+.^_`|~-]+\z/
       }
 
@@ -510,20 +510,20 @@ module Rack
           ## already present, in rack.hijack_io.
           io = original_hijack.call
           HijackWrapper.new(io)
-          ## 
+          ##
           ## rack.hijack_io must respond to:
           ## <tt>read, write, read_nonblock, write_nonblock, flush, close,
           ## close_read, close_write, closed?</tt>
-          ## 
+          ##
           ## The semantics of these IO methods must be a best effort match to
           ## those of a normal ruby IO or Socket object, using standard
           ## arguments and raising standard exceptions. Servers are encouraged
           ## to simply pass on real IO objects, although it is recognized that
           ## this approach is not directly compatible with SPDY and HTTP 2.0.
-          ## 
+          ##
           ## IO provided in rack.hijack_io should preference the
           ## IO::WaitReadable and IO::WaitWritable APIs wherever supported.
-          ## 
+          ##
           ## There is a deliberate lack of full specification around
           ## rack.hijack_io, as semantics will change from server to server.
           ## Users are encouraged to utilize this API with a knowledge of their
@@ -535,10 +535,10 @@ module Rack
           io
         end
       else
-        ## 
+        ##
         ## If rack.hijack? is false, then rack.hijack should not be set.
         assert("rack.hijack? is false, but rack.hijack is present") { env['rack.hijack'].nil? }
-        ## 
+        ##
         ## If rack.hijack? is false, then rack.hijack_io should not be set.
         assert("rack.hijack? is false, but rack.hijack_io is present") { env['rack.hijack_io'].nil? }
       end
@@ -557,7 +557,7 @@ module Rack
       ## <tt>rack.hijack</tt> to an object that responds to <tt>call</tt>
       ## accepting an argument that conforms to the <tt>rack.hijack_io</tt>
       ## protocol.
-      ## 
+      ##
       ## After the headers have been sent, and this hijack callback has been
       ## called, the application is now responsible for the remaining lifecycle
       ## of the IO. The application is also responsible for maintaining HTTP
@@ -566,7 +566,7 @@ module Rack
       ## HTTP/1.1, and not Connection:keep-alive, as there is no protocol for
       ## returning hijacked sockets to the web server. For that purpose, use the
       ## body streaming API instead (progressively yielding strings via each).
-      ## 
+      ##
       ## Servers must ignore the <tt>body</tt> part of the response tuple when
       ## the <tt>rack.hijack</tt> response API is in use.
 
@@ -579,7 +579,7 @@ module Rack
           original_hijack.call HijackWrapper.new(io)
         end
       else
-        ## 
+        ##
         ## The special response header <tt>rack.hijack</tt> must only be set
         ## if the request env has <tt>rack.hijack?</tt> <tt>true</tt>.
         assert('rack.hijack header must not be present if server does not support hijacking') {
diff --git a/lib/rack/lobster.rb b/lib/rack/lobster.rb
index 195bd945..b7d15278 100644
--- a/lib/rack/lobster.rb
+++ b/lib/rack/lobster.rb
@@ -12,7 +12,7 @@ module Rack
     I8jyiTlhTcYXkekJAzTyYN6E08A+dk8voBkAVTJQ==".delete("\n ").unpack("m*")[0])
 
     LambdaLobster = lambda { |env|
-      if env["QUERY_STRING"].include?("flip")
+      if env[QUERY_STRING].include?("flip")
         lobster = LobsterString.split("\n").
           map { |line| line.ljust(42).reverse }.
           join("\n")
@@ -26,7 +26,7 @@ module Rack
                  "<pre>", lobster, "</pre>",
                  "<a href='#{href}'>flip!</a>"]
       length = content.inject(0) { |a,e| a+e.size }.to_s
-      [200, {"Content-Type" => "text/html", "Content-Length" => length}, content]
+      [200, {CONTENT_TYPE => "text/html", CONTENT_LENGTH => length}, content]
     }
 
     def call(env)
diff --git a/lib/rack/methodoverride.rb b/lib/rack/methodoverride.rb
index 062f3d67..7d2b56eb 100644
--- a/lib/rack/methodoverride.rb
+++ b/lib/rack/methodoverride.rb
@@ -11,11 +11,11 @@ module Rack
     end
 
     def call(env)
-      if allowed_methods.include?(env["REQUEST_METHOD"])
+      if allowed_methods.include?(env[REQUEST_METHOD])
         method = method_override(env)
         if HTTP_METHODS.include?(method)
-          env["rack.methodoverride.original_method"] = env["REQUEST_METHOD"]
-          env["REQUEST_METHOD"] = method
+          env["rack.methodoverride.original_method"] = env[REQUEST_METHOD]
+          env[REQUEST_METHOD] = method
         end
       end
 
diff --git a/lib/rack/mock.rb b/lib/rack/mock.rb
index 3c02c1fe..217ae0f7 100644
--- a/lib/rack/mock.rb
+++ b/lib/rack/mock.rb
@@ -91,15 +91,15 @@ module Rack
 
       env = DEFAULT_ENV.dup
 
-      env["REQUEST_METHOD"] = opts[:method] ? opts[:method].to_s.upcase : "GET"
+      env[REQUEST_METHOD] = opts[:method] ? opts[:method].to_s.upcase : "GET"
       env["SERVER_NAME"] = uri.host || "example.org"
       env["SERVER_PORT"] = uri.port ? uri.port.to_s : "80"
-      env["QUERY_STRING"] = uri.query.to_s
-      env["PATH_INFO"] = (!uri.path || uri.path.empty?) ? "/" : uri.path
+      env[QUERY_STRING] = uri.query.to_s
+      env[PATH_INFO] = (!uri.path || uri.path.empty?) ? "/" : uri.path
       env["rack.url_scheme"] = uri.scheme || "http"
       env["HTTPS"] = env["rack.url_scheme"] == "https" ? "on" : "off"
 
-      env["SCRIPT_NAME"] = opts[:script_name] || ""
+      env[SCRIPT_NAME] = opts[:script_name] || ""
 
       if opts[:fatal]
         env["rack.errors"] = FatalWarner.new
@@ -108,10 +108,10 @@ module Rack
       end
 
       if params = opts[:params]
-        if env["REQUEST_METHOD"] == "GET"
+        if env[REQUEST_METHOD] == "GET"
           params = Utils.parse_nested_query(params) if params.is_a?(String)
-          params.update(Utils.parse_nested_query(env["QUERY_STRING"]))
-          env["QUERY_STRING"] = Utils.build_nested_query(params)
+          params.update(Utils.parse_nested_query(env[QUERY_STRING]))
+          env[QUERY_STRING] = Utils.build_nested_query(params)
         elsif !opts.has_key?(:input)
           opts["CONTENT_TYPE"] = "application/x-www-form-urlencoded"
           if params.is_a?(Hash)
diff --git a/lib/rack/recursive.rb b/lib/rack/recursive.rb
index b431d4dd..17d17dfd 100644
--- a/lib/rack/recursive.rb
+++ b/lib/rack/recursive.rb
@@ -14,8 +14,8 @@ module Rack
       @url = URI(url)
       @env = env
 
-      @env["PATH_INFO"] =       @url.path
-      @env["QUERY_STRING"] =    @url.query  if @url.query
+      @env[PATH_INFO] =         @url.path
+      @env[QUERY_STRING] =      @url.query  if @url.query
       @env["HTTP_HOST"] =       @url.host   if @url.host
       @env["HTTP_PORT"] =       @url.port   if @url.port
       @env["rack.url_scheme"] = @url.scheme if @url.scheme
@@ -39,7 +39,7 @@ module Rack
     end
 
     def _call(env)
-      @script_name = env["SCRIPT_NAME"]
+      @script_name = env[SCRIPT_NAME]
       @app.call(env.merge('rack.recursive.include' => method(:include)))
     rescue ForwardRequest => req
       call(env.merge(req.env))
@@ -51,8 +51,9 @@ module Rack
         raise ArgumentError, "can only include below #{@script_name}, not #{path}"
       end
 
-      env = env.merge("PATH_INFO" => path, "SCRIPT_NAME" => @script_name,
-                      "REQUEST_METHOD" => "GET",
+      env = env.merge(PATH_INFO => path,
+                      SCRIPT_NAME => @script_name,
+                      REQUEST_METHOD => "GET",
                       "CONTENT_LENGTH" => "0", "CONTENT_TYPE" => "",
                       "rack.input" => StringIO.new(""))
       @app.call(env)
diff --git a/lib/rack/request.rb b/lib/rack/request.rb
index 602d0d64..e6e46e9d 100644
--- a/lib/rack/request.rb
+++ b/lib/rack/request.rb
@@ -18,10 +18,10 @@ module Rack
     end
 
     def body;            @env["rack.input"]                       end
-    def script_name;     @env["SCRIPT_NAME"].to_s                 end
-    def path_info;       @env["PATH_INFO"].to_s                   end
+    def script_name;     @env[SCRIPT_NAME].to_s                   end
+    def path_info;       @env[PATH_INFO].to_s                     end
     def request_method;  @env["REQUEST_METHOD"]                   end
-    def query_string;    @env["QUERY_STRING"].to_s                end
+    def query_string;    @env[QUERY_STRING].to_s                  end
     def content_length;  @env['CONTENT_LENGTH']                   end
 
     def content_type
@@ -116,10 +116,10 @@ module Rack
     def delete?;  request_method == "DELETE"  end
 
     # Checks the HTTP request method (or verb) to see if it was of type GET
-    def get?;     request_method == "GET"     end
+    def get?;     request_method == GET       end
 
     # Checks the HTTP request method (or verb) to see if it was of type HEAD
-    def head?;    request_method == "HEAD"    end
+    def head?;    request_method == HEAD      end
 
     # Checks the HTTP request method (or verb) to see if it was of type OPTIONS
     def options?; request_method == "OPTIONS" end
@@ -173,7 +173,7 @@ module Rack
     # Content-Type header is provided and the request_method is POST.
     def form_data?
       type = media_type
-      meth = env["rack.methodoverride.original_method"] || env['REQUEST_METHOD']
+      meth = env["rack.methodoverride.original_method"] || env[REQUEST_METHOD]
       (meth == 'POST' && type.nil?) || FORM_DATA_MEDIA_TYPES.include?(type)
     end
 
diff --git a/lib/rack/response.rb b/lib/rack/response.rb
index 12536710..7f0c6b71 100644
--- a/lib/rack/response.rb
+++ b/lib/rack/response.rb
@@ -20,11 +20,13 @@ module Rack
   class Response
     attr_accessor :length
 
+    CHUNKED = 'chunked'.freeze
+    TRANSFER_ENCODING = 'Transfer-Encoding'.freeze
     def initialize(body=[], status=200, header={})
       @status = status.to_i
       @header = Utils::HeaderHash.new.merge(header)
 
-      @chunked = "chunked" == @header['Transfer-Encoding']
+      @chunked = CHUNKED == @header[TRANSFER_ENCODING]
       @writer  = lambda { |x| @body << x }
       @block   = nil
       @length  = 0
@@ -72,8 +74,8 @@ module Rack
       @block = block
 
       if [204, 205, 304].include?(status.to_i)
-        header.delete "Content-Type"
-        header.delete "Content-Length"
+        header.delete CONTENT_TYPE
+        header.delete CONTENT_LENGTH
         close
         [status.to_i, header, []]
       else
@@ -98,7 +100,7 @@ module Rack
       @length += Rack::Utils.bytesize(s) unless @chunked
       @writer.call s
 
-      header["Content-Length"] = @length.to_s unless @chunked
+      header[CONTENT_LENGTH] = @length.to_s unless @chunked
       str
     end
 
@@ -142,11 +144,11 @@ module Rack
       end
 
       def content_type
-        headers["Content-Type"]
+        headers[CONTENT_TYPE]
       end
 
       def content_length
-        cl = headers["Content-Length"]
+        cl = headers[CONTENT_LENGTH]
         cl ? cl.to_i : cl
       end
 
diff --git a/lib/rack/runtime.rb b/lib/rack/runtime.rb
index 1bd411fd..83d6c0ca 100644
--- a/lib/rack/runtime.rb
+++ b/lib/rack/runtime.rb
@@ -12,13 +12,14 @@ module Rack
       @header_name << "-#{name}" if name
     end
 
+    FORMAT_STRING = "%0.6f"
     def call(env)
       start_time = Time.now
       status, headers, body = @app.call(env)
       request_time = Time.now - start_time
 
       if !headers.has_key?(@header_name)
-        headers[@header_name] = "%0.6f" % request_time
+        headers[@header_name] = FORMAT_STRING % request_time
       end
 
       [status, headers, body]
diff --git a/lib/rack/sendfile.rb b/lib/rack/sendfile.rb
index 8e2c3d67..4a9b428b 100644
--- a/lib/rack/sendfile.rb
+++ b/lib/rack/sendfile.rb
@@ -116,7 +116,7 @@ module Rack
         when 'X-Accel-Redirect'
           path = F.expand_path(body.to_path)
           if url = map_accel_path(env, path)
-            headers['Content-Length'] = '0'
+            headers[CONTENT_LENGTH] = '0'
             headers[type] = url
             obody = body
             body = Rack::BodyProxy.new([]) do
@@ -127,7 +127,7 @@ module Rack
           end
         when 'X-Sendfile', 'X-Lighttpd-Send-File'
           path = F.expand_path(body.to_path)
-          headers['Content-Length'] = '0'
+          headers[CONTENT_LENGTH] = '0'
           headers[type] = path
           obody = body
           body = Rack::BodyProxy.new([]) do
diff --git a/lib/rack/server.rb b/lib/rack/server.rb
index d9f971d7..ea1ad82b 100644
--- a/lib/rack/server.rb
+++ b/lib/rack/server.rb
@@ -310,7 +310,7 @@ module Rack
 
         # Don't evaluate CGI ISINDEX parameters.
         # http://www.meb.uni-bonn.de/docs/cgi/cl.html
-        args.clear if ENV.include?("REQUEST_METHOD")
+        args.clear if ENV.include?(REQUEST_METHOD)
 
         options.merge! opt_parser.parse!(args)
         options[:config] = ::File.expand_path(options[:config])
diff --git a/lib/rack/showexceptions.rb b/lib/rack/showexceptions.rb
index 731aea49..60999e64 100644
--- a/lib/rack/showexceptions.rb
+++ b/lib/rack/showexceptions.rb
@@ -39,8 +39,8 @@ module Rack
       [
         500,
         {
-          "Content-Type" => content_type,
-          "Content-Length" => Rack::Utils.bytesize(body).to_s,
+          CONTENT_TYPE => content_type,
+          CONTENT_LENGTH => Rack::Utils.bytesize(body).to_s,
         },
         [body],
       ]
diff --git a/lib/rack/showstatus.rb b/lib/rack/showstatus.rb
index 6892a5b7..4426310a 100644
--- a/lib/rack/showstatus.rb
+++ b/lib/rack/showstatus.rb
@@ -3,7 +3,7 @@ require 'rack/request'
 require 'rack/utils'
 
 module Rack
-  # Rack::ShowStatus catches all empty responses and replaces them 
+  # Rack::ShowStatus catches all empty responses and replaces them
   # with a site explaining the error.
   #
   # Additional details can be put into <tt>rack.showstatus.detail</tt>
@@ -19,7 +19,7 @@ module Rack
     def call(env)
       status, headers, body = @app.call(env)
       headers = Utils::HeaderHash.new(headers)
-      empty = headers['Content-Length'].to_i <= 0
+      empty = headers[CONTENT_LENGTH].to_i <= 0
 
       # client or server error, or explicit message
       if (status.to_i >= 400 && empty) || env["rack.showstatus.detail"]
@@ -35,7 +35,7 @@ module Rack
 
         body = @template.result(binding)
         size = Rack::Utils.bytesize(body)
-        [status, headers.merge("Content-Type" => "text/html", "Content-Length" => size.to_s), [body]]
+        [status, headers.merge(CONTENT_TYPE => "text/html", CONTENT_LENGTH => size.to_s), [body]]
       else
         [status, headers, body]
       end
diff --git a/lib/rack/static.rb b/lib/rack/static.rb
index 41aec7f3..75e1e555 100644
--- a/lib/rack/static.rb
+++ b/lib/rack/static.rb
@@ -107,7 +107,7 @@ module Rack
     end
 
     def call(env)
-      path = env["PATH_INFO"]
+      path = env[PATH_INFO]
 
       if can_serve(path)
         env["PATH_INFO"] = (path =~ /\/$/ ? path + @index : @urls[path]) if overwrite_file_path(path)
diff --git a/lib/rack/urlmap.rb b/lib/rack/urlmap.rb
index d75f5a22..c62baecc 100644
--- a/lib/rack/urlmap.rb
+++ b/lib/rack/urlmap.rb
@@ -41,7 +41,7 @@ module Rack
     end
 
     def call(env)
-      path = env['PATH_INFO']
+      path = env[PATH_INFO]
       script_name = env['SCRIPT_NAME']
       hHost = env['HTTP_HOST']
       sName = env['SERVER_NAME']
@@ -66,7 +66,7 @@ module Rack
         return app.call(env)
       end
 
-      [404, {"Content-Type" => "text/plain", "X-Cascade" => "pass"}, ["Not Found: #{path}"]]
+      [404, {CONTENT_TYPE => "text/plain", "X-Cascade" => "pass"}, ["Not Found: #{path}"]]
 
     ensure
       env['PATH_INFO'] = path