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PORails/Dragonfly/Apache - Rack::Cache - how to use X-Sendfile?
 

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    <p>I'm using <a href="https://github.com/markevans/dragonfly" rel="nofollow">Dragonfly</a> to serve processed images for my Rails app. Dragonfly relies on <a href="https://github.com/rtomayko/rack-cache" rel="nofollow">Rack::Cache</a> for future visits to those processed images, so that <a href="http://markevans.github.com/dragonfly/file.Caching.html" rel="nofollow">Dragonfly won't have to process those images again</a> and again, thus wasting CPU time.</p> <p>My problem starts here: if I'm right that sending a file via Rack::Cache still busies a Rails process, then viewing a page of 30 images, even if these images have a small file size, will tie up the Rails processes pretty quickly. If a couple more visitors come to see that page, then they will experience very slow response times. How do I get these files served via X-Sendfile?</p> <p>I've set the following in <code>production.rb</code>, but I know these are for the assets from Rails, not the Dragonfly files:</p> <pre><code>config.serve_static_assets = false config.action_dispatch.x_sendfile_header = "X-Sendfile" </code></pre> <p>I know that Rack::Cache somehow supports X-Sendfile (probably through <a href="https://github.com/rack/rack/blob/master/lib/rack/sendfile.rb" rel="nofollow">Rack::Sendfile</a>) because it <a href="https://github.com/rtomayko/rack-cache/blob/master/test/entitystore_test.rb#L124" rel="nofollow">produces a body that responds to <code>#to_path</code></a>. However, I don't know how to enable this. When I check files that come from Rack::Cache, I don't see any X-Sendfile information:</p> <pre><code>Date: Wed, 02 Nov 2011 11:38:28 GMT Server: Apache/2.2.3 (CentOS) X-Powered-By: Phusion Passenger (mod_rails/mod_rack) 3.0.9 Content-Disposition: filename="2.JPG" Cache-Control: public, max-age=31536000 Etag: "3174d486e4df2e78a5ff9174cacbede5787d4660" X-Content-Digest: c174408eda6e689998a40db0aef4cdd2aedb3b6c Age: 28315 X-Rack-Cache: fresh Content-Length: 22377 Status: 200 Content-Type: image/jpeg </code></pre> <p>I know, based on <a href="https://github.com/rails/rails/issues/1822" rel="nofollow">posts around the net</a>, that I'm supposed to see something like:</p> <pre><code>X-Sendfile: /path/to/file </code></pre> <p>In the end I don't know if its Dragonfly or Rack::Cache (or both) that I have to configure. <strong>How do I get either Dragonfly and/or Rack::Cache to serve files via X-Sendfile</strong>?</p> <p>Info about my setup:</p> <ul> <li>Rails 3.1.1</li> <li>Passenger 3.0.9</li> <li>CentOS</li> <li>Sendfile module is installed, as far as I know. I have <code>XSendFile On</code> and <code>XSendFilePath /path/to/app</code> specified in my virtualhost configuration, and Apache doesn't complain about the directive <code>XSendFile</code> not existing.</li> </ul> <p>Thanks!</p> <p><strong>UPDATE Nov 6, 2011</strong></p> <p>Based on <a href="https://groups.google.com/d/topic/rack-cache/goQ6uAnVGL8/discussion" rel="nofollow">this old update</a>, as long as <code>Rack::Sendfile</code> is placed in front of <code>Rack::Cache</code>, then X-Sendfile will be used. I did that, and this is how <a href="https://gist.github.com/1339975" rel="nofollow">my middleware looks like</a>. The files, however, <a href="https://gist.github.com/1339985" rel="nofollow">still don't have the X-Sendfile tag</a>. Again, I don't know if that is a sure-fire way of determining if X-Sendfile is enabled, so I checked the Passenger queue. It seems that the queue is greatly encumbered when I visit a page.</p> <p><strong>UPDATE Nov 7, 2011</strong></p> <p>It seems this is purely a Rack::Cache and Rails 3.1 issue. While Rack::Cache supports the use of X-Sendfile through Rack::Sendfile (like I mentioned above, Rack::Cache, when using the <a href="https://github.com/rtomayko/rack-cache/blob/master/lib/rack/cache/entitystore.rb#L83" rel="nofollow">Disk</a> EntityStore since that responds_to <code>to_path</code> since the <a href="https://github.com/rtomayko/rack-cache/blob/master/lib/rack/cache/entitystore.rb#L105" rel="nofollow">body it returns is a subclass of File</a>), Rails 3.1 uses its own storage solution. Rails 3.1 uses <a href="https://github.com/rails/rails/blob/3-1-stable/activesupport/lib/active_support/cache/file_store.rb#L81" rel="nofollow">ActiveSupport::Cache::FileStore</a>, which is set by default, if you don't specify anything in your <code>production.rb</code> file.</p> <p>The problem with FileStore is that the body it returns, to be part of the response to be sent upstream, because that body doesn't respond to <code>to_path</code>. The body is an instance of <a href="https://github.com/rails/rails/blob/3-1-stable/activesupport/lib/active_support/cache.rb#L532" rel="nofollow">ActiveSupport::Cache::Entry</a>. You can see <a href="https://github.com/rails/rails/blob/3-1-stable/activesupport/lib/active_support/cache/file_store.rb#L81" rel="nofollow">here</a> that when the FileStore is asked to read a cached file, it reads it via <code>File.open('/path/to/file') {|f| Marshal.load(f) }</code> which returns an instance of Entry. The value that ultimately gets passed upstream and back to the client, is <a href="https://github.com/rails/rails/blob/3-1-stable/activesupport/lib/active_support/cache.rb#L324" rel="nofollow">Entry#value</a>.</p> <p><strong>My questions</strong></p> <p>To help me decide whether I should patch this, or to get Rails to use Rack::Cache's own Disk store instead, I have some questions:</p> <ol> <li>What's the reason Rack::Cache's own storage solutions weren't used for Rails 3.1? Why does Rails have its own?</li> <li>Is there a reason Marshal is used? Is there a reason that a bytestream of data should be sent back instead?</li> </ol> <p>I got in deeper than I usually go, and will be surprised if I understood things correctly. I hope to find an answer!</p>
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    1. COI've asked around, send an email to Rails user group, but haven't found an answer. Yes, Varnish sounded like overkill at the beginning but to make all the changes I need to get X-Sendfile working, it'd be simpler to install Varnish.
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