The X-Accel-Redirect HTTP header is used on Nginx servers to allow applications to serve static files directly from Nginx. For those familiar with X-Sendfile, X-Accel-Redirect is the Nginx implementation. Using this header to serve static files from a Rails application has some great benefits, like not requiring a file to be streamed through the Passenger and Rails stacks - which can be costly. The setup is quite simple.

  1. Create a controller to serve the file. 
    class BlogsController < ApplicaitonController
  def show
    @blog = Blog.find(params[:id])
    response.headers["X-Accel-Redirect"] = @blog.path
    response.headers["Content-Type"] = "application/json"
    render :nothing => true
  end
end
  2. Write an nginx location block into nginx.conf. server { ... location /blogs/ { internal; root /data; } }
  3. Serve the file. 
    blog = Blog.create(:path => "/blogs/my_first_blog.json")
uri = URI.parse("http://localhost:3000/blogs/#{blog.id}.json")
Net::HTTP.get uri

The request goes something like this:

  • The HTTP GET request goes to Nginx, Passenger, and finally to your Rails BlogsConroller#show action.
  • The BlogsController#show action responds with two HTTP headers (X-Accel-Redirect and Content-Type).
  • The X-Accel-Redirect header is trapped on the return trip by Nginx.
  • Nginx matches the X-Accel-Redirect header with the location /blogs/ block.
  • Nginx serves the file directly from /data/blogs/my_first_blog.json.
  • Nginx also persists the Content-Type HTTP header and passes it to the client.

An additional benefit of this method is that you can use the controller to check file permissions. For example:

class BlogsController < ApplicaitonController
  def show
    @blog = Blog.find(params[:id])
    if current_user.can_access?(@blog)
      response.headers["X-Accel-Redirect"] = @blog.path
      response.headers["Content-Type"] = "application/json"
      render :nothing => true
    else
      redirect_to permission_denied
    end
  end
end