_iframe_redirect.erb.html

<script>parent.changeParentUrl('<%= url %>');</script>

document.rb

def post_process
  redis = Redis.new
  redis.set("pdf_progress:state:#{progress_token}", 'working')
  pages.each do |page|
    redis.set("pdf_progress:msg:#{progress_token}", "Converting page #{i} out of #{total}")
    # Actually post-process the file....
  end
  redis.set("pdf_progress:state:#{progress_token}", 'done')
rescue
  redis.set("pdf_progress:state:#{progress_token}", 'error')
end

documents_controller.rb

class DocumentsController
  def new
    @document = Document.new
    @uuid = UUID.generate :compact
  end

  def create
    progress_token = params[:document][:progress_token]
    @document = current_user.documents.new(params[:document])

    respond_to do |format|
      if @document.save
        format.html {
          render :partial => 'iframe_redirect', :locals => { :url => document_url(@document.token) }
        }
      end
    end
  end

  def progress
    progress_token = params[:token]
    redis = Redis.new
    render :json => {:state => redis.get("pdf_progress:state:#{progress_token}"),
                     :msg => redis.get("pdf_progress:msg:#{progress_token}")}
  end
end

new.erb.html

<script>
    var uuid = '<%= @uuid %>';
    var interval = null;
    var req = null;

    // Called when the user submits the form
    function startUpload() {
        interval = window.setInterval(function () {
            updateUploadProgress();
        }, 1000);
        return true;
    }

    // Helper since parent document.location is read-only from iframe
    function changeParentUrl(url)
    {
        document.location = url;
    }

    // Update the upload progress bar each second
    function updateUploadProgress() {
        if (req != null)
            req.abort();

        req = new XMLHttpRequest();
        req.open("GET", "/documents/progress/upload", 1);
        req.setRequestHeader("X-Progress-ID", uuid);
        req.onreadystatechange = function () {
            if (req.readyState == 4 && req.status == 200) {
                var upload = eval(req.responseText);

                document.getElementById('message').innerHTML = "Uploading...";

                if (upload.state == 'done' || upload.state == 'uploading') {
                    bar = document.getElementById('progressbar');
                    w = 400 * 0.30 * upload.received / upload.size;
                    bar.style.width = w + 'px';
                }

                if (upload.received == upload.size || upload.state == 'done' || upload.state == 'error') {
                    window.clearTimeout(interval);
                    interval = window.setInterval(function () {
                        updateConversionProgress();
                    }, 1000);
                }
            }
        }
        req.send(null);
    }

    // Inform the user about the conversion progress
    function updateConversionProgress() {
        if (req != null)
            req.abort();

        req = new XMLHttpRequest();
        req.open("GET", "/documents/progress?token=" + uuid, 1);
        req.onreadystatechange = function () {
            if (req.readyState == 4 && req.status == 200) {
                var upload = eval('new Object(' + req.responseText + ')');

                document.getElementById('message').innerHTML = upload.msg;

                if (upload.state == 'done' || upload.state == 'error') {
                    window.clearTimeout(interval);
                }
            }
        }
        req.send(null);
    }
</script>

<% form_for(@document, :url => "/documents/?X-Progress-ID=#{@uuid}", :html => {:multipart => true, :target => "uploadframe",
                                                                               :onsubmit => "startUpload();"}) do |f| %>
    <%= f.error_messages %>
    <%= f.hidden_field 'progress_token', :value => @uuid %>
    <%= f.file_field :pdf_file %>
    <%= f.submit 'Upload' %>
<% end %>

<div id="progress" style="width: 400px; background-color: silver;">
    <div id="progressbar" style="width: 1px; height: 10px; background-color: black;">
    </div>
</div>
<div id="message"></div>

<iframe id="uploadframe" name="uploadframe" width="100%" height="100%" frameborder="0" border="0" src="about:blank"></iframe>

nginx.conf

http {
    upload_progress pdf_uploads 256k;

    server {
        passenger_enabled on;
        passenger_use_global_queue on;

        location ^~ /documents/ {
          # Set the size limit to 100m for big uploads
          client_max_body_size 100m;

          # We need to repeat that line so that track_uploads works
          # (track_uploads must be the last registered handler, 
          #  so we need to register Passenger before track_uploads)
          passenger_enabled on;

          # Tracking data expires 30s after the upload finishes.
          track_uploads pdf_uploads 30s;
        }

        # report tracked uploads
        location ^~ /documents/progress/upload {
          report_uploads pdf_uploads;
        }
    }
}