Streaming Files in Rails

Rails has send_data and send_file. The latter works with blobs of data and the former works with files on the server. Blobs of data only work if you already have that in memory and files breakdown once you have multiple servers. What if you want to stream out to the browser as you read in from the database?

You can create a streamer object to hand off data as you fetch it from the database. You just need to set the response body to an object that response to each.

Put this in your controller:

def download
  report = StoredReport.first

  headers["Content-Type"] = "text/csv"
  headers["Content-disposition"] = "attachment; filename=test.csv"

  self.response_body = StoredReportStreamer.new(report)
end

Be sure to include the header stuff to mark it as the correct type and as a download.

Then for the streamer object use this:

class StoredReportStreamer
  attr_accessor :report

  def initialize(report)
    self.report = report
  end

  def each
    report.rows.each do |row|
      yield row
    end
  end
end

That will yield each row of the report out to the browser.

Thanks to this blog post for the tip: http://blog.sparqcode.com/2012/02/04/streaming-data-with-rails-3-1-or-3-2/