• Apache 2
  • ModRewrite
  • Ruby on Rails

When you have a form to upload a file, you may want to limit the file size. If you check for the size within the application - we have already lost the battle.

The better way (IMHO) is to use Apache ModRewrite to check for the size of a POST request and redirect with GET to error handling action

RewriteCond %{REQUEST_METHOD} POST
RewriteCond %{HTTP:Content-type} multipart/form-data
RewriteCond %{HTTP:Content-length}  >500000
RewriteRule ^(.*?)$ %{REQUEST_URI}?form_data_too_big=on [R=303,L]

How it works

  1. Only POST requests
  2. Only mutipart/form-data POST
  3. Check HTTP Request header against limit. Add MIME envelop overhead (5Kb for example)
  4. Redirect with GET to the same URL with one parameter

Let say you have a controller with action process_upload, and the URL for the action is http://www.myserver.tld/file/process_upload.

def process_upload
  if request.get? && params[:form_data_too_big]
    return error_response_for_file_to_big
  end
 ....
end

In that way you can work with respond_to_parent or json or javascript