This will cause problems when concurrent downloads occur (see http://grails.org/doc/latest/ref/Services/scope.html).  Instead of instance variables, each call in to this service should create a "context map" containing this set of variables, and operate on that.

Also, the fact that there are quite a few of these vars is a bit of a code smell - you might be able to kill two birds here by introducing a `DownloadStats` class (or some such, you can probably come up with a better name).