In our web application, for each http-request there is a lot of computation that happens on back end. Output can vary from 10 sec - 1 Hour. In the mean time when it is computed, "Waiting.." is shown on the website for the respective user.

But it so happens, that a user might cut down the service in between. So what all can be done on the back end so that the computation can be stopped in between to save resources? What different tactics can be applied here?

And if better (instead of killing the thread directly), then a graceful termination policy should make wonders.