On restarting the Site Server service, do active Site Server jobs fail?
The jobs don't fail but, on restarting the service, the existing/active jobs restart from the beginning again. This is because Site Server does not know where to pick up the job again.
For example: If the existing job 'A' collected 200 files and the service was restarted, then the job 'A' would kick off again from the beginning and collect the data into a new AD1. The previous AD1 is of no use and never gets processed or inserted into the application.
This information is helpful to understand how recycling the Site Server service (or all services) affect ongoing jobs.