Well, the system should be transactional in the sense that it needs to
fetch the data and do something with it, and if it fails along the way,
it should roll back what it was trying to do, and consider the data
'not fetched'.  The super simple real world example: the web server in
the hosting farm needs to send a fax.  A computer in the office has a
fax machine.  Once the application code has handed off the fax to the
transport mechanism, it needs to be absolutely sure that at some point
the fax is handed off to the office machine, and put in the send queue
(although at that point I guess it's up to the fax software and we
can't really control if the fax actually arrives where it's supposed
to, but we get regular reports from it, so I guess we're ok from that
point of view).

There are a few other processes of this type - sending data from the
office to the server, for instance.  They're all asynch, and all need
to be atomic.

Thanks
Dave