If the network is down each POS station should still be able to function without interruption.
At the moment the current POS system uses an inhouse developed message queing system to communicate with the ERP/Retail backend. A POS station submits each transaction (and other relevant messages) to a local queue, from where it is sent to the back-end system. If the network is down the messages just stay queued until the network is back up again.
The central backend system uses the same queing technique to submit price updates etc. to each POS station.
You may want to check out the Spread Toolkit, at www.spread.org, whose Python wrapper lives at http://www.python.org/other/spread/doc.html. It's used by the Zope Replication Service, for example.
Cheers,
Evan @ 4-am
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