When connecting POS and eCommerce, how stock update be managed between 2 systems?

Stock are managed in detail mainly in POS systems, stocks can be divided location wise and warehouse wise. So when POS and eCommerce is integrated, stock should sync from POS to eCommerce according to business requirement. eCommerce can show a combined stock; if eCommerce has location wise stores, then it can store location wise inventory. But always POS should maintain stock/inventory and that should sync to eCommerce.