SQL-Ledger offers a nice inventory control module (a.k.a. Goods & Services) that provides inventory count, and making its data available to our online store allows real-time inventories and more. For our servers that are at different locations, this could be done by querying the SL server in real time, but a lost of communications could disable our production system in this synchronous setup. So we use Slony-I to replicate data.