Offline-first POS: designing for the worst network day

A point-of-sale system is judged on its worst day, not its best. When the network drops mid-sale, the till still has to ring up the order. We design POS software offline-first: every transaction is written locally and queued, then reconciled against the server the moment connectivity returns, with conflict rules that never double-charge or lose a sale. This post breaks down the sync model, the local store, and how we test it by pulling the plug on purpose.