pos-systemslisted

# POS systems A POS must keep selling when the network, the server or a device fails, and every sale must end up correct in the books and with the tax authority. Load payments-and-money too. It governs amounts, idempotency and card data. ## Offline-first - **The local store is the source of truth at the till.** Sales, receipts and shift state are written durably on the device first (SQLite/Room/Core Data, with WAL and transactions), then synced. - **Sync queue:** - operations get client-generated UUIDs and are idempotent on the server; - retry with backoff; - the server keeps an ordering per device; - conflicts resolve by explicit rules (catalog: server wins; sales: append-only, never overwritten). - **Time:** the device clock drifts. Store both the device time and the server receive time. Never trust device time for security decisions. - **Catalog and prices** are cached with a version. The receipt snapshots the price, tax and discount that applied at sale time. - Test by killing the process mid-sale, cutting the network mid-payment, filling the disk, and restoring from backup. ## Fiscalization and receipts - **Receipts are immutable.** Mistakes are fixed with a refund or correction receipt, never by editing. Fiscal documents are numbered sequentially without gaps. - The fiscal rules depend on the jurisdiction. **Identify it and ask.** In Russia, 54-ФЗ applies: - an online cash register (ККТ) with a fiscal drive (ФН); - transmission through an ОФД; - the f