When the internet drops, the cashier keeps selling. Sales upload once the connection is back, and never twice.

What works offline and what needs a connection

Works without internet: selling at the cashier by scan or from the grid, invoices, purchase orders, receipt printing, and the stock deduction on the device. Waits for a connection: the financial reports, the morning brief and its cards, Dash, syncing between branches, and signing in itself. Closing a shift also waits until every sale on the device has uploaded, so no drawer is closed on incomplete numbers.

How a sale uploads

The upload queue sends transactions in batches of 25, and only one browser tab uploads at a time even when the app is open in several. If a send fails it retries after 200 milliseconds and doubles the wait up to a cap of 30 seconds, with random jitter so every device does not hit the server at the same moment. The reply "already exists" counts as success. A transaction the server refuses for a reason inside the transaction itself, such as an item with zero stock in a shop that does not allow negative stock, is not retried: it appears in the recovery drawer with the reason, and the owner decides what to do with it.

Protecting what is on the device

• The app asks the browser for persistent storage, so pending sales are not evicted when space runs low
• At 80% of the available storage it warns you; at 95% it locks input until sales have uploaded and usage falls back to 85%, so a half-written sale can never occur
• The time of a sale comes from the server's clock rather than the device's: the offset is saved at the first connection, so a device with the wrong date cannot misdate sales
• Stock is deducted on the device immediately and reconciled with the server at upload

For how many days?

There is no time limit; it works until the browser's storage on the device fills up. One sale is a few kilobytes, so a full day of selling in a busy shop comes to under one megabyte.