At a mountain attraction, the network can disappear for the length of a ride. The POS still has to sell.
Local first
Every sale is written to local storage before anything touches the network. The ticket is issued from the device, with a locally generated identifier that cannot collide with another terminal.
Sync when we can
A queue replays pending sales to the server once the connection returns. Each sale is idempotent, so a retry never creates a duplicate.
Honest status
The agent always sees whether the terminal is online and how many sales are waiting to sync. No surprises at the end of the day.