What the till does when the endpoint cannot be reached
An integration of this kind is judged during an outage rather than on a good day. Connectivity at a retail site fails for ordinary reasons: a cut fibre, a router restart, a power event, a provider incident on a Saturday afternoon with a queue at every counter. The design requirement is that trading continues and that a complete, ordered record survives the interruption in a form that can be transmitted and verified once the connection returns. Anything that stops the till instead is an operational failure, whatever else it achieves.
Queue behaviour is where the engineering effort belongs. The local store must survive a power cut rather than living in memory, each transaction needs an identity assigned at the till before any transmission is attempted, and the receipt has to be unambiguous about the state of the sale so that neither the cashier nor the customer is guessing. On recovery, transmit in the order raised, at a rate the service tolerates, with queue depth shown on a screen a supervisor actually looks at.
Recovery introduces the risk of reporting the same sale twice, because a transmission can succeed while its acknowledgement is lost in transit. Resubmission logic therefore keys on the local identity so a repeat is recognised rather than sent again, and every attempt is logged with the response received. Our boundary is the same throughout this work. We design for continuity and evidence. Whether any delay in transmission carries a consequence for the business is a question for the client's tax adviser, and the answer belongs in the runbook before it is needed.
- A durable local queue that survives power loss, with an identity assigned at the till
- Ordered transmission on recovery, paced so the service is not flooded by a backlog
- Queue depth displayed where a supervisor can see it during trading hours
- Resubmission keyed on local identity, so a lost acknowledgement does not report a sale twice
- Every attempt and response logged, giving a defensible record of what was sent and when