- Written agreement on who owns each record
- Before code, we list every record type and which system is its master, and ask your finance or operations lead to confirm it. Most integration trouble begins with two systems that both think they own a customer.
- Safe to repeat
- Timeouts happen after the other side has acted. Each record keeps the other system's ID, and a retry checks before creating, so a dropped connection doesn't produce a second invoice.
- Failures reach someone who can act
- A missing tax code goes to finance in plain language with a retry button, and doesn't sit in a developer's log until month end.
- A check that proves the two sides agree
- Scheduled comparisons of totals between systems catch drift early. On a UK telecom billing platform we work on, this takes the form of nightly balance checks and a month-end checklist.