Fifth rule.
Every entity lives in exactly one place.
A customer. A building. A contract. A request. A supplier. Each one has one home, and everything else points to it. Not a copy. A pointer.
The moment the same fact lives in two places, one of them is wrong. You just don’t know which yet. The phone number in the CRM says one thing, the spreadsheet says another, the email signature a third. Three people act on three truths and all of them are doing their job.
A man with one watch knows the time. A man with two is never sure.
This is not a software problem. It starts with a decision nobody wants to make: which system is the record for this entity? Say it out loud. Write it down. From now on, the contract lives here. If it’s not here, it isn’t signed. If the address changed, it changed here first.
Everything else becomes a view. The dashboard reads from the source. The report reads from the source. The chatbot reads from the source. Views can be wrong for a minute. The source can’t.
The word to be suspicious of is sync. When two systems need to be kept in sync, you’ve already lost. You have two sources, and a job that fails on weekends. Better to have one source and many readers.
It gets harder as you grow. Every new tool arrives with its own idea of what a customer is, and it wants to own it. Say no. The tool can read. It can’t own.
The payoff is quiet. Arguments about “which number is right” stop, because there’s only one number. When a fact is wrong, you fix it once. And every process built on top, every report and every automation, inherits the correction without knowing it happened.
One entity, one home. Everything else is a window into it.
