Single source of truth

A single source of truth is one designated place where a given fact is maintained, so that every person and system reads the same authoritative value rather than a copy of it.

In practice almost no organisation has one source of truth for everything, and chasing that goal wastes years. The CRM is authoritative for customer state, the HR system for who works here, the ticketing system for delivery. Each is a source of truth for its own domain.

The gap is the reasoning between them. Why this customer is on a non-standard rate, why that integration was never switched on, which exception was approved and by whom. That reasoning is rarely the system of record's job, so it ends up in threads and in people's heads.

The realistic aim is one source of truth per domain plus one governed place for the reasoning that connects them, with every answer naming the record it came from.