Single Source of Truth (SSOT)

What the principle actually requires

For each field, one system is authoritative and every other copy defers to it. The common misreading — that everything should live in one platform — is neither achievable nor necessary, and pursuing it usually delays the decision that would actually help.

Building the map

1. List the fields that appear in more than one system. This alone is usually revealing and takes less time than expected.

2. For each, identify where the value is created or verified by the party who owns it. Authority should follow the point of truth, not convenience.

3. Name the authority explicitly and record it somewhere durable. An unwritten convention is not a governance decision.

4. Define the direction of sync and what happens on conflict. "Last write wins" is a choice; it should be a deliberate one.

5. Instrument divergence. Sample records periodically and count where copies disagree with the authority. Divergence rate is the health measure for the whole arrangement.

Why this matters more than tooling

Where no authority is designated, disagreement between systems is resolved by whoever argues most persuasively or most recently. The same disagreement then recurs, because nothing was decided — only settled. Designating authority converts a recurring argument into a lookup.

Where it goes wrong

• Declared but not enforced: an authority named in a document while integrations continue to write both ways.

• Chosen for convenience rather than provenance, so the authoritative system holds a stale copy of a value maintained properly elsewhere.

• Applied at system granularity, producing an unwinnable argument about which platform is 'the' source when the answer differs field by field.

• Set once and never revisited, so an acquisition or a new billing system silently creates a second authority.