single source of truth
expressionops cultureprogramming
The principle that each fact should have exactly one authoritative home, with everything else derived from it.
Two authoritative spreadsheets is how organizations institutionalize Segal's law. In practice SSOT means declaring which system wins - the IPAM, the repo, the CMDB - and making every copy provably downstream. Configuration drift is what happens when nobody declared.
A single source of truth is one authoritative record for a fact, with everything else deriving from it rather than maintaining its own copy. The alternative is several places holding the same fact, drifting apart, and nobody knowing which is right.
The failure it prevents is specific and familiar. An employee's department lives in the HR system, the directory, the ticketing tool and a spreadsheet; three of them are stale, and every downstream decision that reads the wrong one is wrong in a way that is hard to trace. The same shape produces conflicting asset inventories, contradictory network diagrams and the deployment that used the other config file.
The discipline is choosing the authority deliberately and deriving everything else, which is harder than it sounds because it means someone loses the ability to edit their local copy. It is also worth holding alongside Segal's law: a single source of truth is the right pattern for a fact you control, like who works here, and the wrong instinct for a measurement of the physical world, where a second independent reading is how you learn the first one is wrong.
Also known as: SSOT