The single source of truth

DRY — Don't Repeat Yourself — is misremembered as a rule against typing the same lines twice. Its real claim is about knowledge: every piece of knowledge has a single, authoritative, unambiguous representation in the system. The enemy is not a repeated line; it is a repeated fact, restated in places that can disagree.

The fix is a single source of truth: one representation every reader derives from, so a change happens in exactly one place and reaches every reader at once. When a fact lives in many places instead, the copies form a drift surface — the set of sites that can fall out of agreement when a change reaches some and not others.

A fact restated in many places carries two bills. The first is a correctness bill — a surface that can drift. The second is a maintenance bill — the work and the uncertainty of changing the fact later. An agent makes the copies cheap to produce; it does not pay either bill.

A fact in k places has k−1 sites that can fall out of agreement. With one source there is nothing to keep in sync; with eight copies there are seven ways for the truth to splinter. The hero figure above is exactly this: change the fact at the source, and every duplicate that was not updated is now a stale site — quietly wrong.

Changing the fact means editing every copy of it. The more copies, the more edits — and the lower the confidence that all of them were found and updated consistently. Each extra duplicate is one more chance to miss one. Step the slider below: the bill is the edit count, and the confidence that the system stays consistent after a change falls as the copies multiply.

Take one fact on the Portal — the learning platform this course builds. The branded id format, “a 14-character TSK… id that decodes to a type”, is a single piece of knowledge. It must have one home: the Portal.ID module. The store, the Telegram bot, and the student dashboard all call Portal.ID; none re-derives the format.

Ask an agent for “an id” in three surfaces and it will re-derive “14 characters” three times. Change the format later — to fifteen — and the two surfaces it was not pointed at break in silence. The repair is not cleverness; it is naming the authority and pointing the agent at it.

# Duplicated knowledge: the literal "14" restated in three surfaces.
# Change the format and every forgotten copy is a stale, silent bug.
if byte_size(id) == 14, do: store_user(id)      # store/
if byte_size(id) == 14, do: reply(id)           # bot/   (a copy)
if byte_size(id) == 14, do: render(id)          # dashboard/ (a copy)

# One authority: the format lives once, in Portal.ID. Everyone derives from it.
id = Portal.ID.generate(:user)        # => "USR0NbAb1xcFCy"
if Portal.ID.valid?(id), do: store_user(id)   # the rule has one home

The illustrative ids and helpers follow the Portal's established branded-id convention (Portal.ID.generate/1, decode/1), taught in the companion Functional Programming in Elixir course.

DRY is not a typing rule; it is a knowledge rule, and the agent era sharpens it. Generation makes duplicates free to produce and unchanged to reconcile, so the only stable answer is one authoritative home for each fact, with the agent pointed at it rather than asked to re-derive it. That is the first of three re-weighted principles — knowledge has a home; next, a unit gets a precise specification.

Primary sources for DRY and the single source of truth, and where it connects in the course.