Six months from now, someone will look at your code and ask: why is it like this? Maybe it will be a new team member. Maybe it will be you. The code will tell them what it does. It will not tell them why. Why this library and not that one. Why this architecture. Why this tradeoff.
Without the why, the next person will make the same mistake you already considered and rejected. Or they will undo a deliberate decision, not knowing it was deliberate. Or they will spend three days investigating before arriving at the same conclusion you reached in an hour, because you had context they do not have.
Documentation is not about explaining how the code works. The code explains that. Documentation is about preserving the reasoning that the code cannot express.
Architecture Decision Records
ADRs are the simplest tool for this problem. An ADR is a short document that records a single architectural decision: what you decided, why, and what alternatives you considered.
The format is deliberately lightweight:
- Title — a short noun phrase describing the decision
- Context — the situation that prompted the decision, the constraints, the forces at play
- Decision — what you chose to do
- Consequences — what follows from this decision, both good and bad
That is it. An ADR should fit on one screen. It should take fifteen minutes to write. It lives in the repository, versioned alongside the code it describes, so it is always findable and always in sync with the branch it belongs to.
The power of ADRs is not in any individual record. It is in the trail. Over time, your ADR directory becomes a narrative of how your system evolved and why. New engineers can read it chronologically and understand not just the architecture, but the reasoning that shaped it.
The cost of oral tradition
Every team has knowledge that lives in people’s heads. Why the deploy pipeline has that extra step. Why the user table has that weird column. Why the billing service talks to the legacy API instead of the new one.
This knowledge is fragile. It leaves when people leave. It distorts as it is passed from person to person. It is unavailable at 3am when the on-call engineer is debugging an incident and the person who knows the answer is asleep.
Oral tradition is not a knowledge management strategy. It is an accident. The question is not whether you can afford to write things down. It is whether you can afford not to.
This does not mean you need a wiki with a hundred pages that nobody reads. It means you need decisions recorded where people will find them — in the repository, near the code they affect, in a format that is quick to write and quick to read.
What to document
Not everything. Documentation has a maintenance cost, and stale documentation is worse than no documentation because it is actively misleading. Document the things that are stable enough to be worth recording and important enough to be worth finding:
- Decisions — why you chose Postgres over MongoDB. Why the API uses REST instead of GraphQL. Why the service boundary is drawn here and not there.
- Tradeoffs — what you gave up and what you gained. Every architectural choice has costs. Make them explicit so the next person understands the constraints.
- Rejected alternatives — what you considered and why you did not choose it. This is the most underrated form of documentation. It prevents the next engineer from proposing the thing you already evaluated.
- Context that will be lost — if a decision only makes sense because of a constraint that might not be obvious later (a regulatory requirement, a vendor limitation, a temporary performance bottleneck), write it down. The decision will outlive the context.
Do not document how the code works. If the code needs a paragraph to explain its behavior, the code should be rewritten, not annotated. Do not document processes that change monthly — they will be wrong by the time anyone reads them.
A habit, not a project
Documentation is not something you do once during a planning phase and then forget about. It is a habit — a small act performed regularly, at the moment the decision is made, when the context is still fresh.
Finished a design discussion? Write the ADR before you write the code. Chose a library after evaluating three options? Spend ten minutes recording why. Made a tradeoff under time pressure? A two-sentence note now saves two hours of archaeology later.
The cost is low. Fifteen minutes per decision. The return is enormous: a team that can understand its own history, onboard new members quickly, and avoid repeating mistakes that were already solved.
Write it down. Your future self will thank you.