Design Decision Log

A design decision log (also called an Architecture Decision Record in software contexts) is a persistent, structured record of significant engineering decisions made during a project. Each entry captures: the decision objective, the alternatives considered, the analysis performed, the decision made, and the reasoning behind it. The log creates an institutional memory that survives personnel turnover and prevents teams from re-litigating decisions that were already made with full information.

The value of a decision log compounds over time. During initial development, it keeps the team aligned by making implicit decisions explicit. During design reviews, it provides reviewers with the reasoning behind choices rather than just the choices themselves. During debugging, it reveals which design decisions created constraints that later caused problems. During product evolution, it prevents engineers from repeating failed approaches that were already tried and rejected.

Effective decision logs are concise and honest. The alternatives section should include options that were seriously considered, not strawmen included to justify the predetermined choice. The reasoning section should acknowledge trade-offs and limitations of the selected approach. A decision log entry that says 'We chose X because it is the best option in all ways' is not a useful record — it omits the trade-offs that will matter when the product is in field service.