Sustainable design

Most of a system’s cost is paid after it works. Sustainable design optimises for the years of change that follow, not the sprint that ships.

What it means in practice

• Replaceability over reuse. A component that can be deleted and rewritten in a week is worth more than one that is cleverly shared and therefore impossible to remove.
• Boundaries that absorb change. When a requirement moves, the number of components that must move with it is the real measure of the design.
• Decisions carry their reasoning. A recorded decision that states its context and the alternatives can be revisited when the context changes; an undocumented one is re-argued from scratch every year.
• Deletion is a first-class operation. Nothing is sustainable if the only supported direction is add.

Also: what it runs on

Sustainability is not only maintenance. Wasteful architecture has an energy bill and a hardware footprint — an over-provisioned, chattery design costs carbon as reliably as it costs money. Fewer round trips and fewer moving parts tends to be the right answer for both.

The test

For any component, ask: what would it take to remove this? If the answer is unknown, that is the finding.