Trello: what you will build

A board backend: boards with members, lists inside them, cards inside those, and a drag-and-drop ordering that does not rewrite every row every time somebody moves a card two places up.

By the end you will have:

• Boards with roles — owner, editor, viewer — enforced by named policies rather than if statements scattered through handlers
• Lists and cards, with a move endpoint that touches one row
• An activity log, written in the same transaction as the change it records

The hard part

Ordering. A card list has an order that users rearrange constantly. Storing position: 0, 1, 2, 3 means moving one card renumbers everything after it — a write per card, per drag, and two people dragging at once produce a list that is wrong for both.

The fix is to store positions as gaps rather than counts, so a move is a single write. That works until it doesn’t, and chapter 4 measures exactly when: 42 drags into the same gap before double-precision floats can no longer produce a value between two neighbours. Then it fixes that too.

That number is measured, not estimated.

The shape

src/
  schema.ts             boards, board_members, lists, cards, activity
  boards.ts             requireRole — the permission check
  policies.ts           named policies, registered once
  routes/boards/
    index.post.ts       create
    [id]/lists.post.ts  add a list
    [id]/cards.post.ts  add a card
    cards/[id]/move.patch.ts   the interesting one

Before you start

bun create theoven boards --db sqlite --auth basic
cd boards && bun install

The schema →