Conflicts

Stop two people booking the same 3pm slot, taking the same username or buying the last item in stock. Each protection is a precondition: an option on a call you already make.

Uniqueness with keyed creates

Derive the key from the thing that must be unique. The second writer gets a 409; your own retry returns your record (existing: true); deleting the record frees the key. Keys are 1–120 characters of letters, numbers and : _ . @ / -.

await bookings.create({ who: email }, { key: "slot:2026-07-15T15:00" })

Limited stock: N units anyone can buy

Claim units with keyed creates, and on conflict try the next unit. When every unit is taken, the item is sold out. This holds under every safety rule, because no rule lets a person write another person's records.

await orders.create({ item: 42 }, { key: "unit:item42:1" })  // then :2 … :N

Atomic counters

Atomic ops run against the current state on the server. An op that would pass the floor or ceiling returns 409 and leaves the record untouched.

await products.update(id, { stock: { decrement: 1, floor: 0 } })

An object in a patch is an atomic op only when its keys are increment or decrement (plus an optional floor or ceiling) and every value is a number. Anything else is stored as plain data.

Shared editing with versions

Pass back the version you read. A stale save gets a 409 and leaves the other person's edit in place. Re-read, reapply your change, and retry.

await pages.update(id, { body }, { ifVersion: page.version })

One per person, limited supply

A booking that must be unique and capped per person needs two keyed creates: claim the resource first, then the per-person cap. If the second 409s, delete the first, which frees its key:

const slot = await bookings.create({ when }, { key: "slot:" + when })
try {
  await quotas.create({ ref: slot.id }, { key: "user-day:" + user.userId + ":" + day })
} catch (e) {
  if (e.code === "conflict") { await bookings.delete(slot.id) }  // give the slot back
  throw e
}

What happens if…