One engineer.
The whole system.

// A partial unique index allows one live "ok" per ticket:
// two doors at once, one gets in. Retries write nothing.
export const checkInTicket = async (input: CheckInInput) => {
  const ticket = await findTicket(input);
  const ticketId = ticket?.id ?? null;
  const resultKey = getResultKey(ticket, input.eventId);

  let checkIn =
    resultKey === "ok" && ticket
      ? await insertLiveCheckIn(input, ticket.id, …)
      : await insertCheckIn(input, resultKey, ticketId);

  if (!checkIn) {
    const replay = await db.query.checkIn.findFirst({
      where: { idempotencyKey: input.idempotencyKey },
    });

    // Not a retry: another door was faster
    checkIn =
      replay ??
      (await insertCheckIn(input, "alreadyCheckedIn", ticketId));
  }