Record business facts on the server

How to do it

Ad blockers, closed tabs and failed networks drop browser events, and a reload can send one twice. Emit order_completed from the code that commits the order, or from the confirmed payment webhook, never from the thank-you page. Write it to an outbox in the same transaction, so a rolled-back order is never counted. Give each event an ID derived from the fact, such as the order ID, and drop duplicates on that ID. Stamp it with the time the fact happened, and compare counts by that time, not by arrival.

Example

-- Daily reconciliation: must return 0 rows
SELECT day, o.orders, e.events FROM
  (SELECT (created_at AT TIME ZONE 'UTC')::date AS day, count(*) AS orders FROM orders GROUP BY 1) o
  FULL JOIN (SELECT (occurred_at AT TIME ZONE 'UTC')::date AS day, count(DISTINCT event_id) AS events
             FROM events WHERE name = 'order_completed' GROUP BY 1) e USING (day)
WHERE o.orders IS DISTINCT FROM e.events AND day < current_date - 1;

await db.transaction(async tx => {
  const order = await tx.orders.insert(cart);
  await tx.outbox.insert({ kind: 'analytics', payload: { event_id: `order-${order.id}`, name: 'order_completed', plan: order.plan, amount_cents: order.totalCents } });
});

with db.transaction() as tx:
    order = tx.orders.insert(cart)
    tx.outbox.insert(kind="analytics", payload={
        "event_id": f"order-{order.id}", "name": "order_completed",
        "plan": order.plan, "amount_cents": order.total_cents})

How to check

Place 20 test orders with an ad blocker on. The events table holds exactly 20 order_completed rows, matching the orders table. A daily query compares both counts and alerts on any difference.