Send one message per event

How to do it

Give each event and recipient a key, such as order-1042:shipped:user-77, with a unique constraint, as in send each message once. Order channels from least to most intrusive, such as in-app, then push, then email. Schedule the next channel after a wait, such as 15 minutes, and cancel it when the in-app item is read. Only critical types may use several channels at once.

Example

const ESCALATION_WAIT_MS = 15 * 60 * 1000;

export async function deliver(event, userId) {
  const key = `${event.id}:${userId}`;
  const [first, ...rest] = await channelsFor(userId, event.type);
  if (!(await db.deliveries.insertIfNew({ key, channel: first }))) return;
  rest.forEach((channel, step) =>
    queue.add('escalate', { key, channel }, { delay: ESCALATION_WAIT_MS * (step + 1) }));
}
// The escalate worker skips the job when the in-app item for this key is already read.

from datetime import timedelta

ESCALATION_WAIT = timedelta(minutes=15)

def deliver(event, user_id):
    key = f"{event.id}:{user_id}"
    first, *rest = channels_for(user_id, event.type)
    if not db.deliveries.insert_if_new(key=key, channel=first):
        return
    for step, channel in enumerate(rest, start=1):
        queue.enqueue_in(ESCALATION_WAIT * step, escalate, key, channel)

How to check

Trigger a shipment and read it in the app within the wait. One in-app item exists, and no push or email is sent. Trigger the same event twice and the count stays at one.