Skip to content
sysgeeks

Design a Notification System, stage 4 of 11: decide

When are preferences applied?

A user turns off comment emails. Notifications already planned for them are sitting in a queue behind an announcement and may not be sent for an hour. The requirement says preference changes take effect immediately.

System so far· 4 parts
123SERVICEProduct servicesLOG / STREAMEvent streamWORKERNotificationplannerDATABASENotifications DB

Select a component to see what it is responsible for and which state it owns.

  1. 1Product services → Event stream: Domain events via outbox
  2. 2Event stream → Notification planner: Events, at least once
  3. 3Notification planner → Notifications DB: Insert notifications under dedupe keys
  • Asynchronous
  • Request / response

What you need to know

0 of 2 checks done

  1. A notification is planned at one moment and sent at another. Normally the gap is seconds. Behind a large announcement, or during a provider outage, it can be an hour.

    Anything decided at planning time can be out of date by sending time. The question is which decisions must be checked again just before the irreversible step.

  2. Think first

    A user turns off comment emails at 10:00. Three comment emails for them were planned at 09:55 and are queued behind an announcement until 10:40. If preferences are checked only at planning time, what happens?