Design a Notification System

The reflex is to reach for a social notification system: an event happens, look up followers, drop a card in each feed, hand a push to Apple and Google. Shopify's version is a different animal, and naming the difference early is what separates a commerce answer from a generic one. These are transactional messages tied to the order lifecycle. An order confirmation, a shipping update, a "your package is out for delivery," an abandoned-cart nudge. A buyer expects the confirmation the moment they check out, and a lost one is not a missed like, it is a support ticket and a dent in trust. So durability and reliability lead. Raw latency matters less than being sure the message goes out at all.

Two things reshape the problem in ways a feed design never has to handle. First, almost every event has two audiences. One order fires a confirmation to the buyer and a new-order alert to the merchant, from the same event, on different channels with different rules. Second, this runs multi-tenant across millions of stores that share one pipeline, so one merchant's viral moment cannot be allowed to delay another merchant's confirmations. That fairness question is the heart of the design. It maps straight onto how Shopify already isolates stores. We build the plain fan-out first because the interviewer expects it. The back half goes to the parts that decide the round: per-tenant fairness under a flash sale, the delayed conditional abandoned-cart email, coalescing a merchant's order storm, and knowing whether an email really arrived.

The Shopify angle

A Shopify notification is a step in a commerce transaction, aimed at both sides of a sale and rendered from a template the merchant controls, so it looks nothing like a social broadcast. Shopify's mission is to make commerce better for everyone, and the notification is the message that keeps a purchase moving: the buyer's confirmation and shipping updates, the merchant's new-order and low-stock alerts. A detail worth naming is that merchants author these messages themselves.