Design Abandoned Cart Recovery

Problem Statement: Abandoned Cart Recovery

Design an abandoned-cart recovery system: detect when a shopper leaves items unpaid, then nudge them back through email, SMS, or push with a personalized cart deep link. The stakes are large — Shopify merchants lose roughly 70% of carts — so recovery is revenue-critical marketing infrastructure, not "a cron job that blasts generic emails."

Frame it as an event-driven lifecycle

What interviewers want to hear is recovery modeled as an event-driven lifecycle tied to cart mutations, checkout-funnel signals, and order completion — not a nightly SQL export of stale carts. The platform reacts to events in near real time: a cart goes idle, a reminder is scheduled; the shopper starts checkout, reminders pause; an order is paid, all sends are suppressed. It must respect consent, suppress after purchase, and attribute recovered revenue to the campaign that earned it.

The invariant that defines the system: purchase suppression

The single hard invariant is purchase suppression — never send a recovery message after the cart has converted. The failure mode that defines a bad design is batch-only email without real-time purchase suppression, which produces the embarrassing "you left something in your cart!" email after the shopper already bought it. That one mistake erodes trust and brand more than a missed recovery ever could. Everything in the architecture — the real-time event pipeline, the state machine, the cancel-on-order.paid path — exists to make purchase suppression reliable. Recovery is worthless if it nags a customer who already paid.