Design a 'Store Pickup' Feature

Problem statement

Design a Store Pickup feature (industry term: BOPIS — Buy Online, Pick Up In Store) that lets customers browse products online, see real-time stock at nearby stores, reserve items, pay, receive a "ready for pickup" notification, arrive at the store, check in, and collect their order from a designated pickup area.

This is not a simple order-status feature. The system must:

1. Show real-time per-store inventory for thousands of locations, each with tens of thousands of SKUs.
2. Reserve inventory atomically the moment a customer commits, preventing overselling across concurrent web, mobile, and in-store POS channels.
3. Select the optimal store based on distance, stock availability, pickup capacity, and promised time window.
4. Orchestrate a store-side workflow: pick, stage, and mark ready — visible to store associates on handheld devices.
5. Notify the customer when the order is ready, with a pickup window and instructions.
6. Handle customer arrival: geofence detection or manual check-in, identity verification, and handoff confirmation.
7. Maintain an audit trail of every state transition for disputes, returns, and analytics.

Why the problem is distinctive

A pure e-commerce warehouse system ships to an address. A store pickup system must coordinate physical retail operations with digital order flow. The store is simultaneously a retail floor serving walk-in customers and a micro-fulfillment center serving online orders. Inventory is shared. A customer can buy the last unit in-store while another customer is mid-checkout online.

The consistency challenge is sharper than typical e-commerce: inventory counts must be accurate enough to prevent overselling, but querying 4,700 stores × 100K SKUs in real time for every product page view is not feasible without aggressive caching and event-driven propagation.

Public operating baseline

Walmart reported handling over 75 million BOPIS orders in fiscal year 2023 across 4,700+ U.S. stores. Target fulfilled approximately 45 million Drive Up and Order Pickup orders in 2023 across ~1,950 stores. Best Buy processes roughly 40% of online orders through store pickup across ~1,000 locations. These are public earnings-call figures and establish that the category operates at massive scale.

Design assumptions for this answer

Unless tied to a citation, every number below is an explicit design assumption:

• 2,000 stores in the network.
• 80,000 SKUs per store on average.
• 500,000 BOPIS orders per day at steady state.
• 3× peak multiplier during holiday seasons (Black Friday, curbside surge).
• 15-minute SLA from order placement to "picking started" at the store.
• 2-hour default pickup window after "ready" notification.
• 500 concurrent store-associate device sessions per region.

The four architectural planes

1. Inventory plane: real-time stock levels, reservations, releases, and reconciliation across channels.
2. Order plane: order lifecycle, payment, pickup scheduling, state machine, and customer communication.
3. Store operations plane: pick lists, staging, associate workflows, handheld device sync, and capacity management.
4. Customer experience plane: store selection UI, tracking, notifications, geofence check-in, and verification.

A strong interview answer keeps these planes separate. The inventory plane can degrade to cached reads without blocking order creation. The store operations plane can continue picking even if the customer-facing notification service is down. The customer experience plane can show stale tracking without corrupting the order state.