Design an E-Commerce Cart & Inventory Reservation

Commerce, Payments & LedgersMediumdesign-ecommerce-checkout

Design an E-Commerce Cart & Inventory Reservation

Design cart-to-order for a large catalog: persistent carts across devices, inventory counts that are read constantly and decremented atomically, and checkout that survives partial failure.

Cart Session StateInventory ReservationEventual ConsistencyCompensating Actions

Traffic & Capacity Estimates:

50M SKUs · 500k active carts · 2k orders/second · 1000:1 read:write on inventory

Functional Requirements

•Persist a cart per user across devices and sessions, merging guest carts on login.
•Show stock availability on product pages and reserve stock at checkout.
•Create an order atomically with reservation, payment, and confirmation.
•Release reservations automatically when checkout is abandoned.

Non-Functional Requirements

•Product page reads must be served from cache — inventory freshness of a few seconds is acceptable there.
•Reservation at checkout must be exact — overselling is not.
•A payment failure must release inventory within seconds, not at the next sweep.

Back-of-the-Envelope Math

500k active carts * 5 KB = 2.5 GB — small enough to live entirely in the cache tier with durable backup.
2k orders/s * 3 items = 6k atomic inventory decrements/second, concentrated on popular SKUs.

Key Architectural Trade-offs

Displayed stock can be stale and cached; reserved stock cannot — splitting the two is what makes a 1000:1 read ratio affordable.
Reserve-on-add-to-cart protects the buyer and strands inventory; reserve-at-payment maximizes availability and produces 'sold out at checkout'.
Cart in the cache is fast and loses data on eviction; cart in the database survives and costs a write on every interaction — write-behind splits the difference.

Click or drag a component onto the canvas, then connect the handles to draw the data flow.

3 nodes · 2 edges

Components · 35

Client & Edge4

Compute & Gateway7

Storage & Caching11

Messaging & Streaming6

Coordination & Ops5

Intelligence2

React Flow