Problem Statement & Context
What we are building and why it matters
Problem Statement & Context
A shopping cart service stores the intent to purchase before checkout. It must feel instantaneous, survive device changes, and protect inventory from overselling — and it is one of the highest write-QPS surfaces in all of commerce, handling both guest sessions and logged-in accounts.
The cart is a bridge
The cart sits between discovery and checkout, and its job is to freeze intent without freezing inventory. It captures item snapshots, promotions, and delivery options so pricing does not drift between add-to-cart and payment, while deliberately not hard-reserving stock on every tap (which would let abandoned carts starve real buyers).
Why it is deceptively hard
A cart looks like a key-value store but combines five hard problems at once: session management (guest vs registered + merge on login), concurrency (the same cart edited on phone and laptop), inventory validation (soft hints in cart, hard checks at checkout), pricing correctness (price + promo snapshots), and high availability (cart downtime kills conversion directly). The load-bearing idea is a two-speed consistency model: the cart itself is fast and eventually-consistent (AP), while the checkout handoff is strongly-consistent (CP) — speed where mistakes are cheap, correctness where they cost money.
Scale anchors
- 100M+ daily active shoppers
- 10–20% of page views are cart mutations
- 5–10× spikes during sales
These drive cache sizing (the cart lives hot in memory) and the write-behind durability strategy.
Key Highlights
- •A shopping cart stores the INTENT to purchase before checkout — instant, surviving device changes, protecting inventory; one of the highest write-QPS surfaces, handling guest sessions + logged-in accounts
- •The cart is a bridge between discovery + checkout: freeze INTENT without freezing INVENTORY — capture price/promo snapshots so pricing doesn't drift, while NOT hard-reserving stock on every tap (which would let abandoned carts starve real buyers)
- •Deceptively hard: combines session management (guest/registered + merge-on-login), concurrency (phone + laptop), inventory validation (soft in cart / hard at checkout), pricing correctness (snapshots), and high availability (downtime kills conversion)
- •The load-bearing idea is a TWO-SPEED consistency model: the cart is fast + eventually-consistent (AP), the checkout handoff is strongly-consistent (CP) — speed where mistakes are cheap, correctness where they cost money; anchors 100M+ DAU, 10-20% of views are mutations, 5-10× sale spikes
Section Rescue Kit
Buzzwords to use:
Safe statements:
- "I will treat the cart as a write-heavy, conversion-critical system."
- "I will separate guest and logged-in cart flows explicitly."