Design a shopping cart service. How would you handle inventory changes while items are already in users' carts?

E-commerceCartInventoryLLD

How I frame it

Cart is intent to buy, not a stock reservation. Inventory can hit zero while item sits in cart for days. Validate at checkout reserve, not on every cart add.

Data model

carts(id, user_id, updated_at)
cart_items(cart_id, sku, qty, price_snapshot, added_at)
Cart in Redis for speed; persist to DB on checkout or periodic flush.

Handling inventory changes

On cart load: Batch availability check — mark items Unavailable / Only 2 left. Do not silently remove without UX message.
Price snapshot: Store price_at_add; at checkout show "Price updated ₹X → ₹Y" if changed.
Reserve at checkout: Atomic hold (flash-sale pattern) before payment — cart qty > available → block with clear error.
No long-lived holds in cart: Prevents inventory locked by abandoned carts.

User added iPhone (stock 10) Monday
  Friday stock 0 → cart shows "Unavailable at checkout"
  Checkout → reserve() fails → suggest remove / notify-me

Walkthrough

User adds 2× headphones Monday (stock 50). Tuesday flash sale sells out. User opens cart Wednesday — batch check marks "Out of stock." User proceeds anyway → checkout calls reserve(sku, qty) → fails atomically → clear error with remove-item CTA. No payment captured, no oversell.

Weak answer

"Reserve inventory when item is added to cart." Abandoned carts lock stock for hours; flash sales die. "Remove unavailable items silently" — user confused at checkout with missing items they expected.

Follow-up questions

Merge cart on login across devices?

Union SKUs; re-validate availability on merge; higher qty wins or sum with cap at available stock message.

Promo price ends while in cart?

Checkout uses current price engine; show delta. Promo locked only if explicit voucher held with TTL.