E-Commerce Platform (Amazon)

Phase 1 — Clarify & Scope

Interviewer: Design an e-commerce platform like Amazon. What do you want to clarify?

Candidate: This is a massive domain — I want to understand the scope:

1. Are we designing the full platform or a specific subsystem? I'd suggest focusing on the core: product catalog, shopping cart, order placement, and inventory management.
2. What's the scale — users, orders per day, catalog size?
3. First-party inventory (Amazon owns the stock) or marketplace (third-party sellers)?
4. Are we including fulfillment/shipping, or just the order system?
5. How critical is consistency for inventory — can we oversell (like airline overbooking), or must we be exact?

Interviewer: Full platform. Focus on product catalog + search, shopping cart, order placement, and inventory management. 300 million active users, 10 million orders per day, 500 million product SKUs. Both first-party and third-party sellers. Include inventory management. No overselling — don't show "Add to Cart" if out of stock.

Candidate: Let me size this:

$\text{Order rate} = \frac{10\text{M orders/day}}{86{,}400} \approx 116 \text{ orders/sec average}$

$\text{Peak orders (10× average)} = 1{,}160 \text{ orders/sec (Prime Day)}$

$\text{Product views} = 300\text{M users} \times 20 \text{ views/day} / 86{,}400 \approx 69{,}444 \text{ views/sec}$

$\text{Catalog size} = 500\text{M SKUs}$

Cart sessions: 300M users × 30% with active carts = 90M active cart sessions.