Claude Sonnet 5 vs DeepSeek V4 Pro for code review

## The concurrency failure, step by step

Imagine stock for SKU "WIDGET" is 10, and two customers try to buy at nearly the same instant. Customer A wants 8, Customer B wants 5. Each request lands on a different instance behind the load balancer.

1. **Instance 1** (serving A) reads the stock row: qty = 10.
2. **Instance 2** (serving B) reads the stock row: qty = 10. (This happens before Instance 1 has written anything back.)
3. Instance 1 checks: is 10 ≥ 8? Yes → proceeds.
4. Instance 2 checks: is 10 ≥ 5? Yes → proceeds.
5. Instance 1 writes: q…

Here’s the breakdown.

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**The concurrency failure — a step-by-step story**

Imagine two customers, Alice and Bob, both trying to reserve the last item of a popular SKU that has only **1 unit** left in stock.

1. **Alice’s request** reads the current stock: `SELECT qty …` → finds `1`.
2. **Bob’s request** reads the current stock: `SELECT qty …` → *also* finds `1`, because Alice’s request hasn’t written anything yet.
3. Both requests check: `1 >= qty_needed`. Both pass the check with `true`.
4. **Alice’s request** runs the `UPDATE`, setting s…