You horizontally scaled your database to 10 shards, but 90% of traffic still hits just one of them. Writes queue, latency spikes, and one node is on fire while the others idle. This is the hot partition problem — and it's all about key design.
Network timeout on a payment request. Client retries. Customer gets charged twice. This is the most expensive bug in fintech — and it's completely preventable with idempotency keys. Here's the complete implementation.