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.
You shard by user ID. 80% of writes go to 20% of shards because your top customers are assigned to the same shards. Or you shard by date and all writes go to the current month''s shard. Uneven distribution turns a scaling solution into a bottleneck.