A payments company is designing the ledger behind a new global wallet product. Balance updates arrive from users in North America and Europe and must be strongly consistent worldwide; a transfer must never be visible as committed in one region and missing in another. The schema is relational, the team wants to keep writing SQL with joins and secondary indexes, write throughput is expected to outgrow what a single primary instance can absorb, and the business has signed off on a 99.999% availability target for the ledger.