Your fintech scales beyond one region. A user in Lagos wants ledger reads from a Lagos-region replica (sub-100ms latency); a user in Nairobi wants the same from Nairobi. Easy enough for read replicas: stream WAL from primary to followers. But WRITES are tricky, you cannot allow two regions to independently post journal entries against the same user (double-spend on rebalance). The pattern: a SINGLE WRITE REGION (the primary), READ REPLICAS in other regions for low-latency reads. Writes must go to the primary even from Nairobi, taking the cross-region latency hit. Or: SHARD by user-id so each user lives in ONE write region and reads from the local replica. This lesson posts a write that flows through the primary-write topology.