Two-Phase Commit

1. The Incident

The store adds gift-card transfers between customers. Customer A, stored on shard 2, sends $50 to customer B on shard 5. The sharding lesson's warning now applies because one logical operation changes two databases without one transaction covering both.

The first implementation subtracts 50 from shard 2 and then adds 50 to shard 5. Thousands of transfers complete successfully through these two sequential writes.

At 2:37 AM, the application server stops after committing the subtraction but before sending the addition