Step 8: Failure Modes and Operations

Most systems lose time when they fail. A payment system can lose money, and in either direction. A customer can be charged for an order the merchant never hears about, or a merchant can ship goods for a charge that never went through. Every failure in this lesson is judged by one question: does any money end up in the wrong place, and if so, how is it found and put back?

Three mechanisms from earlier steps answer most of these failures. The transaction row is written as pending before the bank is called, so every attempt leaves a record