Steps 1 and 2 are necessary because if the client directly sent the credit and debit instructions, it would require an atomic commit across those two partitions to ensure that either both or neither happen. To avoid the need for a distributed transaction, we first durably log the request as a single message, and then derive the credit and debit instructions from that first message. Single-object writes are atomic in almost all data systems (see “Single-object writes”), and so the request either appears in the log or it doesn’t, without any need for a multi-partition atomic commit.
