[crdrost]

I think you are objecting to a poorly chosen example and that is okay. It _is_ a poorly chosen example.

The exact pedantry that you are making is kind of poorly chosen, in double-entry bookkeeping you would say that you record the transaction proper either once or n + 1 times, depending on how pedantic you are being. (Double-entry bookkeeping discovered the feature that in filesystems is called “journaling,” you always write first to the transaction log, which is the “1” if you answer 1, then you update the n account ledger pages involved in the transaction with their individual deltas plus the transaction log ID.) A real bank also allows accounts to go negative all the time, which is another criticism.

The “event sourcing” is actually just the transaction log, which is not double entry, which is why I say that the pedantry is poorly chosen. The ledger, in combination with “closing the books for the year,” actually instantiates a generic construction for persisting data structures, see lectures at [1], where you take periodic snapshots of a value to limit your worst-case reconstruction time while storing a tractable buffer of deltas. The point of the ledger thus is to denormalize the data to make it efficient to answer the question, “how did this client's balance change over time?” without seeking over the whole transaction log.

[1] https://courses.csail.mit.edu/6.851/spring21/lectures/