An installment loan is decided once. The schedule is fixed at origination and the rest is bookkeeping. A revolving line is decided every day: the borrower draws, repays, and draws again, and the balance the interest is computed on is different at the end of the day than it was at the start.
That is the whole difficulty. Nothing about a revolving product can be precomputed. There is no schedule to sign off, only a set of rules that will be applied to events nobody has seen yet.
The cycle is the unit of truth
A revolving product does not run on a loan clock. It runs on a cycle: a cut-off day, a statement, a grace period, a due date, and a minimum due. Each of those is a definition, and each one moves money. Change the cycle day from 1 to 5 and you have changed how much interest a borrower pays, how long their grace lasts, and when they fall delinquent. It looks like a scheduling preference. It is a pricing change.
The minimum due is the sharpest example. Moving it from 5.00% of the balance to 4.50% is a two-character edit that lengthens the life of every revolving loan on the book and increases the interest every borrower pays. A change that consequential should be a signed version with an effective date, not a value someone updated in a settings screen.
Interest on a balance that will not sit still
Interest on a moving balance is not the annual rate divided by twelve. It is accrued against what was actually owed on each day of the cycle, which means a draw on the 3rd and a repayment on the 27th produce a different charge than the same two amounts in the opposite order. The engine accrues per day, in integer minor units, against the balance the ledger says existed on that day.
Allocation is the second rule that decides real money. When a payment arrives, what does it pay first: fees, accrued interest, or principal? Any order is defensible. Only one of them is yours, and it has to be written down, versioned, and applied the same way to every payment, because the order determines how fast the balance falls and therefore how much interest accrues next cycle.
Seq Event Amount Hash
0415 ALLOCATE −1,404.90 4d92…e3
0416 ACCRUE +680.00 b04e…77
0417 ORIGINATE +12,000.00 8f3a…c1Why append-only matters most here
On an installment loan you can usually reconstruct what happened from the schedule. On a revolving line you cannot: the balance is a consequence of an ordered stream of events, and if any one of them is edited after the fact, every number downstream of it is now a fiction.
So nothing is edited. Every accrual, allocation, fee, and reversal is appended to a hash-chained calculation ledger, and a correction is a reversing entry that leaves the original in place. When a borrower disputes a balance eighteen cycles later, the answer is not a reconstruction. It is the record, replayed against the exact product version and market rules that were in force on each of those days.