Ledgerline

Ledgerline reconciles payments for mid-market lenders. Their month-end close had grown into a six-hour nightly batch that failed often enough to need a person watching it, and the people watching it were the people who had built it.

We replaced the batch with an event-sourced ledger that reconciles continuously, and handed the team the runbook for it. This page is a placeholder write-up on a real template — the numbers below are illustrative until the engagement is cleared for publication.

What the batch actually cost

The six hours were not the problem. The problem was that a failure at 03:00 meant the close did not happen, and nobody knew until someone opened a dashboard at 08:00.

Deploy frequency followed

Once the close stopped being an event, releases stopped being scheduled around it. Both series below are drawn on one Y scale, so the change is not flattered by the axis.

The shape of the ledger

Every movement is an append-only event. Balances are a fold over that log, so a replay is a read rather than a repair.

-- Balance is a fold, never a stored mutable number.
SELECT account_id,
       SUM(amount) FILTER (WHERE posted_at <= $1) AS balance
FROM ledger_events
WHERE account_id = $2
GROUP BY account_id;

Handover

Three of their engineers ran the system in production before we left, each taking one on-call rotation with us on standby and not called.

They turned up with a written plan on day four and never revised the date. The handover was a repo our team could actually read.