Ledger

Ledger started as a Rails monolith with balances computed on read. By the time I joined, reconciliation ran for four hours every night and finance had a spreadsheet to correct the corrections.

We moved to an append-only double-entry event log in PostgreSQL, with projections rebuilt from the log on demand. Every balance became reproducible from first principles, and the nightly batch turned into a continuous consumer.

The migration ran dual-write for six weeks with automated diffing between old and new balances, so the cutover itself was a config flag rather than an event.