Ledger Engine

An append-only ledger where every balance is a fold over immutable entries. Reads are served from projections that can be dropped and rebuilt from the event log at any point, which turned month-end reconciliation from a manual audit into a command.

The interesting constraint was ordering: entries arrive out of band from four upstream systems, so the engine assigns a total order at write time and refuses any entry that would leave an account unbalanced, rather than repairing it later.