Append-Only Billing Engine

Most billing tools keep a single mutable "current balance." ISP Digital keeps an immutable, append-only ledger — one row per customer per period where due rolls forward by formula (Due = previous due + this bill − paid). Your full financial history is always reconstructable, and corrections never edit the past: a reversal row is appended and the change cascades forward through every later period so the chain always reconciles.

Underneath sits real concurrency engineering. Two-level deduplication locks — a global per-period lock plus a per-customer lock — mean the nightly Hangfire run, a manual mass-generate, and a new-customer signup can all fire at once and still produce exactly one bill per customer per month. New customers forward-fill from their joining date, and a last-day-of-month catch-up sweep mathematically guarantees no month is ever skipped.