How I built a billing system with a real double-entry ledger in Node + PostgreSQL

Short summary

Most billing systems fail because they store balance as mutable data, losing auditability when errors occur. The solution is a double-entry ledger: an immutable append-only log where every transaction is recorded and balance is always derived, never stored. This guarantees consistency and makes corrections trivial—applying 500 years of accounting wisdom to modern code.

• •Storing balance as mutable state causes unrecoverable errors when payments duplicate or entries need correction
• •Double-entry ledger pattern: record all transactions immutably, derive balance on-demand from posted entries only
• •Enables reversible posting/un-posting, auditable history, and easy corrections without manual cascade fixes

Generated with AI, which can make mistakes.