19-Usage-Billing-EngineReal-Time Metered Billing & Rating Engine

Idempotent consumption telemetry ingestion, multi-tiered graduated rating, append-only double-entry ledgers, and automated billing cycle invoicing.

F1: Usage Event Store

Granular metered usage records with multi-dimensional metadata, tenant isolation, and composite unique idempotency constraints.

High-throughput ingestion pipeline validating schema formats, clock drift windows, and eliminating duplicate billing retries.

Evaluates raw usage against graduated brackets, volume tiers, overages, and flat fees calculated in exact integer cents.

Append-only accounting journal strictly enforcing the fundamental invariant: sum of debits equals sum of credits.

Locks meter cycles, aggregates line items, applies discounts/taxes, and assigns gapless sequential invoice numbers.

Two-step authorization/capture, idempotent charge retries, and automated clock-driven dunning workflows.

Billing period states (OPEN/CLOSING/CLOSED) and differential testing proving pre-aggregates match raw events.

Second-exact pro-rata credit and charge calculations using Python Decimal ROUND_HALF_UP arithmetic.

Deny-by-default role-based access control, SHA-256 API key authentication, and automated PII scrubbing.

Fast Vanilla JS & CSS dark-mode dashboard displaying live revenue KPIs, invoice explorers, and line item inspectors.

Comprehensive financial systems design deep dive modeling 100,000 events/second and multi-terabyte ledger scaling.

All 275 verification checklist items thoroughly answered with implementation file paths, tests, and trade-offs.