Project Pitch: Real-Time Usage & Metered Billing Engine

30-Second Elevator Pitch

"I designed and built 19-Usage-Billing-Engine, an enterprise-grade metered billing, rating, and invoicing platform in Python FastAPI, PostgreSQL, and React. It solves the critical financial risk of usage-based pricing by providing an idempotent ingestion pipeline that guarantees exactly-once billing under network retries. It features a multi-tiered rating engine supporting graduated and volume pricing calculated to exact integer cents, an append-only double-entry ledger that strictly enforces debit-credit conservation, and automated billing cycle reconciliation with gapless invoice numbering."

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2-Minute Technical Interview Pitch

"As SaaS companies transition from flat subscriptions to usage-based models, their billing infrastructure faces severe reliability challenges: duplicate events caused by network retries, floating-point rounding errors on micro-transactions, and complex mid-cycle plan upgrades requiring exact pro-rata math.

To solve these challenges, I engineered a high-throughput, audit-proof billing platform.

At the edge, in metering/ingestion.py, the ingestion pipeline handles high-volume telemetry events like API calls or GPU compute seconds. Every event is checked against a cryptographic idempotency key index in PostgreSQL. If client network timeouts cause retries, our system recognizes duplicate keys in constant time and returns the cached confirmation, preventing double-billing without dropping valid traffic.

In rating/engine.py, the rating engine evaluates accumulated raw usage against dynamic customer price books. It supports flat rates, per-unit metering, volume discounts, and graduated slab tiers—where the first 1,000 units might be free, the next 10,000 cost 2 cents, and beyond costs 1 cent. All mathematical operations are calculated in integer minor units (cents) or fixed-point Decimal with ROUND_HALF_UP, eliminating floating-point rounding drift.

In invoicing/manager.py, when a billing period closes, the invoice manager locks customer meters, aggregates usage rollups, applies promotional discounts and taxes, and assigns a gapless sequential invoice number (INV-YYYY-XXXX) using row-level locking.

Crucially, every financial mutation is synchronized with an append-only double-entry ledger in models/ledger.py. Every ledger transaction enforces that the sum of debits strictly equals the sum of credits across at least two entries, guaranteeing accounting compliance.

The frontend provides an interactive billing console with real-time KPI metrics and invoice inspection. The entire architecture is verified by 87 automated tests covering timestamp drift rejection, differential rollup reconciliation, and dunning schedules."