Billing Engine

A lightweight, modular, backend‑only billing engine built with FastAPI, SQLAlchemy, and a clean service‑layer architecture. It ingests usage events, aggregates them, prices them, and generates invoices — all without any frontend or dashboard.

This project is ideal for demonstrating backend engineering skills such as:

• API design
• Background job processing
• Database modeling
• Clean architecture
• Usage metering
• Invoice generation
• Scheduling and workers

✨ Features

Record usage events (e.g., API calls, storage, workflows)

Daily usage aggregation (group + sum raw events)

Pricing engine (simple per‑unit pricing, easily extendable)

Invoice preview (estimate charges before billing period ends)

Invoice generation (creates real invoice records)

Clean service layer (routers → services → models)

Background‑safe aggregation (manual or scheduled)

SQLite for simplicity (swap for Postgres easily)

📁 Project Structure

billing_engine/
│
├── app/
│   ├── routers/
│   │   ├── usage.py
│   │   ├── invoice.py
│   │   └── aggregator.py
│   ├── services/
│   │   ├── usage_service.py
│   │   ├── invoice_service.py
│   │   └── aggregator_service.py
│   ├── models.py
|   ├── aggregator.py
|   ├── pricing.py
│   ├── schemas.py
│   ├── deps.py
│   └── database.py
│
├── tests.py/
|   ├── conftest.py
|   ├── test_aggregator.py
|   ├── test_api.py
|   ├── test_invoice.py
|   ├── test_usage.py
├── scheduler.py
├── main.py
├── worker.py
└── README.md
└── LICENSE.md
└── CONTRIBUITING.md

🚀 Getting Started

Create & activate a virtual environment

python -m venv venv
source venv/bin/activate   # macOS/Linux
venv\Scripts\activate      # Windows

Install dependencies

pip install -r requirements.txt

Start Redis (required for the worker)

On Windows (WSL recommended):

redis-server

Start the FastAPI server

uvicorn main:app --reload

Your API is now live at:

http://127.0.0.1:8000
Swagger UI:

http://127.0.0.1:8000/docs

Start the background worker

In a new terminal:

rq worker

Start the scheduler (optional)

In another terminal:

python scheduler.py

• This triggers daily aggregation automatically.

🧪 How to Use the API (Step‑by‑Step) Swagger UI

• Record usage

• POST /usage

• Example:

• json { "customerid": "cust123", "metric": "api_calls", "units": 500 }

  Run aggregation manually

POST /aggregate/run

• This groups + sums raw usage into daily totals.

Preview an invoice

GET /invoice/preview/{customer_id}

• Returns estimated charges for the current billing period.

Generate a real invoice

POST /invoice/{customer_id}

• Creates an invoice record in the database.

🧠 How It Works (Conceptual Overview)

Usage Ingestion Raw events are stored exactly as they happen:

• customer

• metric

• units

• timestamp

• This keeps ingestion fast and append‑only.

Aggregation

A background job periodically:

fetches raw events

groups them by metric + customer

sums units

writes aggregated rows

This reduces billing load and keeps invoices fast.

Pricing Engine

A simple per‑unit pricing dictionary:

PRICING = {
    "api_calls": 0.0005,
    "storage_gb": 0.25,
    "workflows": 0.10,
}

You can easily extend this to:

tiered pricing

free allowances

per‑customer overrides

Invoice Preview

Reads aggregated usage so far and returns:

usage totals

estimated amount

billing window

No DB writes.

Invoice Generation

Reads aggregated usage, calculates the final amount, and writes an invoice row.

📦 Tech Stack

• FastAPI — API framework

• SQLAlchemy — ORM + models

• SQLite — local development DB

• RQ (Redis Queue) — background worker

• Redis — job queue backend

• Uvicorn — ASGI server

🧱 Database Models

• UsageEvent Raw usage events.

• AggregatedUsage Daily totals per metric.

• Invoice Final billed amounts.

• Built By Roy Peters 😁