Transaction Tracking Service (B4 — FastAPI Greenfield)

A lightweight transaction tracking system built with FastAPI. It records credit/debit transactions and computes a running balance.

balance = sum(credits) - sum(debits)

---

Project Structure

B4/
├── app/
│   ├── __init__.py
│   ├── main.py        # FastAPI app + entry point (API layer)
│   ├── models.py      # Domain model + TransactionType enum
│   ├── schemas.py     # Pydantic request/response schemas (validation)
│   ├── routes.py      # HTTP routes (no business logic)
│   ├── services.py    # Business layer (create/list/balance)
│   └── storage.py     # Storage layer (in-memory, swappable)
├── tests/
│   └── test_transactions.py
├── requirements.txt
├── pytest.ini
└── README.md

Layering: API (routes.py) → Business (services.py) → Storage (storage.py). Routes contain no business logic; they delegate to the service.

---

Installation

cd B4
python3 -m venv .venv
source .venv/bin/activate        # Windows: .venv\Scripts\activate
pip install -r requirements.txt

Run

source .venv/bin/activate
uvicorn app.main:app --reload

Then open the interactive docs at http://127.0.0.1:8000/docs.

Test

source .venv/bin/activate
pytest -v

---

API

POST /transactions

Create a transaction.

Request

{ "amount": 100, "type": "credit", "description": "salary" }

Response 201 Created

{ "id": 1 }

GET /transactions

List all transactions.

Response 200 OK

[
  {
    "id": 1,
    "amount": 100.0,
    "type": "credit",
    "description": "salary",
    "timestamp": "2026-06-16T09:35:57.701020Z"
  }
]

GET /balance

Response 200 OK

{ "balance": 500 }

---

Example Requests

# Create a credit
curl -X POST localhost:8000/transactions \
  -H 'Content-Type: application/json' \
  -d '{"amount":100,"type":"credit","description":"salary"}'
# -> {"id":1}

# Create a debit
curl -X POST localhost:8000/transactions \
  -H 'Content-Type: application/json' \
  -d '{"amount":40,"type":"debit","description":"lunch"}'
# -> {"id":2}

# List
curl localhost:8000/transactions

# Balance
curl localhost:8000/balance
# -> {"balance":60.0}

# Invalid (amount must be > 0) -> 422
curl -X POST localhost:8000/transactions \
  -H 'Content-Type: application/json' \
  -d '{"amount":0,"type":"credit"}'

# Invalid (type must be credit|debit) -> 422
curl -X POST localhost:8000/transactions \
  -H 'Content-Type: application/json' \
  -d '{"amount":50,"type":"transfer"}'

---

Validation

Validation is enforced by Pydantic at the API boundary (app/schemas.py):

Field	Rule
amount	required, > 0
type	required, must be credit or debit
description	optional (defaults to "")

Invalid requests return 422 Unprocessable Entity with a field-level error body.

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Notes

• Storage is in-memory — data resets on restart. The storage layer is isolated, so a real database can be added without touching routes or services.
• Timestamps are stored in UTC.