Leave Management API (Flask + SQLAlchemy)

A simple leave-management application with frontend built using React and backend built with Flask, Flask-SQLAlchemy, and SQLite.

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Important Instructions

-While applying for leave as a user, please apply in the format DD-MM-YYYY as start date and similarly for end date. -Please wait for 1-2 minutes after the application starts as the backend goes to sleep after some time of inactivity due to deployment on Render. Will be fixed in upcoming updates

✨ Features

• User management (create & list users)
• Apply for leave, list all leave requests
• Check leave balance
• Admin: approve/reject requests and adjust balances
• Admin: cancel all leave requests for a user
• Minimal HTML tester page to exercise the API without Postman

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Tech Stack

• Backend: Python, Flask, Flask-SQLAlchemy, Flask-CORS
• Database: SQLite (file-based; can swap to PostgreSQL for production)
• Tester UI: Plain HTML + Fetch API
• WSGI/Proxy (prod): Gunicorn + Nginx/Caddy (or Render/Heroku)

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📁 Project Structure

config.py        # App & database configuration (Flask, SQLAlchemy, CORS)
main.py          # Route definitions (REST API)
models.py        # SQLAlchemy models & serializers
mydatabase.db    # SQLite database file (auto-created on first run)
index.html       # Minimal API tester UI

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⚙️ Local Setup

1) Create venv & install dependencies

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

pip install -U pip
pip install -r requirements.txt

2) Run the API (SQLite)

python main.py
# Server: http://127.0.0.1:5000

Tables are auto-created on first run.

3) Optional: Run with Postgres (docker-compose)

cp .env.example .env
# Edit .env if needed, then:
docker compose up --build

• API: http://127.0.0.1:5000
• Adminer: http://127.0.0.1:8080 (PostgreSQL, server db, user/pass in .env)

4) Try the HTML tester

Open index.html and set:

const API = "http://127.0.0.1:5000";

Then click the buttons to hit endpoints.

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🚏 REST API (quick reference)

Base URL (local): http://127.0.0.1:5000

• List users: GET /users
• Create user: POST /create_users (form-data: username, email, department, joining_date, [role], [leave_balance])
• Apply leave: POST /apply_leave (form-data: username, start_date, end_date, reason)
• List leave requests: GET /show_leaves
• Get leave balance: GET /get_leave_bal?username={username}
• Approve/Reject leave (Admin): POST /update_leave_balance (form-data: leave_id, status)
• Cancel all leaves for a user (Admin): POST /cancel_all_leave (form-data: username)

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🛡️ Validation & Security

• Validates joining date, overlapping leaves, leave balance, and user existence.
• Plan to add:
  • Marshmallow or Pydantic schemas
  • AuthN/AuthZ (JWT)
  • Global error handling
  • Rate limiting and request size limits

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📊 Scaling & Improvements

• DB: Migrate to PostgreSQL + Alembic migrations
• Runtime: Gunicorn workers behind Nginx/Caddy; connection pooling
• Caching: Redis
• Background jobs: Celery/RQ for notifications/reports
• Observability: Logging, Prometheus/Grafana, Sentry
• Documentation: OpenAPI (flasgger or Flask-Smorest)

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🚀 Deploy (Render/Heroku)

• Procfile: web: gunicorn main:app
• Set DATABASE_URL env var if using PostgreSQL; fallback is SQLite.

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✅ License

MIT (add LICENSE file)

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📑 Files Included

requirements.txt

Flask>=2.3
Flask-SQLAlchemy>=3.1
Flask-Cors>=4.0
gunicorn>=22.0
python-dotenv>=1.0
psycopg2-binary>=2.9