🌍 What Is This Project?

This is a clean, modular URL Shortener API built using:

• ✅ FastAPI for the web API
• ✅ PostgreSQL as the main database (with SQLite fallback if Postgres is unavailable)
• ✅ Base62 encoding for short, unique, and human-friendly short codes

---

💡 What Does the API Do?

It has 2 main endpoints:

🔹 POST /shorten

• Input: A long URL (e.g., https://www.google.com)
• Output: A short URL (e.g., http://localhost:8000/bM9)

✅ If the same URL is sent again, it returns the existing short URL (no duplication).

---

🔹 GET /{short_code}

• Input: A short code from the URL (like bM9)
• Output: Redirects to the original long URL

---

🧠 How Does It Ensure Uniqueness?

Short code generation is deterministic and unique:

• When a new URL is added, it's assigned a unique auto-incrementing ID in the database.
• This ID is converted into a Base62 string (digits + A–Z + a–z).

This ensures:

• 🔒 Codes are always unique
• ❌ No random collisions
• 📏 Codes are shorter over time (first few are just 1–2 characters like 1, a, B, z, etc.)

✅ Even if a user sends the same URL again — it reuses the original short code.

---

📁 Project Structure

url_shortener/
├── app/
│   ├── main.py         # FastAPI app entry point
│   ├── database.py     # Sets up DB connection (Postgres or fallback to SQLite)
│   ├── models.py       # DB table definition using SQLAlchemy
│   ├── schemas.py      # Request/Response format validation (Pydantic)
│   ├── utils.py        # Encodes integer ID to Base62
│   └── routers/
│       └── url.py      # Main routing logic: shortening and redirecting
└── requirements.txt    # All needed packages

---

🔐 Fallback to SQLite

If DATABASE_URL is not set (e.g. in .env or environment variables), it automatically uses:

sqlite:///./url_shortener.db

This helps:

• ✅ Run the project locally without setting up Postgres
• 🔁 Still allows switching to Postgres in production (just set the DATABASE_URL)

---

🛠️ How to Run the Project

1. Clone the repo or create files as shown

2. Create a virtual environment (optional but recommended)

python -m venv venv
source venv/bin/activate  # or `venv\Scripts\activate` on Windows

3. Install dependencies

pip install -r requirements.txt

If requirements.txt doesn't exist yet, use:

pip install fastapi uvicorn sqlalchemy psycopg2-binary

4. Run the app

uvicorn app.main:app --reload

---

🧪 Test the API

🔸 POST /shorten

POST http://localhost:8000/shorten
Content-Type: application/json

{
  "original_url": "https://openai.com"
}

Response:

{
  "short_url": "http://localhost:8000/bM9"
}

---

🔸 GET /bM9

Open in browser:

http://localhost:8000/bM9

It redirects you to:

https://openai.com

---