Overview
We’ll build a simple URL shortener backend using Python (Flask) to demonstrate key concepts:
- Generate a short URL key.
- Store mapping in a database (we’ll use an in-memory dictionary for simplicity).
- Redirect short URLs to original URLs.
Step 1: Setup
JavaScript
Step 1: Setup
from flask import Flask, request, redirect, jsonify
import string
import random
app = Flask(__name__)
# In-memory store for URL mappings: short_key -> long_url
url_db = {}
# Characters used for short keys
CHARSET = string.ascii_letters + string.digits
KEY_LENGTH = 6 # short key lengthStep 2: Generate a unique short key
JavaScript
def generate_short_key():
while True:
key = ''.join(random.choices(CHARSET, k=KEY_LENGTH))
if key not in url_db:
return key
Step 3: API to shorten URL
JavaScript
@app.route('/shorten', methods=['POST'])
def shorten_url():
data = request.get_json()
long_url = data.get('long_url')
# Simple validation
if not long_url:
return jsonify({'error': 'Missing long_url'}), 400
# Generate short key
short_key = generate_short_key()
# Store mapping
url_db[short_key] = long_url
short_url = request.host_url + short_key
return jsonify({'short_url': short_url}), 201
Step 4: Redirect short URL
JavaScript
@app.route('/<short_key>')
def redirect_url(short_key):
long_url = url_db.get(short_key)
if long_url:
return redirect(long_url)
else:
return jsonify({'error': 'URL not found'}), 404
Step 5: Run the app
JavaScript
if name == 'main':
app.run(debug=True)How this works:
- When you POST a JSON like
{ "long_url": "https://example.com" }to/shorten, the server generates a 6-character key and stores it. - It returns a shortened URL like
http://localhost:5000/a1B2c3. - When you visit
http://localhost:5000/a1B2c3, it redirects tohttps://example.com.
Limitations & Improvements
- This example uses an in-memory dictionary, so URLs are lost when server restarts.
- Real systems use persistent databases like Redis, DynamoDB, or SQL.
- Key collisions are handled by retrying key generation.
- No rate limiting or authentication.
- No analytics or custom aliases.
- No input URL validation or security checks.
