How to Deploy a Full-Stack App for Free: Vercel + Railway + MongoDB Atlas

User → Vercel (Next.js Frontend + API Routes)
                    ↓
          MongoDB Atlas (Database - Free Tier)
                    ↓
          Railway/Render (Background Jobs - Optional)

mongodb+srv://username:password@cluster0.xxxxx.mongodb.net/mydb

MONGODB_URI=mongodb+srv://username:password@cluster0.xxxxx.mongodb.net/mydb
NEXTAUTH_SECRET=your-random-secret-string
NEXTAUTH_URL=http://localhost:3000

typescript
// lib/mongodb.ts
import { MongoClient } from 'mongodb';

const uri = process.env.MONGODB_URI!;
let client: MongoClient;
let clientPromise: Promise<MongoClient>;

if (process.env.NODE_ENV === 'development') {
  if (!(global as any)._mongoClientPromise) {
    client = new MongoClient(uri);
    (global as any)._mongoClientPromise = client.connect();
  }
  clientPromise = (global as any)._mongoClientPromise;
} else {
  client = new MongoClient(uri);
  clientPromise = client.connect();
}

export default clientPromise;